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[OM] Re: ?simple? problem driving me nuts... calculating magnification f

Subject: [OM] Re: ?simple? problem driving me nuts... calculating magnification from focal length and focusing distance
From: Jim Brokaw <j.l.brokaw@xxxxxxxxx>
Date: Wed, 02 Nov 2005 21:12:23 -0800
on 11/2/05 2:00 AM, jking@xxxxxxxxxxxxxx at jking@xxxxxxxxxxxxxx wrote:

> 
> ok, I may be about to open myself up to calls of idiot but here goes
> anyway....
> 
> I would like to calculate how much light loss there is depending upon the
> lens focusing distance and lens focal length. This is bcause I mainly use
> an incident light meter and want to be able to compensate for the light
> loss at close focusing distances e.g. macro but also taking photos of
> statues at close range etc.
> 
> I found a nice equation that can calculate the light loss based upon the
> magnification ratio and the tamron 90mm macro lens has course
> magnification marks on it. Fine. I converted the light loss ratio into
> stops of light loss, ploted and plotted a graph. I found that until you
> get to about 1:8 the light loss is less than 1/3 of a stop. By 1:1 its two
> stops and very significant. Great what about my other lenses that only
> have focusing distance and not magnification ratios marked on them...
> 
> Intuition says to me it must be possible to get the magnification ratio
> from the focal length and the focusing distance but everything I tried did
> not work. Part of the problem was that I had little additional information
> about my other lenses other than minimum focusing distance and for some of
> them the magnification ratio at that distance so I was playing with three
> variables, focal length, minimum distance and mag ratio. I got out my copy
> of ansel adams the camera and used his equations for working out image to
> nodal point vs magnification and subject to nodal point vs magnification
> and I could find no match between them and the markings on the tamron!
> HELP
> am I crazy? (probably) can it be done... I guess so since macro lenses
> have magnification ratios on them as well as focusing distances...
> 
> 
> James

Hi James -- This is a perfect Moose question, and I'm not reading ahead so
maybe he's already answered it much better but I will attempt it anyway.

Basically, with 'normal' lenses, the focal length is based on infinity
focus. To get to 1:1 magnification you need to extend the lens from the film
by another focal length. This is where the phrase "double-extension bellows"
comes from that you might find discussed for view cameras.

So when you rack out a lens that can go to 1:1 the lens (really the rear
focus node I think, which -may- be somewhere around the aperture blades)
goes from, say, 50mm away from the film to 100mm away from the film. The
diameter of the aperture stays the same though. So the effect is that you
lose a couple of stops of light along the way. Say the aperture is 25mm in
the example I set above. At infinity focus you are at f2.0, (50mm/25mm) but
at 1:1 focus you are at f4.0  (100mm/25mm) thus the true aperture at 1:1
focus is f4.0 even though your lens still says f2.0 on the aperture ring.

For instance, with the 50/2.0 Zuiko macro lens, the 100mm total focus
extension to get to 1:1 focus is based on the 25mm available from the lens
helical (which gets you to 1:2) and then add on a 25mm extension tube (and
the helical covers the range from 1:2 to 1:1) for the last bit of focus
extension. Actually I think the 50/2.0 Zuiko macro uses moving/floating
elements and thus changes actual magnification and effective focal length as
you focus, kind of like a special-case zoom lens, but that gets really
complicated...

All of this changes with magnification in the lens -- so it doesn't exactly
apply with telephoto macro lenses. I think. Or maybe it could, but more as a
case of ratios than as a direct linear progression. I remember reading about
this way back when, and feeling very thankful for TTL light metering. And
feeling that TTL flash was the -only- way to use flash in macro situations.
-- 

Jim Brokaw
OM-'s of all sorts, and no OM-oney...




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