magnification formula

[marcel]

Hello everyone:

A short question: a while back I remember reading about a simple formula for calculating magnification, but I did a search and can't find it. It used bellows extention and focal lenght... Any ideas?

Thanks!!!!!

marcel

[David A. Goldfarb]

Much easier just to put a ruler at the subject position in the scene and compare it to the width of the film frame. So if you are using a 4x5" camera with the back oriented vertically and you can see 8" of ruler placed horizontally in the scene, the magnification is 1:2. If you can see 3" of ruler, the magnification is 4:3, etc. This method works conveniently with any camera, any format.

Bellows extension only affects exposure significantly if magnification is greater than 1:10. For factors of 1:3 to 1:10, I usually just estimate. I have a table taped to the back of my camera to convert the magnification factor quickly to exposure factors.

[Ernest Purdum]

There is a mathematical formula, but being a computational incompetent, I think the easy way is as follows. To get to a lifesize image, you need to extend to two focal lengths. After that, each further focal length you extend will get you one even number of magnification. With a four inch lens, you need eight inches extension to get to 1:1, twelve inches for twice lifesize, sixteen inches for 3X magnification and so on, until you run out of bellows.

[Michael Kadillak]

Whatever procedure you employ to correct for bellows extension when making a photomacrograph, I wanted to share a piece of information relative to a prior post on diffraction that I feel has bearing here.

Many of us always feel that because of the fact that the depth of field in these types of shots is already marginal, we nearly always want to use the smallest aperture possible for obvious reasons. However, when the bellows extension is twice the focal length the adverse effect of diffraction is twice the effect of diffraction on the same lens focused at infinity. For example, if you used f45 for a shot of twice the bellows extension of the lens the effective f stop for diffraction purposes would be f90. The further the extension, the greater the net effects of diffraction. If you have determined your personal limit for lens diffraction, make sure that you do not unintentionaly exceed it.

Cheers!

[Jay DeFehr]

Image size divided by subject size equals magnification.

[Michael S. Briggs]

One source of the equation that Marcel seeks is David Jacobson's Lens Tutorial. One location is http://www.photo.net/learn/optics/lensTutorial. The particular equation that Marcel requested is M = (Si - f) / f, where Si is the image to lens (rear principle point) distance and f is the focal length. Be sure to use the same units for both, such as mm.





It is amazing how resistant people are to simple mathematics. Suppose you want to know the maximum magnification your camera could do with a lens you are considering buying. Are you going to buy the lens to find out?

[otzi]

" It is amazing how resistant people are to simple mathematics."

Sorry, but I'm one of those with empty head and full hands when it comes to mathematics. A case of 'In theory there is no differance between theory and practice but in practice there is.'

[Guy Boily]

Here's a very simple tool that I found not too long ago and it really works well. Check this site for the info: http://www.salzgeber.at/disc/ www.guyboily.com

[David A. Goldfarb]

I certainly would do the math for the example mentioned (trying to determine the maximum possible magnification with a given lens and camera), but in the field, it's much quicker to estimate the magnification and use a table or to use something like the QuikDisk.

[Tracy Storer]

Hello Marcel,

M=magnification (image size divided by subject size, as noted above)

F=focal length

(M+1)F=bellows extension

((1/M)+1)F=lens to subject

Is this what you were looking for? I've found this useful again and again. With practice, you can do it (and other math) in your head.

[Leonard Evens]

Several people have given the answer, but let me give it again in words.

You divide the distance from the subject to the lens by the focal length. Then subtract one. Finally, take the reciprocal to find the magnification.

Alternately, you can do the following, which is a bit easier. After focusing on the subject, find the total bellows extension, and divide by the focal length. Then subtract one. That is the magnification. In practice, this is easier to use since you don't need to measure the distance to the subject. You make your measurements along the rail. However, if you wanted to know the magnification for a given subject distance and focal length without actually setting up the camera, you would use the other method. The point about not having to buy the lens before deciding if it will work is well taken.

