Double-Gauss Lenses

This is a continuation from the previous tutorial - symmetrical lenses.

In the early 1800s, Gauss described a telescope objective comprising a pair of meniscus lenses with one having positive power and the other negative power. An interesting aspect of his lens is that the spherochromatism is essentially constant.

Although this lens found little acceptance, in 1888, Alvan Clark of Massachusetts placed a pair of the Gauss lenses around a central stop to create a high-aperture, wide-field-of-view lens.

This lens form is known as the Double-Gauss lens and is the basis of almost every high-aperture lens developed to date. An example of this lens was patented by Richter in 1933 and can cover a field of view of \(\pm45°\) at F/6.

In 1896, Paul Rudolph of Zeiss developed the Planar which reduces the often serious oblique spherical aberration and the separation of the astigmatic foci at intermediate field angles.

Rudolph placed a buried surface into the thick negative elements to control the chromatic aberration. A buried surface is defined as the interface between two glasses that have the same refractive index \(n_d\) at the central wavelength, but have significantly different Abbe numbers.

Such a surface has no effect upon the monochromatic aberrations or the lens system power, but does allow the inclusion of a wide range of chromatic aberration to compensate for that caused by the rest of the lens. 

Many Double-Gauss lenses are symmetrical; however, it was discovered that if the lens was made unsymmetrical, then an improvement in performance could be realized.

This lens form is often called the Biotar. A large portion of 35-mm camera lenses are based upon this design form or some modification thereof. Figure 25 shows the configuration of the Leica Summitar introduced in 1939.

It is the general nature of meniscus lens systems of this type to exhibit little coma, distortion, or lateral color; however, oblique spherical aberration is often observed to increase to significant levels as the field angle increases.

Oblique spherical aberration can be recognized in transverse ray plots as the S shape of spherical aberration, but with the S becoming increasingly stronger as the field angle increases.

As the aperture is increased beyond about F/8, the outer negative elements must be thickened dramatically and achromatic surfaces must necessarily be included. 

The next tutorial introduces Petzval lenses.