97. Single Lenses. The simple non-achroma tized lens was used in the camera obscura before the discovery of photography. To obtain a relatively large field, W. H. Wollaston (1812) recommended the use of a convergent meniscus lens (Fig. 65), with stop in front, having an aperture of Fix', and covering a field equal to the focal length (angle 6o'). This image, it must be understood, has every possible aberra tion and chromatic aberration, and in particular requires the adjustment after focussing sug gested in the note to § 46. In some fixed focus cameras (§ 87) this is made once for all by the maker. After being abandoned for many years, single lenses have come back into favour as the simplest form of unachromatized lens. Artistic photography calling for a small angle of field, the piano-convex form is to be preferred to the meniscus, the convex surface being turned towards the object, and behind the lens is placed the diaphragm having an aperture which may be as much as F/8. This gives a very satisfactory image at the centre of the field (about 15°), with very little spherical aberration, suitable for a, bust portrait. Chromatic blur may be mini mized (after correction of focus) by using a filter absorbing the ultra-violet and, if need be, a little violet.
The first photographs made by Daguerre were taken with a single lens made by Ch. L. Chevalier (9o° at F/32) in his " rapid wide angle landscape lens." 98. Petzval Portrait Lens. This lens (Fig. 69), the first to be calculated by a mathematician, was only moderately successful when first brought out (1840). J. Petzval set out, by a proper choice of curvatures and thicknesses, to correct aberrations, and particu.
lady curvature of the field, with out the necessity of using a small stop, so as to shorten the very long exposures required in the Daguerreotype process. But this aim could only be accom plished at the expense of angle of field, the diameter of useful field being only about one-third the focal length (angle 20° to 25°), with an effective aperture of F/3-4 to F/3.6, subsequently enlarged to F/2-4 by H. Zincke in Although this angle of field was quite sufficient for the bust portrait, as experience has since shown, the tendency of the time was towards a very large angle of field, and the (1830), partially corrected for achromatism (Fig. 66), which, with stop F/14 (afterwards increased to F/12), covered sufficiently well a circle of diameter equal to half the focal length. At a smaller stop, F/70, the field was half as large again.
This form of lens was improved and brought to its present form of single achromatic lens only in 1857 by Thomas Grubb. In his lens (Fig. 67) it is seen that the arrangement of Chevalier was reversed, the convergent lens this time facing the object. With an aperture, usually F/16, the field reached with complete correction of achromatism and partial correction of spherical aberration for the central rays. By increasing the number of cemented lenses to three (Fig. 68), J. H. Dallmeyer (1865) was able to extend the aperture to F/11 and field to 70° photographer would almost have liked the lens to portray objects behind it ! In his first type Petzval had intentionally left in a little chromatic aberration, which subse quent opticians adopting this type corrected in the endeavour to make the image a little more homogeneous.
The Petzval portrait lens, more or less modi fied, is still in use in a large number of portrait studios. Perhaps it would also be used by amateurs if it had not been for the tradition of mounting it in an extremely cumbrous mount. It is frequently also used for projection work.
At the time of the reaction against the exag gerated correction of lenses for portraiture, lenses of the Petzval type were successfully used (de Pulligny, 1904) with the back combination removed and replaced by a simple convergent meniscus of the same diameter but almost double the focal length. The whole lens can be used at about F/5. The chromatic fringe is then about one-third to one-quarter that of an anachromatic lens of the same focal length and relative aperture.
99. Rectilinear Lenses. Apart from some combinations abandoned as soon as tried, photo graphers had really only the single achromatic lens (more or less corrected but always possessing lens has been abandoned by almost all opticians of repute since the appearance of the anastigmat, and the glass polishers who make these lenses nowadays are not burdened with much know ledge of optics, i.e. these lenses still fitted to many cheap cameras are, for the most part, very inferior to what they were formerly, in spite of the high-sounding names such as " simili-anastigmat " which are sometimes given to them. Their " nominal " aperture rarely exceeds F/8, and their effective aperture F/9.
If the aperture of a rectilinear lens is increased beyond its normal limits, spherical aberration is introduced, and gives the image a softness comparable, to a certain degree, with that given distortion), and the portrait lens, the field of which was too limited for other uses than portraiture. Need was felt for lenses covering a field comparable with that of the single lenses but perfectly corrected for distortion, which consequently could be used for copying and architectural photography. This correction was sought for in a symmetrical arrangement of the components, following that already used by Wollaston for microscope objectives. In 1866 H. A. Steinheil brought out his symmetrical aplanat (Fig. 70), and soon afterwards the recti linear of Dallmeyer appeared, almost identical to the smallest detail. However, while this lens was indeed rectilinear, it was only very slightly aplanatic, so that it is the name given it by Dallmeyer that has become general for this class of lenses, which all opticians endeavoured more or less successfully to improve. In its original form it covered a field of 45° at F/8, and 60° at F/30. Steinheil succeeded in increasing the aperture to Fh and even F/6 without much sacrifice of field (40. These lenses obtained considerable popularity, which has never entirely ceased.