Cameras

This is a continuation from the previous tutorial - Liquid Crystals.

 

1. Introduction

Thanks to technical progress and vigorous competition , the camera buyer faces a difficult challenge in making a choice. This chapter will attempt to reduce the difficulty by asking the buyer to consider the final image; its purpose, its audience, and its appearance.

Next, some of the more recent technical features are discussed. These include the intriguing ability to select objects in a scene for focus and/ or exposure measurement by tracking the position of the user’s eye. Finally, various types of cameras and their accessories are described.

In terms of technical sophistication, a moderately priced 35 mm snapshot camera made today would astonish a photographer who was suddenly time shifted from the 1950’s. Consider the automation of exposure, focus, film loading, winding, rewinding, plus flash exposures from a tiny integral electronic flash unit no bigger than a spare roll of film.

The net result, for the snapshooter, is a higher percentage of ‘‘good’’ pictures per roll of film than ever before. The specialist also profits, particularly when the basis and limits of the feature are understood.

A good share of these technical features have been incorporated in the more advanced cameras ; sometimes just because it can be done. Looking beyond this, the most basic technical camera ever made, the view camera, remains virtually unchanged for the past century. It is to photography what the wooden match is to fire making.

Portions of this chapter are adapted from the author’s recent book, ‘‘Camera Technology: The Dark Side of The Lens’’ (Academic Press, 1992). The author acknowledges, with thanks, the permission granted by Academic Press to use certain material from that book in this chapter.

 

2. Background

Imagine the first camera as nothing more than a tent with a small hole in the side casting an image upon the opposite wall. From this accidental version of a ‘‘pinhole’’ camera to today’s ‘‘smart’’ cameras, we find a cornucopia of ingenuity embracing optics, mechanics, electronics, and chemistry.

The variety of cameras ranges from one tiny enough to be concealed in a man’s ring to one large enough for several people to walk around in without obscuring the image. The price range of cameras stretches from under five dollars for a disposable model (complete with film) to several thousand dollars (without film).

Cameras have recorded images of the deepest ocean trenches and the surface features of Jupiter’s moons.

 

 

There are cameras that can freeze a bullet in midair or compress the germination of an acorn into a few minutes. From intimate portraits of bacteria to a \(360^\circ\) panoramic view of the Grand Canyon, there’s a camera for any task. 

Nonetheless, there is a common denominator: all cameras produce an image. This image may be the end product, or it may be converted in some way to the final image intended for viewing , as shown in Fig. 1.

To choose the best camera for a given task, the properties of this final image should be determined first.

 

3. Properties of the final image

1 . Appearance

• a . Black-and-white
• b . Color
• c . High contrast
• d . Continuous tone

2 . Smallest detail to be resolved

3 . Type of display

• a . Audience population
• b . Viewing conditions

(2) Ambient illumination

c . Display choices

4 . Distribution

By considering the properties listed, we’re obliged to visualize the final image through the viewer’s eyes. Esthetics aside, we’ll assume that the prime purpose of the final image is to convey information to the viewer.

 

4. Film Choice

The appearance of the final image affects the choice of a camera by the kind of film required to produce that appearance. There are some films that are not available in all sizes.

Other films are available in certain sizes only by special order. The availability of some films in some sizes changes over time, so check with your supplier before you select a camera for which film may be scarce.

Most film makers will be glad to send you their latest data on their current films, but be prepared for changes, because this is a very competitive field. New 35-mm color films in particular seem to come out with every change in the seasons.

 

5.Resolving Fine Detail

If the information in the final image is to be of any use, it must be legible to its detector, which we’ll assume to be the human eye. Figure 2 shows that for high-contrast detail

 

 

viewed under at least 50 foot-candles (office lighting), the eye has an angular resolution of about one minute of arc. This means that we can resolve about seven line-pairs per millimeter (LP / mm) at a distance of 250 mm.

Since most photographic images exhibit moderate contrast and are viewed in moderate light, a more conservative limit of resolution would be 3.4 minutes of arc, which is good enough to resolve a pattern of two LP/mm at 250 mm.

In most cases, the final image is a magnification of the primary image formed in the camera. All else being equal, there is a practical limit to the extent of this magnification, after which the structure of the film, residual lens aberrations, focus inaccuracy, and/or diffraction effects begin to obscure fine image details.

Suppose then, that for some film we set a practical limit of magnification at 10X. Based on the visual resolution limit given previously, the smallest detail in the primary image could be 20 LP/mm, each line 0.025 mm wide.

Looking at it another way, if you want to photograph fine details and display the image legibly at a distance of 250 mm from the viewer, choose a film that will clearly resolve at least 20 LP/mm and is capable of being enlarged 10 diameters without its grain or other structure obscuring the image. Most films in common use today easily satisfy this criterion.

 

6. Film Sizes

In terms of the widest variety of films available, 35-mm ranks number one. The most common format for this film is 24 X 36 mm. Although seldom used today, other 35-mm formats include 18 X 24 mm and 24 X 24 mm.

Next in line for a broad choice of film types is known as medium - format and is sold in 61.5-mm-wide rolls. The shortest rolls are paper-backed and are called 120. Many cameras that accept 120 film will also accept 220 film, which has an opaque paper leader and trailer, but no paper backing over the film. This permits a longer strip of film (more exposures per roll) and better film flatness than 120.

