John Emmons wrote:
by the time you figure out the formula, you could have set up, focused and
exposed the film...;^)

Steve, the best "formula" to use for focusing is to get under the dark cloth
with a focusing loupe and get the scene sharp. Check the near and far, all
the corners, etc.

Keep in mind that you probably won't be able to get the entire scene in
focus with the lens wide open, you'll want to stop the lens down and let
your eyes get accustomed to the light, with a bit of practice you'll be able
to focus.

The math that Leonard uses is undoubtedly correct, but I think you'll find
that in practice it's much better to actually see the image get sharp on the
ground glass.

This is the second response expressing a similar sentiment. I would
agree that in practice one wouldn't focus by setting the film to lens
distance based on the subject to lens distance and calculations. That
seems rather obvious. But there are certainly other reasons for wanting
to know the formula. The most common is when you want to know if you
can focus at a certain distance with the maximum bellows extension your
camera allows, but there are others. Steve's original request was quite
simple and didn't suggest he intended to use that as a primary method of
focusing, although perhaps his subject title could be interpreted that
way. Generally, when someone asks a question like this, I assume he
has a reason for wanting to know the answer, and, if I know the answer,
I respond. Let's give him the benefit of the doubt instead of lecturing
him about what he should be doing.

By the way, my answer is correct.


John Emmons

"Leonard Evens" wrote in message
news
Steve Yeatts wrote:

Is there a formula to calculate the required distance between the lens

board

and the film plane for an object at a given distance from the camera?
Thanks in advance.

If u is the distance from the lens to the subject, v the distance of the
lens from the film plane, and f the focal length, then

1/u + 1/v = 1/f.

So

v = fu/(u - f)

In words, multiply the subject distance by the focal length, and then
divide that by the difference between the subject distance and the focal
length.

But this requires some additional qualification since the lens has an
extent of its own. The subject distance is measured from a point called
the front principal point and the film distance is measured from a point
called the rear principal point. For most lenses, these points are both
close to the center of the lens which in turn is close to the lens
board. But for lenses of telephoto design, both these points may be far
in front of the lens. For many wide angle lenses, the rear principal
point is a short distance in back of the lens board. Usually lens
specifications tell you what the rear flange focal length is. If you
know that and the focal length, you can determine the position of the
rear principal point.


Steve (new to 4x5)