What Depth ol Focus Means.
Suppose norv that you are using your Anas_
tigmat at the full opening./.f.f. An object t0
feet distant x'ill be absolutely sharp, objects
6
and 25
feet distant, rvhile not as sharp,
will
be sharp enough for all ilractical purposes.
Stop your Anastigmat down to /.f1. and
those
objects
each side of ilre exact point of
focus will rnaterially increase in sharpness.
Go
further arrd use stop f.zL, or a still smaller
stop, and cverything from 5 feet on to infinity
will be sharp. It will thus be seen ilrat ilre
srnaller
the stop
the
greatcr the depth of focus,
i. e., the greater ilre power of ilre lens tc,
sharply deline. But it is obvious ilrat with
the small stops the exposure must be cor-
respondingly lengtherrecl.
ANASTIGMAT SPEED.
IJsing a stop of f.tl. or smaller, the advan_
tages of the Anastigmat over ilre \.{eniscus
Achromatic lens
is
an improvement in dclini_
tion and in the corrections of lines, But let
us suppose
that we desire to take a picture on
a cloudy day. What do we find? The /,
value of the lens denotes the relation of the
opening in that lens
to its focal length. Sup-
pose then, that we are using the Nleniscus
Achromatic lens, (3 inch focus), speed
/.u.e,
and an Anastigmat lens, speed, /.?.2 of the
same length of focus, 3 inches. How do they
compare in speed
? To reduce this to its
simplest terms, we will divide ilre focal
length (three inches) in each case b]' the
value. g '.- ll.8:265
3':- 7.7--389
It will thus be seen that in using the l'Ienis-
cus lens the largesb opening is 265-1000
of an
inch in diameter and, rvith the Anastigmat
389-1000
of an inch, I'he amount of ligltt aci-
mitted by a lens in a given time depencls, of
course, upon the area of the opening at th:rt
time being used in that lens. The amount of
light admitted in a given time with the-se
different lenses
would, therefore, be in direct
proportion to the square of their diameters.
flere, then, ornitting the fractions, is the
result :
f,,Ieniscus
Achromatic lens 265 x 265: 102
Anastigmat Ierrs, 389 x 389:1513
We thus find that the speed of the Anastig-
mat is double thespeed of the \'Ieniscus
lens,
it therefore admits twice as much light as Ure
Meniscus in a given amourrt of time. Therein
lies the greatest Anastigmat advantage. But
simply because
it has tltis speed, you don't
ahvays need to use it. The speed must be
uscd rvith discretion, just as
greater care is re-
quired in operating arr automobile than in
operating a bicycle.
Under conditions that would give you good
results
with a lleniscus lens atl.11.3, use stop
/.11, with your Ar.rastigmat, don't use the
largest opening for every occasion use it
only for emergency. Your greatest Anastig-
(18) (
le)