Common formats include (nominal dimensions) 45 X 60mm, 60 X 60 mm, 60 X 70 mm, and 60 X 90 mm. Some medium-format cameras also accept 70-mm film that has a row of sprocket holes along each edge and may be loaded in special cassettes for use in the camera’s large capacity, motorized, interchangeable film magazine.

The large formats , commonly referred to by their sheet film sizes in inches include 4 X 5, 5 X 7, and 8 X 10, to name the most well known. They may not offer as broad a choice of film as the smaller formats, but the most essential films are available for them.

 

7. Display

Choosing the best type of display for the final image should start with the number of people in the viewing audience. For large groups, a projected transparency has the advantage of being visible to the entire audience simultaneously.

This is especially important if you want to use a pointer to single out detail in the image. Image detail should be clearly resolved by everyone in the audience, from the front row (image not too grainy) to the last row (image detail within the visual limits).

In some cases, the best display is both a projected transparency that the lecturer can refer to with a pointer and a print for each viewer to examine closely, regardless of his position in the audience.

For the best viewing of projected images, the only light striking the screen should be that coming through the transparency. In other words, the room should be pitch black. Unless this condition is met, there is no possibility of reproducing the full tonal range, from deepest black to sparkling white, that the image could contain.

When this condition is difficult to satisfy, a self-luminous display may be best. One or more video monitors located at strategic points can provide good image contrast even under office illumination.

The type of monitor may be the conventional cathode ray tube (CRT) or liquid crystal display (LCD). Of the two, the CRT produces a brighter image and is the least expensive. But it is bulky and fragile. The LCD has the virtue of minimal thickness; it’s a flat screen display that can be hung on a wall like a framed photo.

At present, neither type can equal the fine detail and subtle color reproduction of a high-grade projected transparency viewed under the proper conditions. However, the gap in image quality is closing, especially now that high-definition television (HDTV) shows promise to become widely available in the near future.

 

8. Distributing The Image

For many applications, the ease and speed with which an image can be distributed is crucial. Thanks to scanners, fax machines, modems , color photocopy machines , rapid photofinishing plants, self-processing ‘‘instant’’ films, etc., we can send practically any image to practically anyone who wants it in a matter of minutes.

At present, our ability to do this depends on transforming the analog information in the subject into digital information for transmission, reception, manipulation, analysis, storage, and/or display, as indicated in Fig. 1.

 

9. Video Cameras

If speed of acquisition and distribution is most important, we can capture the image on the charge coupled device (CCD) of the widely available camcorder, whose video and audio output signals are available in real time. These video cameras are versatile and moderately priced.

The still-picture counterpart to the camcorder seems to have come to a fork in the road. One path goes to a complete camera system, designed from scratch around the CCD chip and incorporating a miniature magnetic disk drive.

The second path leads to a special video back , designed to replace the standard back of a conventional (film) camera . The video back contains a CCD chip and associated circuitry. In some cases the video back and the recorder, in which hundreds of images can be stored, require an ‘‘umbilical’’ cord between them. Some of the newer designs have integrated the back and recorder into a single (cordless) unit. Some of these backs can store up to 50 images internally.

As the capacity for image storage and/or manipulation grows, we see the emergence of systems within systems, where black box A converts black box B to communicate with computer C as long as you have the right adapter cables D, E, and F. This is typical of many rapidly expanding technologies.

Users of a video back on a conventional film camera will notice an unusually narrow angle of view for the lens in use if the light-sensitive area of the CCD chip is smaller than that of the film normally used in the camera. The reason is that only the central region of the camera’s format is used. The result is that the camera’s lenses perform as though their focal lengths have been ‘‘stretched’’ compared to their performance with conventional film that covers the whole format.

For example, Kodak’s DCS 200 replaces the back of an unmodified 35-mm SLR camera, the Nikon N8008s.

The ‘‘normal’’ lens for this camera’s 24 X 36-mm film format has a 50 mm focal length , producing a (diagonal) angle of view of about \(47^\circ\). The same lens used with the video back produces an angle of view of \(37^\circ\) because the CCD measures only 9.3 X 14 mm . To duplicate the \(47^\circ\) angle of view for this size CCD, a 19.3 mm focal length lens should be used.

Concerning the resolution from CCD images, Kodak’s data for the DCS 200 gives a count of 1.54 million (square) pixels, arranged in a 1012 X 1524-pixel array that measures 9.3 X 14 mm. This gives a pixel spacing of 0.018 mm, which theoretically can resolve 54.4 monochromatic LP/mm.

The color version uses a checkerboard pattern of red, green, and blue filters over the array, so divide the monochrome figure by three to come up with a color resolution of 18.1 LP/mm.

This is quite close to the criterion, discussed earlier, of two LP/mm for a 10X enlargement viewed at 250 mm. A 10X enlargement of the CCD image just described would measure 93 X 140 mm, about the size of a typical snapshot.

 

10. Instant Pictures

For many applications, instant, self-processing film is the best choice. A familiar example is the oscilloscope camera loaded with high-speed film. With minimum, moderately priced equipment, a transient waveform on the scope screen can be captured on the film. Seconds later the print can be examined.

Polaroid dominates this field, which they spawned in 1948. Their range of camera models goes from snapshot to trucksize. They also have special backs which can be used on various cameras to adapt them for use with Polaroid films.

These films range from 35-mm color transparency to 8 X 10-inch (and larger) color print. Included in this variety are black-and-white sheet films that yield both a positive print and a negative.

