This is because a non-achromatic lens bends the rays of light of different colors to different extents, so that the yellow rays which we use for focusing do not come to a focus in the same place as the blue rays which affect the film, because the blue rays are bent more than the yellow.
In 1752, Dollond, an English optician, showed that by combining two different kinds of glass to make a lens he could get the blue rays to focus at the same point as the yellow rays, and lenses made in this way were called "achro-matics," from the Greek words "a" meaning not, and "chroma" meaning color. The best shape of achromatic lens to use is shown in Fig. 31, and since this is also of a "meniscus" or crescent shape the lenses are called meniscus achromatics. If a single achromatic lens is used, it is necessary to "stop it down" so that only a small portion of the lens is used, because the rays which come through the edges do not focus together as well as those which come through the center, and so the image is not quite sharp if the whole lens area is used.
This stopped-down meniscus lens has the effect of producing slight curvature of the edges of the picture, which does not matter in landscape work or portraiture; but if subjects containing straight marginal lines are photographed with such a lens, their outer lines appear slightly curved - so slightly, however, that the effect is negligible unless the image of the subject so crowds the picture area that its outer lines are very near the margins of the picture, as shown by figures 32 and 33, which represent a window sash photographed with a meniscus lens at short range.
If the stop is in front of the lens the curvature is in one direction, and if it is behind the lens the curvature is in the opposite direction, so that if we put two lenses together with the stop between them, the curvature is neutralized and we get a lens which gives no curvature at all.
Such a lens is called a "Rapid Rectilinear"- rectilinear because it gives straight-line images, and rapid because having a focal length half that of either of the component lenses with a stop of the same diameter, it passes four times as much light and only requires one-quarter of the exposure. Rapid Rectilinears are sometimes called by other names, such as "Rapid Aplanats," "Planatographs," and so on. Now, it so happens that the two kinds of glass used in an achromat must fulfill certain conditions to bring the blue and the yellow rays to the same focus, and must fulfill certain other conditions to get a picture which is flat, that is, a picture that is sharp on a flat plate or film; and the ordinary glasses which are used for making achromats will not fulfill all these conditions at once, so that the lenses made with "old" achromats will not give flat field images, the image being saucer-shaped. These lenses are, therefore, said to be "astigmatic," which means that they do not give sharp-point images of points.
Fig. 30. Focus of Blue and Yellow Rays.
Fig. 31. Achromatic Lens.
Fig. 32. Made with Stop in Front of Meniscus Lens
Fig. 33. Made with Stop Behind Meniscus Lens.
Fig. 34. Made with Rectilinear Lens
About thirty years ago, Professor Abbe and Otto Schott, working together at Jena, found out how to make new kinds of optical glass from which lenses could be made which would give flat field images with the blue and yellow rays of the same focus.
By the use of these new glasses the opticians have been able to make lenses that give sharp images on a flat field to the very edge of the picture and, therefore, these lenses are called "Anastigmats," meaning "not astigmatic," but this better defining power can, however, only be obtained by the most careful and skilled work in making the lens, this work being of a far higher quality than that employed on the older types of lenses, which accounts for the higher cost of anastigmats.
Anastigmat lenses can be used with larger stops than any of the older lenses, so that if an Achromatic working at f. 16 requires a 1/5 second exposure, a Rapid Rectilinear working at f.8 will require a 1/20 second exposure, and an Anastigmat working at f.6.3 will require a 1/32 second exposure.
To summarize the advantages and disadvantages of the three types of lenses discussed in the preceding pages:
The single lenses (meniscus and meniscus achromatic) must be used with a relatively small stop, which means that they are somewhat slow. They are fast enough for snapshots in good light, the shutters they are fitted with being adjusted for the making of moderately slow "snaps". The very fact that they require a small stop gives them great depth of focus, however, and for that reason errors in focusing are largely compensated for, resulting in a high percentage of successful pictures.
The Rapid Rectilinear Lenses have more speed than the single lenses, and are also better for architectural work.
The Anastigmat, f.6.3, lenses are about sixty per cent faster than the Rapid Rectilinear lenses and are corrected for the finest definition (sharpness). When used at their full speed - that is, with the largest opening - they require accurate focusing, although it should be borne in mind that both the length of focus and the stop opening affect this matter of depth of focus. That is why the 3A,the largest of the Kodaks, requires more accurate focusing than the smaller ones, and is why, when we get down to the Vest Pocket size, it is possible to use an Anastigmat lens with a fixed focus.
An Anastigmat lens does not require any more accurate focusing than any other lens when used with the same stop. Take, for instance, an average landscape with a prominent object in the foreground. The correct stop would be f.16 and,if the sun were shining, the correct exposure 1/25 of a second. This same stop and exposure should be used with a Single lens, a Rapid Rectilinear or an Anastigmat, and the depth of focus with the same focal length of lens would be the same in all cases - no more accurate focusing would be required with one lens than with another.
But when the light is weak and an Anastigmat is used at its full opening, or nearly its full opening, in order to get a well timed snapshot, there will be a gain in speed but a loss in depth of focus. The object at the focused distance may photograph even sharper than it would with the Single or Rapid Rectilinear lenses, but objects a little nearer the camera or a little farther away will not be so sharp because depth of focus has been sacrificed for speed. And, of course, this same thing is true in using a large stop in order to arrest the motion of moving objects. With a fixed-focus camera working at a fixed shutter speed, all still objects at, say, fifty feet away, would be sharp and, with a good light, fully timed, but moving objects might show a blur. With an Anastigmat lens opened to f.6.3 and a shutter speed of 1/200 of a second, it is possible to arrest moderately fast motion and get a fully timed negative (with good light), but in such case care must be taken to focus accurately.