# Calculating Bellows Extension from Magnification Factors? Anyone?

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• 02-24-2013, 09:33 PM
Prof_Pixel
There are two different equations:

Lens-to-film Distance = (Magnification + 1) X Focal Length

and

Effective Aperture = f-number X (Magnification + 1)
• 02-24-2013, 09:40 PM
Sparky
ahhh okay. so for a 55mm lens in the case of a micro nikkor if i chose to go that way - i'd be looking at (10 +1)x55mm = 605mm

and in the case of an 80mm rodagon:

(10+1)x80mm= 880mm

wow that was easy cheesy.

Although what confuses me about that is - I always understood that ever doubling of a bellows length implied a doubling of image size (and quadrupling of exposure of course)... but in the case of a 55mm lens - 1:1 gives you 110mm and therefore 2:1 gives you 220mm 4:1 is 440mm etc... which doesn't seem to jive with the above formula!

which is wrong here? (not meaning to complicate things - I'm thankful for the formula though)
• 02-24-2013, 10:02 PM
Prof_Pixel
To reproduce something at full size (Magnification = 1), the lens-to-film distance would be ( 1 + 1) X focal-length which is 2 X focal-length (known as double extension). For a 55mm lens, this would be 110mm

To reproduce something at twice full size (Magnification = 2, the lens-to-film distance would be ( 2 + 1) X focal length which is 3 X focal-length. For a 55mm lens, this would be 165mm
• 02-24-2013, 10:04 PM
Prof_Pixel
Quote:

Originally Posted by Sparky
Although what confuses me about that is - I always understood that ever doubling of a bellows length implied a doubling of image size (and quadrupling of exposure of course)... but in the case of a 55mm lens - 1:1 gives you 110mm and therefore 2:1 gives you 220mm 4:1 is 440mm etc... which doesn't seem to jive with the above formula!

Doubling the focal-length only applies to a magnification of 1
• 02-24-2013, 10:12 PM
Sparky
Quote:

Originally Posted by Prof_Pixel
To reproduce something at full size (Magnification = 1), the lens-to-film distance would be ( 1 + 1) X focal-length which is 2 X focal-length (known as double extension). For a 55mm lens, this would be 110mm

To reproduce something at twice full size (Magnification = 2, the lens-to-film distance would be ( 2 + 1) X focal length which is 3 X focal-length. For a 55mm lens, this would be 165mm

Yep - well that makes good sense... thanks professor... :)
• 02-25-2013, 07:25 AM
Dan Fromm
Quote:

Originally Posted by Sparky
well thanks for the responses but I dare say few people have read the question. I'm really not concerned with exposure compensation or subject/object distances. Those (subject/object distances cancel out to become magnification in certain formulae) - but I've been googling for some time and can't find the correct formulae - though I know they exist since I've used them before but it must be 15 years since I have.

I wouldn't really call it 'ultra macro' or anything of the sort. It's just regular 'macro' photography. I figure a micro nikkor would probably work the best... maybe i'll just stick with one of those for everything... but I was just hoping there was a quick formula i could use to derive the extension from the F.L. and the magnification - but I guess it's not too well known...

What format are you shooting? I ask because the MicroNikkors sold for 35 mm cameras don't cover 4x5 at magnifications much below 4:1.

Oh, and by the way, if you're going to use a MicroNikkor at magnifications > 1:1, it should be reversed. My tests of my 55/2.8 MicroNikkor AIS found that it is best at f/4 above 1:1 and that image quality rapidly vanishes as it is stopped down farther.

You and most of the participants in this thread should read a book. Two books, in fact, and the last time I looked the usual places (abebooks.com, alibris.com, amazon.com, ...) showed copies of both at reasonable prices.

Lefkowitz, Lester. 1979. The Manual of Close-Up Photography. Amphoto. Garden City, NY. 272 pp. ISBN 0-8174-2456-3 (hardbound) and 0-8174-2130-0 (softbound).

A thorough discussion of getting the magnification, lighting, and exposure. Especially good on working above 1:1. Extensive bibliography.

Gibson, H. Lou. Close-Up Photography and Photomacrography. 1970. Publication N-16. Eastman Kodak Co. Rochester, NY. 98+95+6 pp. The two sections were published separately as Kodak Publications N-12A and N-12B respectively. Republished in 1977 with changes and without the 6 page analytic supplement, which was published separately as Kodak Publication N-15. 1977 edition is ISBN 0-87985-206-2.

Gibson is very strong on lighting, exposure, and on what can and cannot be accomplished. His books, although relatively weak on getting the magnification with lenses made for modern SLR cameras, provide a very useful foundation for thinking about working at magnifications above 1:10 and especially above 1:1. Extensive bibliography.
• 02-25-2013, 09:14 AM
Prof_Pixel
Quote:

Originally Posted by Dan Fromm

Yup, you were first with the correct answer. Congratulations. However, you've got to admit that the answer was well hidden in a lot of extra words.
• 02-25-2013, 09:53 AM
Dan Fromm
P_P, I won't accept responsibility for the OP's inability to read. I hope he/she/it gets the books, reads them, and learns to think for himself/herself/itself.
• 02-25-2013, 01:38 PM
Prof_Pixel
Quote:

Originally Posted by Dan Fromm
I won't accept responsibility for the OP's inability to read.

If a person asking for help doesn't understand our reply, we really haven't helped that person, have we?

I thought the object here was to help people.
• 02-25-2013, 02:02 PM
cliveh
Quote:

Originally Posted by Sparky
making photographs of small objects...

Why don't you just meter at the focal plane?
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