Camera Work: A Photographic Quarterly (1904 (Heft 7))

ABOUT FOCAL LENGTH.

ALTHOUGH RAPIDITY is a matter of importance
when selecting a lens, especially for portrait-photog-
raphy, it is secondary in ninety-five per cent. of all
cases to the importance of the focal length.
This should not be understood to mean that it
is indifferent whether a lens works at F.6 or at F.15,
for such considerable differences always point to entirely
different classes of objectives, but F.4⁵ or F.5⁵ or
even F.6³ or F.6⁸ becomes a matter of less im-
portance, than the question of the proper focal length
for the purpose we have in view.
Differences in maximum openings of various lenses are only noticeable
when using such maximum openings, which condition in actual practice is
very rarely encountered, and furthermore the influences of different relative
openings can be completely counterbalanced by corresponding modification
of the duration of exposure.
Differences in focal lengths, on the contrary, can not be counterbalanced.
A portrait made with a 12-inch lens may be equally as large as one made
with an 18-inch lens, but the size will be absolutely the only point in common
between two such pictures. Their general appearance, even though they be
taken from the same relative points of view, will be quite different, owing to
the different perspective produced by the different angles embraced by the
two lenses when set to make equal-sized images. A further study of this
subject will lead us to recognize the fact that the longer focal lengths will
always produce the more satisfactory results, with regard to true perspective
in portraiture as well as in landscape-work.
The perspective effect is due to the decreased scale at which objects
situated in receding planes are depicted. Knowing that this scale is directly
proportionate to the distance, it is a simple matter mathematically to com-
pare the results produced by two lenses of different focal length when used
to make images of the same size.
Let us assume for the purpose of this investigation that we have two
8x10 cameras pointed at the same person. One camera is equipped with a
lens of 12-inch focus, the other with a lens of 18-inch focus, both cameras
adjusted to make a bust which will properly suit an 8x10 plate and therefore
one-third of life-size.
In order to reduce in proportion of 1 to 3, the lenses should be placed
at four times their respective focal lengths from the sitter. Let us measure
this from the eyes and place the 12-inch lens at 48 inches distance, and the
18-inch lens at 72 inches distance. The corresponding extensions of the
cameras will be 4 8/3 = 16, and 7 2/3 = 24 inches, respectively; which shows that the
12-inch lens has been racked out 4 inches and actually works as a 16-inch
lens, and the 18-inch lens has been racked out 6 inches and works as a
24-inch lens. The proportion of reduction in the plane of the eyes will


43