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.