Of course, all distances have to be in the same units. To convert, remember that one inch equals 25.4 mm, and that there are 12 inches in a foot.

Let me remark that to use these rules, all you need is some way to measure the distances and simple arithmetic. It is also helpful to have a simple calculator to do the division. If you can't remember the rules, jot them down somewhere and carry it with you. Many of us use some log book to record apertures, speeds, and other information about camera setup, anyway, and that would be a good place to keep such information. There is no abstruse, difficult mathematics. I can't imagine that anyone who is incapable of doing the arithmetic can successfully take pictures with a large format camera. In fairness, however, I should add that for telephoto lenses and some wide angle lenses, the true extension is signficantly different from the distance between the standards. One way around that difficulty, is to measure the extension beyond the infinity setting instead and to add that to the focal length. That will give you the true bellows extension. (You can also do it using the rear flange focal distance, which is usually included as part of the specifications of the lens.)

Of course, the other methods for determining magnification may be more convenient for some people. It is really a matter of taste and what you would rather not think about. For me, it is easier to measure and remember how to do the arithmetic than to set a scale in the scene, fiddle with something on the gg, and then remember to remove the target from the scene. Others may not want to remember how to do it with arithmetic. What I don't understand is the insistence that one method is better or worse than another. It may be easier for you, but that doesn't mean it is easier for the next guy.

[David A. Goldfarb]

Fair enough. I didn't mean to insist that there was only "one true way."

[marcel]

Thank's to everyone for their helpful response. What I am trying to do is maximize resolution in extreme macro (5-50x) range with lenses such as the Photars, Luminars, and Ultra Micro Nikkor's. And from my research their magnification range (optimal) seems very narrow. As a side to my question, is there a "holy grail" type lens in terms of resolution (preferably in the macro range)? I am interested (more like obsesed ;-o) with maximizing resolution in my work. (sinar p2, betterlight 6k2, + various lenses).

Thank's again to the best forum ever!!!

marcel

[Dan Fromm]

Marcel, who initially asked an entirely different question, finally explained what he's really after: "maximize resolution in extreme macro (5-50x) range with lenses such as the Photars, Luminars, and Ultra Micro Nikkor's."

Marcel, each of these lenses has an optimal magnification at which resolution is best and a recommended range of magnifications in which resolution isn't much worse than the best. The question is discussed at some length at http://www.a1.nl/phomepag/markerink/microlen.htm. It is discussed slightly in Brian Bracegirdle's book Scientific Photomacrography. And H. Lou Gibson wrote quite a lengthy analysis of what is possible in Kodak's publication N-12B (if I remember the number correctly) Photomacrography.

Some of the discussion, especially with respect to Tominon lenses for the Polaroid MP-4 system, strikes me as just plain wrong. For example, my 35/4.5 Tominon is best at the highest magnification I've tried it at, and that is outside the range possible on the MP-4, which is rather short. The published recommended ranges for these lenses is what's possible with them on an MP-4 and doesn't seem to reflect testing..

In my experience with these lenses, there's some variation between lenses of the same make/model, so as always testing before acceptance is needed.

As for pursuing holy grails, well, its your time, your money, and your madness. But and however, the emulsions I use, KM (135 and running out of it), EPP, and TMX, limit the fine detail that can be recorded more than choice of lens does. Also, the technique is so exacting that one usually pisses away more image quality by sloppiness than can be gained by using a better lens. There are plenty of lenses around that are good enough, and very little to be gained by getting one that's better than that.

Get the books, also get a copy of Lester Lefkowitz book The Manual of Closeup Photography, and read them.

Also consider whether you wouldn't be better off with, say, a Wild Stereo Microscope with a camera attachment. I've used a Photomakroscope and an M5 with camera attachment. Both are much easier to use than the gear you have in mind and offer better working distance at the same magnification.

Cheers,

Dan