Calculating bellows extension/focal length at different focal distances?

[BetterSense]

Say I have a box camera focused at 12 feet. How much closer to the film plane would I have to move the lens to focus it at infinity? I don't know how to calculate bellows extension for different focal distances, or where to find the formulas to do so.

[SteveKarr]

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[SteveKarr]

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[Peter K]

Say I have a box camera focused at 12 feet. How much closer to the film plane would I have to move the lens to focus it at infinity? I don't know how to calculate bellows extension for different focal distances, or where to find the formulas to do so.
If you don't know the focal-length of the lens it can calculated as follows


focal lenght = image distance / (magnification + 1)

then you can calculate


lens-to-film distance = (magnification + 1 ) * focal lenght.

More also at the LF home page here (http://www.largeformatphotography.info/bellows-factor.html).

Have fun

Peter

[BetterSense]

It's too hard to calculate the magnification. On wikipedia I found

1/S_1 + 1/S_2 = 1/f

My brain sorta-remembers this formula from college physics. S_1 and S_2 are the distances from the rear nodal point to the film and the from the front nodal point to the subject, and I'm not sure what that means. If I do the calculation to find S_1 with f=90 and S_2 = 12 feet then I get 92.3mm. I take this to mean I would need to move the lens 2.3 mm closer to the film to get infinity, but I don't know about the nodal point thing.

[Peter K]

My brain sorta-remembers this formula from college physics. S_1 and S_2 are the distances from the rear nodal point to the film and the from the front nodal point to the subject, and I'm not sure what that means. If I do the calculation to find S_1 with f=90 and S_2 = 12 feet then I get 92.3mm. I take this to mean I would need to move the lens 2.3 mm closer to the film to get infinity, but I don't know about the nodal point thing.
How to find the nodal points is described here (http://www.largeformatphotography.info/forum/showthread.php?t=60087&page=3). But keep in mind the nominal focal lenght, e. g. 90mm isn't always the effective focal lenght. With the Super-Angulon 90mm f/8 it's 90.7mm +/- 1 %. This datas one can find normaly at the lensmaker's website.

For LF practice it's easier to calculate with the distance between the front of the lensboard and the film surface focussed at infinity. For the SA 90mm it's 99.4 mm. Now you can calculate sufficient all you need as a photographer. Like


magnification = focal length / (subject distance - focal lenght)

or


close-up exposure time = (magnification + 1)^2 * time of distant exposure

and many more

Peter