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08-18-2009, 05:22 AM   #1
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Magnification ratio in macro photography

Hi all,
after many searches on the net I didn't found any document that show how the magnification ratio is related to the focel length of a lens.
For example: if I use a 24mm macro extension tube with a 50mm lens, what will be the resulting magnification ratio? Is there a formula that relates macro extension tubes length and the lens focal length?
And when reversing the lens, is there a way to calculate the magnification ratio?

Bye
Jenner

08-18-2009, 07:04 AM   #2
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QuoteOriginally posted by ntx Quote
Hi all,
after many searches on the net I didn't found any document that show how the magnification ratio is related to the focel length of a lens.
For example: if I use a 24mm macro extension tube with a 50mm lens, what will be the resulting magnification ratio? Is there a formula that relates macro extension tubes length and the lens focal length?
And when reversing the lens, is there a way to calculate the magnification ratio?

Bye
Jenner
General rule - a lens focused at infinity will achieve 1:1 when it has its focal length in extension applied.

e.g. 50mm focused at infinity will be 1:1 with 50mm of extension. Focused closer - don't know how to calculate.
08-18-2009, 07:10 AM   #3
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QuoteOriginally posted by ntx Quote
Hi all,
after many searches on the net I didn't found any document that show how the magnification ratio is related to the focel length of a lens.
For example: if I use a 24mm macro extension tube with a 50mm lens, what will be the resulting magnification ratio? Is there a formula that relates macro extension tubes length and the lens focal length?
And when reversing the lens, is there a way to calculate the magnification ratio?

Bye
Jenner
This Pentax Auto Extension Tube guide has some information:

http://www.pentax.ca/pdf/o_manuals/om_AUTOEXTENSIONTUBESET_e.pdf

Phil.
08-18-2009, 07:44 AM   #4
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Magnification ratio is always the ratio of subject to lens distance to lens to focal plane distance

1:1 magnification is achieved on any lens when subject to lens distance is at exactly double the focal length. (assuming you can actually focus that close)

there are formulas on Wikipedia covering the magnification ratio etc, but it is a little bit of a task to work out exact magnifications with different extension tubes and the focus range. The simplest is to work out the magnification at infinity focus, because the lens to focal plane distance is the focal length,

This is why people respond that to achieve 1:1 you need an extension tube that matches the lens focal length.

I went through all the formulas, and developed a spreadsheet that calculates for all my lenses the magnification rations and focusing distances with both extensiopn tubes and close up lenses, but the time invested makes me keep this for myself for the moment

08-18-2009, 01:07 PM   #5
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Attached is a simple excel spreadsheet for calculating the focus distance, magnification ratio and effective aperture for a lens used with extension tubes or a bellows unit. This will take the focus travel of the lens into account provided you give it the right data. It should give acurate results but be warned that the lens to object distance it calculates will be measuerd from somewhere inside the lens and the position will depend on the lens used.

If anyone in interested then I can do something similar for suplementry close up lenses but this will be less accurate as some assumptions need to be made.

It is quite difficult to calculate these parameters when using a reverse mounted lens as there are no convenient points from which to measure as the front principal point of most lenses is not usually well defined.
Attached Files
File Type: zip Macro focussing.zip (2.6 KB, 627 views)
08-19-2009, 10:53 AM   #6
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You might be interested in this article by Peter Zack:

Shooting Macros Section 3 – The Math

.
08-19-2009, 11:18 AM   #7
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QuoteOriginally posted by Lowell Goudge Quote
Magnification ratio is always the ratio of subject to lens distance to lens to focal plane distance

1:1 magnification is achieved on any lens when subject to lens distance is at exactly double the focal length. (assuming you can actually focus that close)....

[snip]
....This is why people respond that to achieve 1:1 you need an extension tube that matches the lens focal length.
I understand that with say a 50mm prime 50mm of extension tubes gets us 1:1.

However, I've always been confused about the way certain zooms act. For instance, Some 70-300 mm zooms have a 1:2 repro ratio while others have a 1:4 ratio and others still have effectively no macro capability at all.

Zooms I think greatly complicate the formula because we may not be certain of what the actual focal length is at various settings.

Regards,
mike
08-19-2009, 02:22 PM   #8
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QuoteOriginally posted by X Man Quote
I understand that with say a 50mm prime 50mm of extension tubes gets us 1:1.

However, I've always been confused about the way certain zooms act. For instance, Some 70-300 mm zooms have a 1:2 repro ratio while others have a 1:4 ratio and others still have effectively no macro capability at all.

Zooms I think greatly complicate the formula because we may not be certain of what the actual focal length is at various settings.

Regards,
mike
You are correct, the thing with zoom lenses, especially new ones, is that some of them offer macro only at minimum focal length, others move the internal elements differently to achieve close focus.

They do not behave normally but because they have internal mechanizms to move lots of elements around (zoom plus focusing) they can offer capabilities of close focus (not really macro)

for a zoom that goes to 200 or 300mm, macro is really a gimick not a feature, but I always maintain that even though I have a true macro lens, I like to always carry one lens that can do close focus because yo never know when an opportunity arises.

Note that to some extent, lenses with internal focusing also may not behave as you expect. The optical formulas really only work for lenses where the entire group is moved. i.e. most older MF lenses. Internal focusing may cause change in true focal length when close focused, but I can't say that for sure, I have not done a test on my samyang 85mm F1.4 (my only IF manual focus lens) yet

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