THE OPTICS OF PHOTOGRAPHIC LENSES 



31 



and the extreme colors fall beyond, so that the curve connecting wavelength (X) 

 with image position becomes bent back on itself as shown in Fig. 25. This means 

 that in the neighborhood of one wavelength the lens is substantially free from chro- 

 matic aberration, that being the "minimum focus wavelength" corresponding to the 

 point m in Fig. 25. For lens systems to be used before the eye, this minimum-focus 

 wavelength should fall at about 0.55 m, which is the peak of the sensitivity curve of 

 the eye, but for photographic work with ordinary plates, it should fall much lower, 

 say at 0.45 n or even less. For use with panchromatic materials having a very broad 

 range of color sensitivity and sometimes even with two definite peaks of sensitivity 

 at different wavelengths, the best position of the minimum-focus wavelength has not 

 yet been decided. The only solution may be to make "apochromatic" lenses, i.e., 

 lenses in which the curve in Fig. 25 is practically flat or in which it has a double bend 

 giving union of three colors at a common focus. Either of these conditions is hard to 

 fulfill and is indeed almost impossible because of the necessity of correcting all the 

 other aberrations at the same time. The longitudinal extent of the chromatic 

 aberration of a lens is not diminished by stopping down, but its seriousness is reduced 



Barrel Original Object Pincushion 



Fig. 26.— Outline of square images produced when distortion is present. 



because the depth of focus becomes increased at smaller aperture and thereby absorbs 

 more of the chromatic aberration. 



Chromatic Difference of Magnification. — This exists in any lens in which the focal 

 length is different in different colors; it should be carefully distinguished from ordinary 

 chromatic aberration in which the position of the focal point varies in different colors. 

 Chromatic difference of magnification produces colored fringes round the outer edges 

 of the image, and in photography these are recorded as blurred outlines. The effect 

 becomes Avorse if the image point lies farther from the center of the picture. It is 

 especially serious in lenses to be used in color photography or in three-color process 

 work, and it is not improved in any way by stopping down the lens. 



In modern miniature cameras with good lenses, this defect is never likely to be 

 large enough to be noticeable, even when taking pictures on Kodachrome film. Its 

 effect in enlargers is liable to be much more serious, and is discussed under process and 

 enlarging lenses below (on page 48 in Chap. III). 



Distortion. — When distortion is present in a lens, the magnification is not constant 

 all over the picture, but the outer parts may be magnified less or more than the central 

 parts. These cases are illustrated in Fig. 26. In the upper part of that diagram are 

 shown images of a uniformly divided scale taken with a lens having these two types of 

 distortion, and in the lower part of the figure are shown the effects of these distortions 

 on the image of a square. In the latter case, since the corners of the square are farther 

 out than the sides, they are magnified relatively less or more than the sides, and hence 

 the square is distorted into a barrel- or cushion-shaped figure. In each case, the 

 perfect square-image is shown dotted. Distortion is unaffected by stopping down 

 the lens, and is very serious in any lens where measurements are to be made on the 

 plate, e.g., in process lenses or lenses for map copying or aerial surveying. Lenses with 



