For a lens of focal length f, and an extension past infinity of e, the magnification M is given by:
M = e / f
The distance of lens to image is: d_i = f + e.
These formulae agree with the ones given earlier in the thread if you do some algebra to rearrange them.
The difficulty with using the formula exactly is that the distance of lens to image is really the distance from rear principal point to the image, not the flange distance or the distance of the aperture from the image. But this is usually only a few mm off.
From this we can see that with your setup of a 50mm lens plus minimum 35mm extension past infinity, it would give a minimum magnification of 35/50 = 0.7x, which is a bit more than you wanted, so the 50mm enlarging lens plus bellows wouldn't be a good choice for duping 35mm onto APS-C.
In general, 50mm lenses don't provide a lot of working distance at magnifications near 1:4 to 1:1. If you want to do that, choices include: getting an SLR macro lens with a normal focus ring and some extension tubes; or using a longer f.l. enlarging lens on a bellows or similar; or buying a focusing helical mount and then messing around with a bunch of different adapters to get the extension within the travel of the focusing helical.
Why do you want to use an old enlarging lens on a new digital camera?
Why do you want to use an old enlarging lens on a new digital camera?
The op need a magnification of 1:7.6 which is in most macro lens range without any extension. The negative that the OP want to digitize is quite large 13x18cm.
This lens is a lot sharper than my Fuji lenses, even with a toilet paper roll extension tube and light leakageAnd I cant see any distorsion at all. And I already own it. But yes, it seems to have some limitations regarding working distance.
Why do you want to use an old enlarging lens on a new digital camera?
Why do you want to use an old enlarging lens on a new digital camera?
I was not familiar with: "extension past infinity" - but it looks like a very useful term. Thanks for that!For a lens of focal length f, and an extension past infinity of e, the magnification M is given by:
M = e / f
The distance of lens to image is: d_i = f + e.
Yes, this is the difficulty I have been having with trying to use the formulas. When we say "distance to lens" it is never clear which exact part of the lens the measurement is made from. While it may be only a few millimeters off, sometimes that few millimeters can be the difference between the lens being useful for my intended purpose -- or not.These formulae agree with the ones given earlier in the thread if you do some algebra to rearrange them.
The difficulty with using the formula exactly is that the distance of lens to image is really the distance from rear principal point to the image, not the flange distance or the distance of the aperture from the image. But this is usually only a few mm off.
I was not familiar with: "extension past infinity" - but it looks like a very useful term. Thanks for that!
Yes, this is the difficulty I have been having with trying to use the formulas. When we say "distance to lens" it is never clear which exact part of the lens the measurement is made from. While it may be only a few millimeters off, sometimes that few millimeters can be the difference between the lens being useful for my intended purpose -- or not.
I was hoping the formulas could be used to avoid the necessity of: buy-it-and-then-try-it; repeat as necessary. But when when the numbers are right on the edge between acceptable and unacceptable, theory fails, and it is back to the practical world. ;-)
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