First, if possible, find the equivalent focus of your lens. If it is made
by a known maker, you will find it in his price list, and if not, you may
calculate it for yourself by the rules given in the various text books,
provided you have a camera of pretty long focus. However, it will be near
enough for our purpose if you get a sharp image of the sun on a piece of
paper, and while you hold lens and paper, get some one to measure the
distance from the paper to the diaphragm aperture, or, in the case of a
single lens, to the center of the lens. Note down this focal length, and
proceed to measure your diaphragms in sixteenths of an inch.
Then, with pen and paper, proceed to divide the diameter of each stop
into the focus, and state the result as a fraction of the focus, thus
f/8. For example, a Ross half plate rapid symmetrical has a focal length
of 71/2 in.; for convenience reduce this to sixteenths=120. A diaphragm
measuring seven sixteenths will give the fraction f/17. Now let us see if
any of these stops correspond with Mr. Burton's. The first two in his
table will only be found in portrait lenses, but we shall probably find
one to correspond with the third, if we are using a doublet lens; with a
single lens we won't find any so large.
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