Hyperfocal Distance

[chris_4622]

Hi,

I'm making a fixed focal length camera. The hyperfocal distance for my lens/fstop combination is 9.6'. In order to set the lens in the correct position I need to know do I set up the film plane 9.6' from the target or some portion of the lens?

thanks,
chris

[Glenn Thoreson]

What I did when I made mine was set up a target to focus on, using ground glass at the film plane, at the hyper focal distance for the largest aperture you plan to use on the lens. I then attached my camera front Lens board) at the point of sharp focus. By using smaller apertures than the starting point, you gain acceptably sharp focus at both farther and closer distances. You may want to use a different aperture as a starting point. My lens is a 90mm f/12.5 Extreme Wide Angle. Closest hyper focal distance I could find for 90mm is 19 feet at f/11. Close enough. That was my focus target to set the depth of the camera. It works fine. By the time I stop down to f/45, everything gets sharp right up to the camera. Almost. :D

[Jeff Conrad]

It's easy to calculate the image distance directly: image distance v = f + Nc, where f is the focal length, N the f-number, and c the acceptable circle of confusion. But it might be just as easy to set the distance visually, using a method similar to what Glenn described.

[Emmanuel BIGLER]

Gilles Barbier from France has built a fixed-focus wooden LF camera as explained here ; sorry, the article is in French, but the images are interesting, G. Barbier uses the whole image circle of the lens and gets circular images on a 8x10" sheet of film.

http://www.galerie-photo.com/barbier-hybis-90.html

For a classical wide-angle lens of 90mm of focal length as used by G. Barbier (a good ol' Angulon-but-not-super), @f/22, taking a value for the circle of confusion "c" = 150 microns, according to Jeff's classical formula, the ground glass should be in principle placed at about 3.3mm ( 1/8") behind the focal plane.
Interesting to note that in principle this offset in totally independant from the focal length in use ! It depends only on the f-number and the diameter of the circle of confusion. For sure, the hyperfocal distance directly depends on the focal length, but in fact the proper setting of the fixed-focus camera for landscape use (images of distant objects detected close to the focal plane) can be considered as a pure depth-of-focus problem, with the ground glass slightly offset from the focal plane. In the depth of focus problem, the classical geometrical ray tracing has no need for the actual value of the focal length, wich seems really counter-intuitive at a first glance..

Since the choice of the proper diameter for the circle of confusion is somewhat fuzzy, no need to be actually very precise. If c=100 microns was considered as the proper value, @f/22 the proper offset distance for the ground glass with respect to the focal plane would be about 2.2 mm. Setting something between 2 and 3 mm only requires (in principle) a schoolboy's ruler and no precise measuring instrument like a caliper, a dial-indicator precise to 1/100-th of a millimeter, or watever ;)

I agree that the experimental / visual method is the best, but it is good to know a rough value for the proper offset "N c" for the ground glass, between 2-3 mm in the previous example, this is so easy to computed for any combination of f-number/circle of confusion.

[Stephen Lewis]

Hi,

I'm making a fixed focal length camera. The hyperfocal distance for my lens/fstop combination is 9.6'. In order to set the lens in the correct position I need to know do I set up the film plane 9.6' from the target or some portion of the lens?

thanks,
chris

Your 'Target' will need to be 9.6' (sic) from the nodal centre of the lens. The 'target' should then be sharp with the zone of apparent sharpness (depth of field) extending from the near focal point to infinity. HTH

[chris_4622]

Thanks for all of the responses.

Glenn I am going to take your suggestion and use a larger aperture for my initial setting.

[Glenn Thoreson]

Chris, using a larger aperture but not necessarily wide open will give you more choices as to what part of the image will be in best focus. There's the fact that you will rarely, if ever, be shooting at the hyperfocal distance, too. You need room to move around with your aperture settings. There's a trade off in building one of these. I don't know enough about math and circles of confusion, other than my own, to figure it out on paper. I'd go nuts. It's a short trip the way it is, so easier was better for me. It works pretty darngood, too. My lens is wide open at f/12.5, so I get pretty good depth from the get go. Good luck with your project. :D