The negative must be stabilized, then washed and dried before being placed in an enlarger or contact printer.

 

11. Critical Features

In many cases, the availability of an accessory such as a Polaroid and/or digital image back is important enough to dictate the choice of a camera. Other factors that may tip the scales in favor of one camera over another might not be discovered until the chosen camera is used for some time.

For example, it may be very useful to have the kind of exposure automation that measures the light reflected from the film plane, before and during the exposure, thus being capable of responding instantly to any change in the scene luminance. There are some cameras that have this capability, yet they lack another feature that may be more valuable for some kinds of photography: the ability to observe the image through the viewfinder of an SLR not just before, but during the exposure.

Most SLRs employ a mirror that swings out of the way just before the exposure begins. This allows the image-forming light to reach the film, but it also blacks out the viewfinder, so that during the crucial instant of the exposure the photographer is momentarily blind.

 

 

Figure 3 illustrates that by using a beam splitter instead of a conventional mirror in an SLR, the problem is eliminated. 

When the advantages of a beam-splitting system are considered , it seems strange that the feature isn’t used more widely. Eliminating the swinging mirror reduces the noise and vibration generated each time an exposure is made.

This can be crucial when the camera is attached to a microscope or telescope. Some SLRs provide for the mirror to be locked in its raised (shooting) position when desired.

 

12. Time Lag

Even more important for some types of photography, substituting a beam splitter for a moving mirror in an SLR should reduce the camera’s time lag. This is the interval between pressing the camera’s trip button and the beginning of the exposure.

It’s a characteristic shared by all cameras and is rarely mentioned in a manufacturer’s specifications for his camera. With few exceptions, time lag has increased in step with camera automation.

Testing 40 different 35-mm SLRs for their time lag resulted in a broad range, with the minimum of 46 ms and the maximum of 230 ms. The average was 120 ms. Figure 4 shows that during this interval, a walker moves about 0.8 ft, a runner about twice as far, a galloping horse about 7.0 ft, and a car going 60 mph moves 10.6 ft.

Various other cameras were also tested for their time lag, with these results:

Minox 35 EL (35-mm ultracompact): 8 ms
Leica M3 (35-mm coupled range finder classic): 17 ms
Hasselblad 500C (6 3 6-cm SLR classic): 82 ms
Kodak Disk 4000 (subminiature snapshot): 270 ms
Polaroid SX-70 Sonar (autofocus instant SLR): 600 ms

 

 

FIGURE 4 Time and motion.

 

13. Automation

Camera automation has taken full advantage of the miniaturization and economy of electronic devices, making two features, autoexposure and autofocus, available in all but the least expensive cameras. This increases the percentage of (technically) good photos per roll of film exposed by the typical amateur.

It’s the amateur photographer that is first served when it comes to most of the significant camera automation features. Curious as this may seem, camera makers prefer to introduce a new concept by offering it first in a model intended for the casual snapshooter.

This generally means large numbers will be produced. If problems with the feature show up, improvements are made and a ‘‘new, improved’’ model follows. Typically, the feature will be scoffed at by the more seasoned photographer who has learned to overcome the difficulties of making a technically good photograph with the most basic equipment.

In time, the new feature is mature enough to be included in the camera maker’s premier model. Eventually, even those that scof fed at the feature in its infancy learn to love it, but only after they discover how to recognize and compensate for its weaknesses, if any.

 

3.1 Autoexposure

Early autoexposure systems measured the average luminance of a scene with a selenium photocell , then regulated the shutter speed and/or f-stop based on the deflection of a galvanometer connected to the photocell. These were known as trapped needle systems and were successful in their prime mission: to produce acceptable exposures in snapshot cameras with the just-available color films, whose exposure error tolerance is much smaller than that of black-and-white film.

Most of the first generation autoexposure cameras using the trapped needle system relied on brute force, requiring a long, hard push to trip the camera. This caused camera motion, resulting in a (correctly exposed) smeared image. Nonetheless, many resourceful photographers used these early autoexposure cameras, bolted together with an intervalometer and electromagnetic tripping system, to create an unmanned camera for surveillance, traffic studies, etc.

Amateur movie cameras eagerly adopted autoexposure systems, which proved to be at least as much, if not more, of an improvement for them as they were in still cameras.

The movie camera autoexposure systems work by regulating the lens opening (the f-stop), either with a galvonometer or a servomotor. With autoexposure, the movie maker can follow the subject as it moves from bright sunshine to deep shade without the distraction of manually adjusting the f-stop.

This same freedom to follow action without the distraction of manually resetting camera and/or lens controls explains the need for autofocus, a feature whose introduction enjoyed greater enthusiasm from amateur movie makers than from still photographers.

Once again, the amateur models were the first to incorporate the feature, but in far less time than it took for autoexposure’s acceptance, autofocus became a standard feature in both the amateur and front-line models from most of the makers of 35-mm cameras.

There are similarities between the automation of exposure and focusing. Both have become increasingly sophisticated as user expectations increase. Paradoxically, in the effort to perfect the making of a routine snapshot, some of the more sophisticated automation intrudes on the process by offering the user certain choices. Instead of simplifying photography, these technological marvels require the user to select a mode of operation from several available modes.

For example, many cameras with autoexposure offer factory-programmed combinations of shutter speed and f-stop that favor: 

• Action: fast shutter speed, wide f-stop
• Maximum depth of field: small f-stop, slow shutter speed
• Average scenes: midway between the first two
• Fill-flash: to illuminate portraits made against the light (backlit)

It comes down to this: if you know enough about photographic principles to choose the best autoexposure program, you will rarely need any of them. But when an unexpected change in the subject occurs, such as a cloud moving across the sun, some form of autoexposure can be valuable.

