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el french
6-Oct-2010, 00:26
Is there a good (easy) method of calculating the image circle of a lens for a particular magnification, or to put it another way, what's the minimum magnification where the image circle of a lens will cover the chosen film size.

The camera and lens in this thread, http://www.largeformatphotography.info/forum/showthread.php?t=67250, is prompting the question. It would seem that this f2 lens should be capable of some extreme macro shots before diffraction set in.

Jack Dahlgren
6-Oct-2010, 00:59
If you know the angle of view of the lens you can use simple trig or geometry.
Tan theta = opposite side / adjacent for a right triangle.
Theta would be half the angle, the adjacent side is the distance from lens to film. Then use the lens formula to figure out the distance and magnification given the focal length.

The difficult thing to figure out is the angle of view of the lens. If it is published by the manufacturer it is pretty simple, but a lot of old lenses might not have this information.

Still, that lens is 42mm so by the time it is a half meter from the film like that it should cover. No idea how sharp it would be though. And the bellows extension compensation factor would make exposures tremendously long.

Emmanuel BIGLER
6-Oct-2010, 04:42
Following Jack's suggestion we could propose a simple formula :

Covered diameter = (nominal image circle) (1 + M)
Where M is the magnification ratio

So at 1:1 the image circle is doubled if we assume that the cone of rays (film side) has the same angle whatever the bellows extension might be.
At M = 1:10, i.e. an object localed at 11 times (1+1/M) the focal length in front of the camera, the gain in image circle is about 10%

The correspondence between the magnification ratio M and the additional bellows extension (beyond the focal point) "ext" is simple : ext = M. f
where f is the focal length ; this formula is universal whatever the lens design might be (i.e. it is valid for telephoto lenses)
Extend by one focal length : M = 1:1, image circle is doubled
Extend by 20% of the focal length : M = 1:5 = 0.20, object is located at bout 6 times (1+1/M = 1+5) the focal length in front of the camera, image circle is increased by 20%

rdenney
6-Oct-2010, 05:19
Extend by one focal length : M = 1:1, image circle is doubled. Extend by 20% of the focal length : M = 1:5 = 0.20, object is located at bout 6 times (1+1/M = 1+5) the focal length in front of the camera, image circle is increased by 20%

Point being that the diameter of the image circle is proportional to the bellows extension. If you increase the bellows extension from infinity focus by 20%, the image circle will increase by 20%. If you increase it by 100%, the image circle increases by 100%. The 4.2cm cine lens, when extended to what appears to be 60 or 80 centimeters, will be extended beyond infinity focus 15 or 20 times its focal length, so the image circle will be increased by a factor of 15 or 20. If the cine lens is designed for 35mm movie film, which has a format of perhaps 18x24mm, the diameter needed to cover it is 40mm. Taking that as the image circle at infinity, a factor of 15 or 20 would increase that to 60 or 80 cm.

Rick "for people not fluent with notation" Denney

ic-racer
6-Oct-2010, 06:46
Hmmm.. thats a 42mm lens. If you guess a nominal 42mm image circle, it would cover 84mm diagonal at 1:1 and 168mm diagonal at 2x mag and so on.

I'd say an f2 42mm lens is slow.

Drew Bedo
6-Oct-2010, 07:55
Why not mount the lens on a camera and find out what-is-what? If you are just testing coverage and not texposing film, ; just mock-up a lense board from heavy cardboard and duct tape. It doesn't have to be light-tight or elegant, just secure enough for this testing session.

After all the calculating is over, you will mount the lens and compose a shot to see if it will work anyway. Why not just get after it and find out?

Let us know how this works out for you.

el french
6-Oct-2010, 22:19
Thanks everyone, that should help when looking for a lens to do macros.

p.s. Drew: you need the lens first :)

Ernest Purdum
9-Oct-2010, 08:43
For macro work, lack of coverage is hardly ever a problem.

Dan Fromm
9-Oct-2010, 09:54
Original poster, don't bother looking for a rare old lens of unknown quality.

If you are bound and determined to work at painfully high magnification with a relatively inexpensive lens, consider using a reversed 25/1.9 Cine Ektar II. Its best aperture at high magnification is f/2.8, stopping down only makes image quality worse.