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# Thread: How to focus to infinity?

1. ## How to focus to infinity?

I guess I never thought of this before but, how does one focus a view camera to infinity??

Although I'm sure the answer will be painfully obvious and I'll be embarrassed I asked, I have to ask.

Do you need to know the focal length of your lens?

2. Actually, this is a quick and dirty way to find the focal length of a lens. Just pick a distant object that is small and easy to determine that it is in focus; like hardwood tree branches in winter or lights, wires, fences, etc. and focus it on the ground glass. When in focus, the camera is focused at infinity for that lens. The distance from the face of the lens board to the plane of the ground glass will be the approximate focal length of the lens.

3. Originally Posted by jbbooks
Actually, this is a quick and dirty way to find the focal length of a lens. Just pick a distant object that is small and easy to determine that it is in focus; like hardwood tree branches in winter or lights, wires, fences, etc. and focus it on the ground glass. When in focus, the camera is focused at infinity for that lens. The distance from the face of the lens board to the plane of the ground glass will be the approximate focal length of the lens.
It'll be the exact focal length of the lens, won't it? Modulo the fact that "infinity" isn't really infinity, which should make a microscopic difference if you picked a nice distant object, and any slop in choosing your points of measurement. Am I missing something that makes it only an approximation?

-NT

4. Nathan-
The rear nodal point of the lens, the distance from which to the film plane is the focal length, is not necessarily on the same plane as the face of the lens board.

5. Originally Posted by pawlowski6132
I guess I never thought of this before but, how does one focus a view camera to infinity??

Although I'm sure the answer will be painfully obvious and I'll be embarrassed I asked, I have to ask.

Do you need to know the focal length of your lens?
Some folding view cameras have infinity stops. Once set, they allow you to rapidly pull the front standard out to a specific point after unfolding the camera. Some people set them just past infinity, so you can find tune the focus, if it needs to be at infinity, after you get the camera set up.

6. View cameras are focused by observing the image on a ground glass at the film plane, which is then removed and the holder containing the film or plate is put in its place.

7. Originally Posted by ntenny
It'll be the exact focal length of the lens, won't it? Modulo the fact that "infinity" isn't really infinity, which should make a microscopic difference if you picked a nice distant object, and any slop in choosing your points of measurement. Am I missing something that makes it only an approximation?

-NT
It really is only a close approximation. I set up my Sinar Norma at infinity (by focusing on a very distant object) and measured the distance from the film plane to see where the lens focal length comes to on the lens, knowing that it's the rear nodal point that's important. I did this for different lenses and discovered that the front of the lens board was correct for all Symmars (symmetrical design), but the aperture ring was correct for all Super Angulons (unsymmetrical design). I added this fact to my field notebook so as not to forget it. So lens design is a contributing factor. Note: This technique is not valid for telephotos designs, which I do not own.

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