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Re: [OM] How to Calculate magnification on reversing ring?

Subject: Re: [OM] How to Calculate magnification on reversing ring?
From: Alan Wood <alan.wood@xxxxxxxxxxxxx>
Date: Fri, 30 Nov 2001 14:59:29 -0000
Bill Clark asked:

>  Anyone know how to calculate the life-size ratio
>  when you reverse a lens?

>  Say we use a 24mm Zuiko, is this 2:1?

It sounds as though you are coupling two lenses, with a 50mm attached to the
body and the 24mm attached to the 50mm via the filter threads.

If that is the case, then the magnification is the focal length of the lens
on the body divided by the focal length of the other lens.  In this case, 50
divided by 24 is approximately 2, so the magnification is 2x (or 2:1).  This
will only be approximate, and you will get a more accurate result by
measuring and calculating as below.

If you add extension tubes, it is probably easiest to focus on a ruler.  If
you can see 12mm horizontally through the viewfinder, you know that the
width of the image on the film is 36mm, so 36 divided by 12 is 3x (or 3:1).

It is more accurate to open the back of the camera, open the shutter, and
see how much of the ruler is visible at the film plane. A piece of ground
glass or frosted paper may make it easier.

If you are reversing your 24mm lens directly to the camera body, then
measuring the visible part of a ruler is probably the easiest way to work
out the magnification.

I hope that helps.

Alan Wood
Documentation Writer / Web Master
Context Limited (http://www.context.co.uk/)
mailto:alan.wood@xxxxxxxxxxxxx
http://www.alanwood.net/ (Unicode, special characters, pesticide names)


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