From where do I measure de focal lenght of the lens? From the plate where the lens is attached?
Hello from France!
This is a frequently asked question
What you want is to set the proper distance between the lens mount and the film.
Hence, to do this, the knowledge of the focal length is not really useful, as strange as it may seem!
What you need is a distance given by lens manufacturers, named
"flange focal distance" or sometimes
"Flange Focal Length" (e.g. in Fuji lenses documentation).
This is the distance between the back side of the shutter and the focal point. The focal point is where an image of a far-distant object is projected sharp.
Now in order to properly set your lens mount in front of your film holder, you have to take into account the film holder's depth, which is for modern film holders compliant with the ANSI standard
- 2" x 3" and 4" x 5" : 5.0 mm +- 180 microns (0.197" +-0.007")
- 5" x 7" : 5.8 +- 250 microns (0.228" + - 0.010")
- 8" x 10" : 6.6 mm +- 400 microns (0.260" + - 0.016")
If you already have a springback with ground glass, simply focus the image of a distant object, no computation is required.
And if you wish to fix-focus at the hyperfocal distance, instead of infinity, starting from the flange focal distance, you'll need to slightly increase the length between the lens and film by a small amount, to be easily computed according to your working f/number, focal length and sharpness criterion.
You can have a look at this article (in French) by Gilles Barbier who built a fix-focus camera for the 5x7" format.
http://www.galerie-photo.com/barbier-hybis-90.html
In this case, focal length was 90 mm, the sharpness criterion was 150 microns, the working aperture f/16, hence the offset required to focus at the hyperfocal distance instead of infinity was 3.3 mm.
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