One of the more helpful refinements of autoexposure is the automatic shift of shutter speed with the focal length setting of a zoom lens. This is based on the time-honored guide that gives the slowest shutter speed that may be used without objectionable image motion from normal body tremor. The rule of thumb is to use the shutter speed given by the reciprocal of the lens’s focal length.

For example, if you’re using a 35- to 105-mm zoom lens, the slowest shutter speed for arresting body tremor will shift as you zoom , from 1 / 35 s to 1 / 105 s (nominal). If the focal length’s reciprocal doesn’t coincide with a marked shutter speed, use the next faster speed. This guide applies to a hand-held camera, not for a camera mounted on a tripod.

Another autoexposure refinement combines a segmented silicon or gallium photocell with a microprocessor to automatically select the best exposure based on the distribution of light reflected from the subject.

It amounts to making a series of narrow-angle ‘‘spot’’ readings of the subject, then assigning weighting factors to the different readings according to their relative importance. The weighting factors are determined by the camera maker based on the analysis of thousands of photographs.

Reduced to its most spartan form, a segmented photocell could have a very small central region, surrounded by a broad field. The user can flip a switch to select the desired reading—the center segment for spot readings, the broad segment for full field readings, or both segments for center-weighted full field readings.

To ensure optimum exposure for a subject, seasoned photographers ‘‘bracket’’ exposure settings by making at least three exposures of the subject. The first exposure obeys the meter’s reading. The next two are one exposure step less and one greater than the first. This exposure bracketing, with some variations, has been incorporated as an on-demand automatic feature in some cameras.

 

3.2 Autofocus

Autofocus, in one form or another, has become a standard feature in camcorders and in most 35-mm cameras. The latter can be divided into two main types: (1) the snapshot ‘‘point – and – shoot ,’’ also known as ‘‘PHD’’ (press here, dummy) and (2) the SLR, spanning a wide range in price and sophistication.

In between, there are several models which can be thought of as ‘‘PHDs on steroids.’’ They have zoom lenses and elaborate viewfinders, making them too bulky to fit easily into a shirt pocket.

There are two main types of autofocus systems, the active and the passive. The active type emits a signal toward the subject and determines the subject’s distance by measuring some property of the reflected signal. The passive type measures subject distance by analyzing the subject’s image.

Active Autofocus Systems.  Nearly every active system uses two windows, spaced some distance apart. The user centers the subject in the viewfinder’s aiming circle and presses the shutter trip button. Figure 5 shows how a narrow infrared beam is projected from one of the windows, strikes the subject, and is reflected back to the second window.

A photocell behind this window detects the reflected beam. The photocell is sensitive to the position of the beam on its surface and relays this information to its associated circuitry to regulate the camera’s focus setting.

Initially , this was a straight-forward triangulation system, using a single infrared beam. But too many users were getting out-of-focus pictures of the main subject when it wasn’t in the center of the picture. The camera’s instruction book gives the solution: center the

 

 

main subject in the finder’s aiming circle, press the trip button halfway down, and hold it there, then recompose the scene and press the trip button all the way to make the exposure. This requires a fair amount of concentration and discipline, so it contradicted the purpose of having an automatic camera—to be free of cumbersome details, relying on the camera to make properly exposed, sharp photos. 

A big improvement was made by projecting three beams from the camera, instead of one. The beams are divergent and the center beam coincides with the finder’s aiming circle. Focus is set on the object closest to the camera.

A very different type of active autofocus is the ultrasonic system used by Polaroid in several models. Basically, it’s a time-of-flight device that’s been compared to sonar and bats. It uses an electrostatic transducer to emit an ultrasonic ‘‘chirp’’ towards the subject.

Based on a round-trip travel time of about 5.9 milliseconds per meter, the time it takes for the chirp to reach the subject and be reflected back to the camera is translated into subject distance and a servomotor sets the focus accordingly.

A significant advantage of the active autofocus systems just described is their ability to work in total darkness. On the minus side is their inability to focus through a pane of glass or on a subject with an oblique glossy surface that reflects the signal away from the camera.

Passive Autofocus Systems.  Passive autofocus systems can be broadly characterized as acquiring two views of the subject, each view coming from a slightly different position, then focusing the lens to make the two views match. In this sense, the system operates just like a coincidence-type of optical range finder, but there are important differences.

With an optical range finder we rely on our ability to see when the two images are perfectly superimposed, so our focusing accuracy depends on our visual acuity. In a passive autofocus system, we relieve our eye of this burden and let the tireless electro-optical technology take over.

For the point-and-shoot camera, a passive autofocus system uses two windows, one whose line of sight coincides with that of the viewfinder’s, and a second window , spaced some distance from the first. A simple, symmetrical optical system behind the windows includes a CCD for each window.

The signal from the first CCD is taken as the reference against which the second CCD’s signal is compared. Differences in the light distribution and/or differences in the relative location of the waveforms causes the control circuit to change the focus setting.

Autofocus SLRs.   Instead of the two windows just described, autofocus SLRs use two bean-shaped segments on opposite sides of the camera lens’s exit pupil . Figure 6 shows how this is done. Two small lenslets are located a short distance behind the geometric equivalent of the camera’s film plane. Each lenslet receives light only from its side of the exit pupil and projects it onto a CCD line array, one for each lenslet.

