THE. ENL.ARGTING LENS OR OBJEC'TIVE.
All that is desirable in the lens, in addition to the greatest possible freedom from faults, such as spherical and chrom atic aberration, astigmatism and curvature of field, is that it shall work with as large an aperture as possible, and that it shall cover sharply to the corner the negative which is being enlarged. The size of the enlargement has no direct connection with its covering power, but, of course, the greater the enlargement the more any defects the lens possesses will be' • apparent. Any of the modern flat field lenses and anastigmats are admirably ' suited for enlarging. It is of most im portancc that the lens should work with a large aperture when using artificial light, and as with such light it is seldom possible to use a stop, it will be seen that a lens of good quality and of the portrait class must be used under these conditions. using a small source of
light, the effect of using a stop is often to cut off a portion of the image, and not to distribute the effect all over the picture, as is usual. If the rays are parallel, as when using daylight, a stop may be used to improve marginal definition. A similar rbsult may be obtained, but to a much lesser degree, by using a diffuser. There is a very common fallacy among beginners that the lens must be one capable of covering an image the size of the enlargement, but although this is true in a sense, it is practically wrong. If the lens is able to produce an image the si:e of the negati re sharply to the corners, that is sufficient, and the same lens may there fore be used for making enlargements of any reasonable dimensions from the same negative.