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04-20-2007, 10:45 AM   #1
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Macro Magnification

Been trying to find a good answer for this...

I have a set of the Asahi Pentax Auto Extension tubes 12mm, 19mm, 26mm (total of 57mm) - I know if I use the whole set on my 50mm, I get slightly more than 1:1 Macro. Now how do I determine the magnification of using less than the full set? is it something like - 50mm/12mm = ~1:4 macro - so about 25% life sized?

How do you determine magnification when reversing a lens directly on the mount? or on the end of Tubes? or on the end of a lens? on the end of a lens on the end of tubes?

I know this is mostly academic and I should just go out and shoot - just trying to get get a feel for it.....


04-20-2007, 12:30 PM   #2
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The magnification terminology when using scientific close-up equipment like bellows and/or microscopes is the ratio of the physical size of the object being photographed to the size of the image it casts on the sensor/film. That's it.

This is completely UNLIKE figuring out 35 mm film lens magnification equivalents when put on a digital camera.

So, on your bellows rails or on the barrel of your macro lens, given the same focal length lens and the same lens extension, the magnification quoted, because it's about how big an image is on the back of the camera, regardless of the size of the sensor, will be the same whether it's a tiny APS-C sensor on a Pentax DSLR, a 35mm film camera, a 6x6 film camera, or an 8"x10" view camera.

04-20-2007, 01:22 PM   #3
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QuoteOriginally posted by ccallana Quote
Been trying to find a good answer for this...

How do you determine magnification when reversing a lens directly on the mount? or on the end of Tubes? or on the end of a lens? on the end of a lens on the end of tubes?
Life-size magnification requires extension that is equal to the focal length of the lens. So a 50mm lens requires 50mm of extension for a life-size image on the sensor or film frame. With 25mm of extension on a 50mm lens, you get 1/2 life size. The lens, without extension tubes or a bellows, have some extension of their own from the focusing mechanism, so you may not need to add the full amount to reach life size.

A reversed lens give lots of magnification, at the expense of inability to focus and manual aperture. I forget the formula for calculating the magnification with a reversed lens, and it depends on the specific lens design. You can also add extension with tubes or bellows with a reversed lens for extreme magnification.

There are tons of books on general photography that discuss this subject, and the basics haven't changed in many years.
04-20-2007, 03:03 PM   #4
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what i do is look thru the the camera at a MM ruler. you will see 23.5mm(just under an inch) in the VF at 1:1 mag. when i use my 105mm with 63mm ET + 2x TC i'm getting somewher around 3-4:1 mag. this is a full frame resize of a dragon flies head.

i know there is a math way of doing it but the ruler trick works pretty well.


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