The relative position of each image on its CCD strip is analyzed by the system’s microcomputer which is programmed to recognize the focus condition as a function of the CCD’s signals. If the signals deviate from the programmed values, the microcomputer issues the appropriate command to the focus motor.

For off-center subjects, it’s necessary to prefocus on them by pressing the trip button halfway, holding it there as you recompose the scene, then pressing all the way on the trip button to make the exposure.

This is asking too much of a photographer shooting any sort of action, and many of them mistrusted their autofocus SLRs. In response, camera makers offered new models with broader CCD arrays to provide a larger central region of autofocus sensitivity. Some of these can be switched between narrow and broad sensitivity regions.

Other refinements to SLR autofocusing include:

• Optimization of camera settings to maximize depth of field

 

 

 

• Prediction of moving subject’s distance at instant of exposure
• Accommodation for horizontal and vertical subject detail
• Focus priority according to position of user’s eye

To optimize depth of field, the user aims the camera at the near point and presses the trip button halfway. This is repeated for the far point. Then the scene is recomposed in the viewfinder and the exposure is made with the actual focus set automatically to some midpoint calculated by the camera’s microcomputer.

For predicting the distance of a moving subject, the subject’s motion should be constant, both in direction and velocity. Under these conditions, the autofocus sensor’s signals can be used to calculate where the subject will be when the exposure is made. The calculation must consider the camera’s inherent time lag.

Early AFSLRs used focus sensors that were shaped to respond to vertical image detail,

 

 

with diminishing response as the detail approached the horizontal, where they were unable to respond. One solution incorporates three sets of lenslets and their CCD detector arrays. One set is laid out horizontally to respond to vertical detail, while the other two sets are vertical and straddle the first to form the letter ‘‘H.’’ The two vertical sets respond to horizontal detail.

Another solution has the individual, rectangular, detector segments (pixels) slanted to respond to both horizontal and vertical details. 

By combining information from the autofocus detector and the focal length tracer in a zoom lens, some AFSLRs can maintain the image size (within the limits of the zoom range) chosen by the user, even as the subject distance changes.

Eye Tracking.   Figure 7 shows how Canon’s model EOS A2E overcomes the need for the subject to be centered in the viewfinder in order to be in focus. Canon devised an eye tracking system that detects what portion of the viewscreen the user is looking at. Using low-power infrared emitting diodes (IREDs) to illuminate the eye, the system is matched to the user by having him / her look at the extremities of the five autofocus aiming patches in the viewfinder’s center. The reflections from the eye are detected by a 60 X 100-pixel CCD array and the resulting signals are stored in the camera’s memory.

At present, the five aiming patches occupy a 15-mm horizontal strip at the center of the finder, but it is possible that this could expand in future models. As it is, the camera’s 16 user-selectable operational modes include one in which both the autofocus and autoexposure systems are commanded by the eye tracking feature.

If the user wants to preview the depth of field, all that’s necessary is to look at a small patch near the finder’s upper left corner (not shown here). This brief glance causes the lens to close down to the f-stop chosen by the autoexposure system.

Because this eye tracking feature is in an SLR, the user can see if it’s working as expected just by looking at the viewscreen image. This indicates if, but not how, it works. To see how it works, I set up a simple experiment to measure the distribution of the light

 

 

 

reflected from my eye as I shifted my gaze between two marks on a wall. The separation between the marks and their distance from my eye were chosen to duplicate the angle swept by the eye when looking from one side to the other of the 15-mm focus patch array on the Canon EOS A2E viewscreen.

As indicated by Fig. 8, the format was nearly filled with the image of my eye. Consistent eye placement was assured with a chin and head rest. Once the image of my eye was recorded on tape, I could play back and pause at any point, then select a line at half screen height and store its waveform in a storage oscilloscope. By superimposing line scan waveforms from the frames showing my gaze from one side to the other, I could easily see the difference and dismissed my skepticism. This novel feature has intriguing possibilities.

 

14. Flash

Many 35-mm cameras feature a built-in electronic flash unit. Some are designed to flash every time the shutter is tripped, unless the user switches of f the flash. Others fire only when the combination of scene luminance and film speed calls for flash.

In some of the more advanced models with zoom lenses, the beam angle emitted by the flash changes in step with the focal length setting of the lens.

 

14.1 Red Eye

In the interest of compactness, the majority of cameras with built-in flash units have the flash close to the lens. The resulting flash photos of people frequently exhibit what is commonly known as ‘‘red eye,’’ which describes the eerie red glow in the image of the pupils of a subject’s eyes.

The red glow is the light reflected from the retina, which is laced with fine blood vessels. Young, blue-eyed subjects photographed in dim light seem to produce the most intense red-eye images.

The effect is reduced by (1) increasing the angle subtended to the subject’s eye by the separation between the centers of the lens and the flash; (2) reducing the subject’s pupil diameter by increasing the ambient brightness or having the subject look at a bright light for a few seconds before making the exposure.

Examples of how some camera makers fight red eye include Kodak’s Cobra Flash , used on several of their point-and-shoot models, and the ‘‘preflash,’’ used on many dif ferent camera makes and models.

