At this point, termed the focus, the sensitive photographic material
is placed for the purpose of obtaining the required picture.

The great disideratum in a photographic camera is perfect lenses.
They should be achromatic, and the utmost
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transparency should be obtained; and under the closest
inspection of the glass not the slightest wavy appearance,
or dark spot should be detected; and a curvature which as much
as possible prevents spherical aberration should be secured.
The effect produced by this last defect is a convergence
of perpendiculars, as for instance; two towers of any building,
would be represented as leaning towards each other; and in a
portrait the features would seem contracted, distorted and
mingled together, so as to throw the picture out of drawing
and make it look more like a caricature than a likeness.
If the lens be not achromatic, a chromatic aberration takes place,
which produces an indistinct, hazy appearance around the edges
of the picture, arising from the blending of the rays.

The diameter and focal length of a lens must depend in a great measure
on the distance of the object, and also on the superficies of the plate
or paper to be covered. For portraits one of 1 1/2 inches diameter,
and from 4 1/2 to 5 1/2 inches focus may be used; but for distant views,
one from 2 inches to 3 inches diameter, and from 8 to 12 inches focal
length will answer much better. For single lenses, the aperture
in front should be placed at a distance from it, corresponding to
the diameter, and of a size not more than one third of the same.
A variety of movable diaphrams or caps, to cover the aperture in front,
are very useful, as the intensity of the light may be modified by them