The doublets described in the last chapter are limited to small apertures (up to about f/4) and a small field of view. If we require a larger field angle and aperture then the Petzval lens, which consists of two positive groups separated by an air space, is a simple solution. Joseph Petzval designed his famous âPortrait Lensâ in 1839. The front lens was a cemented doublet, the rear component an air-spaced doublet. This was probably the first lens ever to be designed by computation, in this case by logarithmic tables.
The limitation of the doublet objective is that astigmatism cannot be corrected in a single component, and this severely limits the field angle that it can cover. We have seen from earlier chapters that for a thin lens at the stop, the astigmatism is given by S 3 =H 2 K, which must always be non-zero. Hence, astigmatism can only be corrected if there are one or more lenses that are not at the stop. Thus, two separated lenses, as shown in the Fig. 9.1, may be employed to generate equal and opposite amounts of astigmatism. In practice, these will not always be single lenses, but achromatic doublets or more complex combinations.
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