The Cobra Flash describes a flash unit whose flashlamp/reflector unit is hinged at the camera’s top. When the camera is not in use, the flash is folded down, covering the lens. To use the camera, the flash is swung up, positioning it further from the lens than would be possible if it had been contained in the camera’s main body. One of their most compact cameras featuring the Cobra Flash is the Cameo motordrive model, which slips easily into a dress shirt pocket when the flash is folded down. When opened for use, the flash is 72 mm above the lens. Test shots were free of red eye when the subject was no more than seven feet away.

Another Kodak approach to the elimination of red eye is their single-use Fun Saver Portrait 35, whose integral electronic flash unit points upward, instead of forward. To use the camera, a simple white plastic panel, hinged at the camera’s top rear edge above the flash is pulled open. It latches at a \(45^\circ\) angle to switch the flash circuit on and direct the light from the flash forward. The result is a diffused beam that appears to originate from a point 100 mm above the lens.

Other makes and models have integral flash units that pop up a short distance when put into play. This may only gain several millimeters of lens-to-flash separation, but my experiments indicate that, as sketched in Fig. 9, for every extra millimeter of separation between the lens and the flash, the (red-eye-free) subject distance can be increased about 30 mm.

Several 35-mm cameras use the preflash method to reduce red eye by emitting a brief, rapid burst of low intensity flashes just before the main flash goes off for the exposure. A variation uses a steady beam from an incandescent lamp in the flash unit. The beam switches on shortly before the flashlamp fires for the exposure. The purpose in both methods is to make the subject’s pupils close down, reducing the light reflected from the eye during the exposure.

The preflash approach has two drawbacks: (1) it drains energy from the camera’s battery, reducing the number of pictures per battery; (2) many times the subject reacts to the preflash and blinks, just in time for the exposure.

 

15. Flexibility Through Features and Accessories

The seemingly endless combinations of operating modes with a camera like the Canon EOS A2E might be taken as an attempt to be all things to all photographers. Another way to look at it is to see it as a three-pound Swiss army knife: you’ll never use all of the tools all of the time, but if there’s the need for some tool, even just once, it might be nice to know you have it.

Many cameras have long lists of accessories. A typical camera system can be thought of as a box with an open front, top, and rear. For the front, the user may choose from as

 

 

many as 40 different lenses. For the top, there may be three or more viewfinder hoods. For the back, choose one of perhaps five image receptacles. 

Then there are the other groups, shown in Fig. 10: flash units, motor drives, close-up hardware, carrying cases, neck straps, lens hoods, filters, remote control cables, transmitters and receivers, mounting brackets, eyepiece magnifiers, corrective eyepiece lenses, cold weather heavy-duty battery packs, and more.

No matter how varied your photographic needs may be, the camera maker wants you to find everything you need in his or her catalog, Possibly, the availability of just one accessory, such as a wide-angle lens with tilt-shift controls for perspective correction, can decide which camera you choose.

 

16. Advantages of Various Formats

In terms of versatility through a broad range of accessories plus the camera’s intrinsic capabilities, it’s hard to beat one of the major brands of 35-mm SLRs. No other type of camera has had as much ingenuity and as many refinements lavished on it for so many years. It’s one of today’s most highly evolved consumer-oriented products.

Accompanying the evolution in optics, mechanics, and electronics, film emulsions have

 

 

improved over the years, making the 35-mm format just as able as the larger formats for most applications. Even so, all else being equal, there is no substitute for ‘‘real estate’’—the precious additional square millimeters of emulsion offered by the many 120-size medium formats . As the data in Fig. 11 shows, some of these are SLRs with systems as extensive as their 35-mm counterparts.

 

17. Large Format: A Different World

When you make the jump from medium-format to large-format, you’re in a different world. You use individual sheets of film, not rolls. Your camera will be used on a tripod or copy stand most of the time. Your photography will be contemplative, careful, and unhurried—perhaps better.

Scene composition and focusing are done with the lens at full aperture. Then the lens is stopped down, the shutter closed, the film holder inserted, its dark slide pulled, the shutter tripped, the dark slide replaced, and the film holder removed.

In a short time you’ll realize that the large-format (view) camera can be thought of as a compact optical bench. As such, it lends itself to special applications that could be difficult for the smaller formats.

 

17.1 View Camera Versatility

To illustrate, suppose you need a picture of a picket fence at some obliquity, with every picket board, from near to far, in sharp focus and with the lens wide open. This calls for the use of the ‘‘Scheimpflug condition,’’ shown in Fig. 12. It requires that the planes containing the lensboard, film, and subject all intersect on a common line. When this condition is satisfied, the entire surface of the subject plane will be in focus, even with the lens wide open.

 

 

The necessary camera movements , involving lensboard and film plane, are standard features of even the most spartan view cameras. These movements are known as swings and tilts. They take just a few seconds to adjust on a view camera and the job doesn’t require a special lens.

You can do it with a smaller format camera too, but you’ll need one of their special (expensive) tilt-shift lenses or a bellows unit with articulated front and rear panels, plus a lens with a large enough image circle. The resulting combination may not retain all of the small-format camera’s features, such as exposure metering, autofocus, etc. 

The view camera’s fully articulated front and rear provide for swing, tilt, rise, fall, and left-right shift. Thanks to this flexibility, objects such as boxes and buildings can be photographed without distortion, and distracting detail near the image borders can be omitted.

It takes first-time users a while to get used to the inverted and reversed image seen on the view camera’s groundglass screen. This can be annoying when shooting a portrait, since an upside-down smile looks like a frown until you accept the fact that even though you understand the basic camera optics, it doesn’t mean you have to enjoy coping with it.

