Richard Knoppow wrote:

What you say below is correct. But there is one minor problem for
perfectionists. You have to know the actual focal length, which may be
different from the nominal published focal length.


Its not hard to find the principal points of a lens. If
you know the focal length its only necessary to focus the
lens exactly at infinity and measure one focal length from
the focal plane toward the lens. By definition that is the
principle point. To measure the front or first principal
point turn the lens around and again focus it for infinity.
Again, measure from the focal plane toward the lens, that
will be the first or front principal point and is the point
to measure object to lens distance to.
If there is no handy target far enough away to
approximate infinity you can focus by autocollimation. You
need a small mirror and a card with a small hole in it, and
a flashlight to place behind the card. Place the mirror over
the lens. Place the card in back of the lens, adjust the
card (or lens) until an image of the illuminated hole is
reflected back to the card. It should be near the hole. When
this image is sharply focused the lens is exactly at
infinity focus from the card. In a view or press camera you
can usually get sufficiently good accuracy by placing a
small light source over the ground glass near the center and
focusing the lens on that.
Since you know the focal length and infinity focus
position you can calculate the amount the lens must be moved
from infinity focus and use that to adjust for some closer
distance. Its still necessary to know the location of the
front principal point so you can measure the distance.

For any lens the distance from the infinity focus point for
any closer distance is:

X = f^2 / u - f

Whe
X = extension from infinity focus
f = focal length
u = subject distance