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SeanEsopenko
14-Feb-2013, 09:02
I'm working on a box camera attachment for the rear portion of a Titan pinhole camera. How do I calculate the distance from the film plane of my 90mm SA F8 when focused on hyperfocal at F32? Where do I measure this from? The back of the lens board? The metal housing of the rear element?

Emmanuel BIGLER
14-Feb-2013, 10:26
Hello from France

the distance you are referring to is listed by the manufacturer and can be defined and measured either between the lens board and the focal point = flange focal distance, or between the last lens vertex and the focal point = back focal distance
You have bothe values here for the vintage super angula 90 mm f/8. there is a tiny difference between two models but for your project you do not have to care that much
http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/super-angulon/data/8-90mm.html

Flange Focal Distance : 99.0mm (older model) or 98.8mm (MC)
Back Focal Length (S'F') : 66.3mm (older model) or 67.0mm (MC) S' = last lens vertex F' = focal point.

With a 90 mm lens, the amount of travel that brings focus from infinity to one meter of distance is about 8.1 mm (f^2/1000)
If you consider a circle of confusion c = 0.1 mm, the hyperfocal distance at N = 16 - f/16 for a 90mm is 90x90/(16x0.1) = 5 metres. the required offset from the focal point required to focus at 5 metres is N.c = 16x0.1 = 1.6 mm. In other terms, instead of allowing 66 mm between the last lens vertex and the film, allow 1.6mm extra, for a total of 67.6 mm.
Of cours ethe best would be ton control the focus with a ground glass..
Good luck !

Jim Jones
14-Feb-2013, 10:26
Calculating the distance from lens to film depends on many variables. Improvise a ground glass frame, determine the correct infinity distance for the lens you have, and apply a correction for the hyperfocal distance.

(edit) Emmanuel thinks better and types faster than me. As he indicates, the hyperfocal distance depends on the Circle of Confusion. The CoC depends on personal preference and the ultimate use of the photographs. If the lens is mounted with the focus correct for infinity, you can experiment by shimming it out for your ideal hyperfocal focus.

C. D. Keth
14-Feb-2013, 10:53
Why not do it practically? Put that lens on a normal view camera. Use a depth of field calculator to calculate hyperfocal distance at f32. Run out a long tape measure and put something at that distance that's really good to focus on. Something like a printed out siemans star (http://www.panavision.asia/uploads/PDFs/Charts/Large_Siemen_Star.pdf). Open up your lens, focus, record the bellows draw. I would still do as Jim suggests and create your mount a tiny bit short so you shim out to the final distance. That will provide a small amount of adjustability.

SeanEsopenko
14-Feb-2013, 13:46
Thanks guys. It's for a fast to set up, run and gun camera. I won't really have time to focus the lens during the shots so I'm building a super light weight camera that's fast to set up and Pre focused. Wont even have ground glass, maybe a view finder. I'm working out how to jury rig a mount fora linhoff lens board, too.