Worse, you’ll ned to drape a dark cloth over the back of the camera and over your head in order to see the image if you’re working in bright light. If you’re claustrophobic, this may bother you.

On the plus side, large-format negatives are frequently contact-printed or only slightly enlarged for the final image. Because the image is large, depth of field and other image properties can be examined easily on the groundglass viewscreen with a small magnifier of modest power—a 4 X loupe works well.

The large negative has another attribute: it lends itself to retouching, masking, and other image manipulations, but these may be lost arts now that clever computer programs are available for doing the same things, provided your image is in digital form.

 

18. Special Cameras

Some photographic tasks call for cameras with special features, such as the ability to form images in near-total darkness or inside of a crowded mechanism. Among the long list of special cameras, we find:

• Aerial
• Clandestine
• Endoscopic
• High-speed
• Periphery
• Sewer
• Stereo (3-D)
• Streak
• Thermal imaging
• Underwater
• Wide-angle

18.1 Aerial Cameras

Aerial cameras come in a variety of sizes and features. Among the more common features are image motion compensation, where focal length, speed, and altitude are factored into the movement of the film during the exposure; a vacuum back to hold the film flat during the exposure; and a calibrated lens so that any rectilinear distortion can be factored into the measurements made of the image.

 

18.2 Clandestine Cameras

Clandestine, or ‘‘spy’’, cameras have been with us since photography was invented. In the broadest sense, any camera that is not recognized as such by the subject being photographed might be considered a successful spy camera.

Many early box cameras were dubbed ‘‘detective’’ cameras because they were much smaller and more drab than a ‘‘real’’ camera with its prominent bellows and sturdy stand.

Cameras have been disguised as books, rings, binoculars, cigarette packs and lighters, matchboxes, portable radios, briefcases, canes, cravats, hats, even revolvers. Of all of them, the classic Minox is probably the best known.

It can be concealed in an adult’s fist, focuses down to eight inches, and is nearly silent. Its smooth exterior and gently rounded corners have inspired the belief among many that it was designed to be concealed in a body cavity with minimal discomfort.

 

18.3 Endoscopic Cameras

Endoscopic cameras use a tiny, short-focal-length lens to form an image that’s transferred by a coherent, flexible fiber-optic bundle to a relay system that forms the image on the detector (film or CCD) in the camera. To illuminate the subject, the coherent bundle may be surrounded by an incoherent ring of fibers optically coupled to a light source at its free end , close to the camera.

Often fitted with a \(90^\circ\) prism on its tip, these cameras are used to photograph inside humans and machines. Another application is shown in Fig. 13: getting close-up views of architectural models from ‘‘ground’’ level. Variations include those without illumination optics but having a very small diameter image bundle to fit inconspicuously in some object for surveillance photography.

 

 

18.4 High-speed Cameras 

High-speed cameras were once defined as being able to make exposures of less than 1/1000s. Today this would include many 35-mm SLRs which have a top speed of 1/10,000 s, a speed equaled by several consumer-grade camcorders.

When shorter exposures are called for, a common, low-cost electronic flash unit can give flash durations as short as 1/32,000 s.

The next step includes the Kerr cell and Faraday shutters, both of which work by discharging a high-voltage capacitor across a medium located between crossed polarizers. This produces a momentary rotation of the plane of polarization within the medium, permitting light to pass through to the detector. Exposure times are in the nanosecond range for these electro-optical/magneto-optical devices.

For exposures in the picosecond range accompanied by image intensification, there’s the electronic image tube. When a lens forms an image on the photocathode at the front of this tube, electrons are emitted. Their speed and direction are controlled by electrodes within the tube.

A secondary image is formed by the electrons as they strike the phosphor screen at the rear of the tube. This image may be photographed, or, if the tube has a fiber-optic faceplate behind the screen, the image can be directly transferred to a film held against the faceplate.

By placing a microchannel plate in front of the phosphor screen, the image can be intensified by a factor of 10,000 or more. A microchannel plate is a thin glass disk riddled with microscopic holes that pierce the disk at an angle.

In Fig.14 the wall surface of each hole is coated with a substance that reacts to the impact of an electron by emitting more electrons. A high voltage across the disk accelerates the stream of electrons. For every electron that enters one of the angled holes, about 100,000 electrons emerge to strike the phosphor screen.

 

18.5 Periphery Cameras

A periphery camera is used to make photos of objects like gas engine pistons, bullets , and other cylindrical objects whose surface detail must be imaged as though the surface was ‘‘unrolled’’ and laid out flat before the camera. Depending on the size of the subject, either it or the camera is rotated about its longitudinal axis at a constant angular velocity.

The image strikes the film moving behind a slit that’s parallel to the axis of rotation. The film’s velocity matches that of the image unless deliberate image compression or elongation is desired.

 

 

 

18.6 Sewer Cameras 

A sewer camera is designed to photograph the inside of pipes, tunnels, etc. It may be thought of as a small underwater camera on a sled. The camera’s lens is encircled by an electronic flashtube and reflector to illuminate the scene. Pictures are made at regular intervals, as judged by distance marks on the cable attached to the sled.

Other cables attached to the camera convey signals to and from the camera. With the miniaturization of video cameras, they have taken over this task, except where maximum resolution is required. This is where film cameras excel.

