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10-24-2010, 10:14 AM   #1
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Effective focal length of a reversed lens

I am googling this over and over, but so far I haven't been able to find an answer.

Quite often, and always for extreme macro purposes, I am using a reversed lens, either my 50mm or 24mm manual lens. Usually I reverse the lens on the body, before I have usually reversed the lens on another lens, such as the wide kit on 55mm or a telephoto lens.

In any case, the usual problem with macro shooting is the lack of light. Having flashes available, it is not a problem, but if I want to shoot something using natural light, it becomes really challenging.

Having in-body SR definitely helps, and having it available I would prefer it working at its optimum for such situations. However, I need to dial in a simple lens characteristic - its focal length.

Now, it is a 24mm lens, right, but it is reversed - how to find the effective focal of a reversed lens?

Most of the math I've seen tackles getting the diopter and magnification - but not the effective focal. Can it be derived from the data above? How to calculate it?

10-24-2010, 10:43 AM   #2
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The focal length of a lens does not change when you reverse it.

The focal length for shake reduction purposes is the lens' focal length times (1+magnification).

This is because shake reduction corrects for rotation of the camera back about the lens' vertical and horizontal axes. The optical lever causing the image to sweep over the sensor is the distance from the lens to the sensor; f(1+m).

The distance is always f(1+m) but m is usually so small it is ignored.

Last edited by newarts; 10-24-2010 at 10:49 AM.
10-24-2010, 11:04 AM   #3
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A related question is the focal length when one lens F' is reversed on another F" with a spacing of D between; it is

F ~F'F"/(F'+F"-D)

The shake reduction focal length entry would be the same as before:

F(1+m) using the new F.

Dave

PS these are estimates based on thin lens theory.
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