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Old Oct 11, 2012, 6:54 AM   #11
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Regardless of their telecentricity?

Certainly you'll agree that a lens that is not very telecentric would create a more magnified image than a lens that is more telecentric. From Wikipedia:
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An image-space (or image-side) telecentric lens produces images of the same size regardless of the distance between the lens and the film or image sensor. This allows the lens to be focused to different distances without changing the size of the image.
Therefore, conversely, lenses that are not telecentric would change the size of the image, and therefore, the magnification of the image.
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Old Oct 12, 2012, 8:58 AM   #12
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The tube contains no optical elements; its sole purpose is to move the lens farther from the image plane. The farther away the lens is, the closer the focus, the greater the magnification, and also the greater the loss of light (requiring a longer exposure time). Lenses classically focus closer than infinity by moving all optical elements farther from the film or sensor; an extension tube simply imposes this movement.
http://en.wikipedia.org/wiki/Extension_tube

I guess the keyword is "classically" here. There doesn't seem to be a hard and fast rule for the magnification factor of a tube. For example my Sigma 105mm f/2.8 Macro will focus from infinity to 1:1 with an extension of only 50mm! It does get to 1:1 as I focused on a scale and showed about 24mm in a Nikon D300 viewfinder.

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Old Oct 12, 2012, 9:23 AM   #13
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Quote:
Originally Posted by TCav View Post
There's no way to know what magnification ratio you'll get with a particular lens on extention tubes without trying it. ...
Quote:
Originally Posted by Bob Nichol View Post
... There doesn't seem to be a hard and fast rule for the magnification factor of a tube. ...
That's my point. At best, the formula you referred to is a useful guide.
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Old Oct 12, 2012, 12:15 PM   #14
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Quote:
Originally Posted by TCav View Post
That's not entirely correct. The purpuse of extension tubes is so that you can focus closer. The effect that prevents focusing at infinity is also what permits focusing closer than the lens' minimum focus distance.

You are correct. I had been thinking of teleconverters, which don't change the focal distance, but do increase magnification. Thank you for the correction.



That's not entirely correct either. Some lenses are more telecentric than others, so the change in the size of a projected image varies from lens to lens. For lenses that are perfectly telecentric, the size of the image circle never changes, however many extension tubes you use.
For a lens to be perfectly telecentric, the exit pupil would have to be as large as the diagonal of the image sensor, and be perfectly collimated. This would make flange to focal plane distance irrelevant, and we could all use adapters for the various lens mounts without a problem. Unfortunately, this isn't the case. A fair example would be my 50mm f/1.4, with an exit lens diameter of about 30mm. (Exit pupil can't be greater than the lens diameter) In order to fill the image sensor of 35mm film, the image circle has to be a bit over 43mm, meaning the image from the lens must diverge.
Using extension tubes creates a larger image circle at the film(sensor) plane, giving greater magnification. For most lenses this has a greater effect on magnification than the closer focal distance. The point is valid.
Bob's formula, which uses the lens' original magnification, is valid, as the flange distance is taken into account.
Sorry I took so long to respond, but life is in somewhat of a state of flux right now.

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Old Oct 12, 2012, 1:35 PM   #15
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I agree with everything you said, except that the light leaving the lens would not be collimated. Light is collimated when all the beams of light are traveling parrallel to one another. If they're parrallel, they never cross. If they never cross, they never focus, and will never project an in-focus image on the image sensor.

While telecentric lenses do have exit pupils that are at least the size of the image sensor, light from all portions of the exit pupil is projected onto all portions of the image sensor, which is why the image is in-focus.

P.S. Good luck with life.
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