 

18.7 Stereo Cameras

Stereo cameras seem to come in and out of vogue with some mysterious rhythmic cycle. The root idea has been around since the dawn of photography and is based on the parallax difference between the views of our left and right eyes. The classic stereo camera mimics nature by using two lenses spaced about 65 mm apart to form two images of the subject.

The two images can be made in other ways. A simple reflection system using four small mirrors or an equivalent prism system placed in front of a normal camera’s lens will form two images of the subject, as shown in Fig. 15. Another method requires that the subject is stationary because two separate exposures are made, with the camera being shifted 65 mm between exposures. In aerial stereo photography, two views are made of the ground, the views made so many seconds apart.

When the images are viewed in a manner that restricts the left and right images to their respective eye, the stereo effect is achieved. Various methods for viewing stereo pairs include projection, where the left and right views are polarized at \(90^\circ\) to one another. The viewer wears glasses with polarizing filters oriented to let each eye see the view intended for it.

Another viewing system is called a parallax stereogram. It (optically) slices the left and right images into narrow, interlaced strips. When viewed through a series of vertical lenticular prisms with a matching pitch, the 3-D effect is seen.

 

18.8 Steak Cameras

Streak cameras are useful for studying relative motion between the subject and camera. They share certain characteristics with the periphery camera described previously, insofar as they match the movement of the film to that of the image coming through a slit at the film plane. Exposure time is determined by how long it takes for a point on the film’s surface to travel across the slit’s width.

The basics of the streak camera are shown in Fig. 16. It would be pointless to use a streak camera without some relative motion between the image and film. Some photographers use a streak camera for creative effects, such as depicting motion tack-sharp at its beginning, then gradually elongating or compressing it, and ending in a smear.

This is done by varying the relative velocity between the image and the film during the exposure, either by moving the camera, the subject, or the film, These motions may be made singly or in combination. Varying the focal-length setting of a zoom lens with the film moving also produces unusual images.

A streak camera’s format has a width defined by the film it uses, but each picture has its own length, limited only by the length of the roll of film. One of the more critical factors to look for in a streak camera is freedom from cogging, a local density variation in exposure while the film is moving at a fixed velocity.

The result of periodic or intermittent speed variations , the cause may be improperly meshed gears, a bad bearing, poor fit

 

 

 

between the film drive sprocket teeth and the film’s sprocket perforations, or the magnetic pole effects of the drive motor.

 

18.9  Thermal Image Cameras

Thermal imaging cameras convert the intrinsic heat of a subject into a visible image. Among their many applications are detection of heat losses from buildings, blood circulation disorders, and surveillance. Some of these cameras produce false color images, in which each color represents a different temperature.

Among the various methods to form visible images of temperature variations, the most direct way is to use a normal camera loaded with film that’s sensitive to the infrared (IR) portion of the electromagnetic spectrum.

In a more elaborate system, a moving mirror scans the subject and, line-by-line,

 

 

projects its image onto a heat-sensitive semiconductor device whose output is proportional to the IR intensity. The output is used to modulate a beam of light focused on the surface of conventional film.

Another version uses the semiconductor to modulate a stream of electrons striking a phosphor screen in an image converter tube. The image can then be photographed.

 

18.10  Underwater Cameras

Underwater cameras come in a wide variety of sophistication, from the disposable costing less than ten dollars, to the high-tech versions costing thousands of dollars. In between, there are dozens of underwater housings designed for specific cameras. Typically, these housings permit the user to change the camera’s settings through watertight couplings. Most of the cameras used in such housings have motorized film advance, autoexposure, and autofocus, so the only external control needed is a pushbutton at one end of a simple electrical switch.

External attachments include flashguns, viewfinders, and ballast weights. The flashgun connections should be carefully examined because they are one of the leading sources of problems. In general, the simpler the connector, the better.

 

18.11  Wide-angle Photography

There are several 35-mm and medium-format cameras designed specifically for wide-angle photography. These include straightforward types which use lenses designed for wide-angle views on larger-format cameras. Essentially these cameras use only a rather long horizontal strip of the broad image circle the lens produces. This type of camera is uncomplicated and rugged.

Panoramic Cameras.  A special kind of wide-angle camera is known as a panoramic camera, and there are two main types: one where the entire camera rotates; the other, where just the lens rotates.

The rotating camera type is capable of a full \(360^\circ\) vista. As the camera turns on its vertical axis, the film is moved past a narrow, stationary slit at the center of the film plane. The motion of the film is matched to that of the image.

Because these cameras rotate slowly, a common prank in photos of large groups is for the prankster to stand at the edge of the group that’s exposed first, then dash behind the group to the opposite edge in time for its exposure, with the result that the same person appears twice in the same photo, once at either edge of the group.

The rotating lens type shown in Fig. 17 produces images of about \(140^\circ\). It works by rotating its lens on a vertical axis coinciding with its real nodal point. The image is swept across the film through a tubular image tunnel at the rear of the lens. The tunnel extends almost to the film surface and has a narrow slit at its end.

The slit is parallel to the axis of rotation and extends over the width of the film. During the exposure the film is held stationary against a cylindrical film gate whose radius equals the focal length of the lens. The slit width, the rotating speed, and the lens opening may be adjusted for exposure control.

The panoramic cameras described here regulate their speed of rotation with precision governing systems to ensure edge-to-edge uniformity of exposure, so they should be kept as clean as possible.

Also, to avoid unpleasant distortion, use care in leveling them and always use the best single camera accessory money can buy: a good, solid tripod.