The first aid I have to suggest is the use of such a table as Mr.
Burton's. Before we do anything more in this direction, we must consider
the influence of the lens and its diaphragms. In theory the single
landscape lens is more rapid than the doublet of equal aperture, but the
difference is so little that it may be disregarded in practice, and my
remarks will apply to both.

The rapidity of a lens depends mainly on its aperture and its focal
length. Thus a lens of twelve inches focus will require four times the
exposure of a six inch, with an equal sized diaphragm, and a quarter inch
diaphragm will require four times the exposure of a half inch when used
in the same lens.

The Photographic Society of Great Britain have recommended that the
diaphragms of all lenses should bear such relation to the focal length
that each should require exactly double the exposure of the next smaller.
Now, if we turn again to Mr. Burton's table, we shall find that it is
constructed on this principle, and that each stop is numbered so as to
show its exposure. Obviously, the most sensible thing would be to get a
set of stops made to correspond with this arrangement, but we will see
how we can construct a table for stops of any size.

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