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Ricardo de Oliveira
21-Jul-2014, 04:47
Hi,
I'm in the process of building a 4x5 PS around a Schneider Angulon 90 f6.8 and found in the auction site a m42 focusing mount that travels 7mm from open to close. How do I calculate if this is enough for focusing from portrait to infinite with my lens?
Sorry by my poor English.
Ricardo

Bob Salomon
21-Jul-2014, 04:52
First you need to find out if the mount will let you reach infinity focus based on the flange focal length of the lens.

Ricardo de Oliveira
21-Jul-2014, 05:21
hi Bob,
flange focal lenght of the lens? I'm afraid i don't get it. How do I check that? I think I need a Lens 101 crash course.
I found data about the lens here:
https://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/angulon/data/6,8-90mm.html
And found that the flange focal distance for a M42 mount is 46.45 mm
Help me digest that.

Ben Syverson
21-Jul-2014, 07:29
Ricardo, what you want is the thin lens equation (http://en.wikipedia.org/wiki/Thin_Lens_Formula#Imaging_properties):

(1 / object distance) + (1 / flange distance) = (1 / ƒ)
Where ƒ is the focal length of the lens.

In this case, want to solve for object distance (minimum focus). We know that the Angulon has a flange focal distance in the range of 89.5–90.0. We'll call it 90.0, plus your maximum travel of 7.0, to get 97.0.

(1 / x) + (1 / 97) = (1 / 90)

1/x = (1 / 90.0) - (1 / 97)
1/x = 0.00080183276059
x = 1247.14mm

So you'll be able to focus down to 1.2m. Not quite portrait distance for a 90mm on 4x5, but enough to get a little background separation at ƒ/6.8.

Good luck! Building cameras is fun. Whatever you do, don't do a Kickstarter!

Ricardo de Oliveira
21-Jul-2014, 12:22
Oh, thank you Ben. That was perfect.
You know I'm doing this to myself just because you guys don't want to send me one of yours to me, here in Rio..
No. Just kidding, I'm doing it because yours is ugly and plastic...;-)