Sure thing. I recently bought a set of auto extension tubes from Tom Scales,
so my directions are handy....
The compensation is affected by *focal length and the extension*.
For 50mm lens, here are some extensions and F-stop Multipliers. These f-stop
factors (FF for short) are mulitiplied by your
indicated aperture, giving the Transmission (t-stop).
7 mm extension = 1.1;
(about 1/3 stop lost)
f8=t9
14mm = 1.3;
(about 2/3 stop lost)
f8=t10
21mm = 1.4;
(one full stop)
f8=t11
1.4x = 1 additional stop
2x = 2 additional stops
4x = 3 additional stops
For example, if your 50mm lens is set to f/8, and you use 21mm of extension,
the factor of 1.4 means that your lens is transmitting
t-stop of 8*1.4= 11
If FL= focal length in mm, and E is extension in mm, and FF is the f-stop
factor, here's the formula:
FF = (FL + E)/FL
A 50mm lens on 21mm of extension gives:
(50 + 21)/50
(71)/50
1.42 ~= 1.4
f/8 * 1.4 = t/11
Since a tele throws its light in a narrower cone, you'd expect to lose less
light. And you'd be right:
With a 135mm lens, you'd need 50mm of extension before you'd lose a full stop!
Professor Lama, still wondering what I thought I'd ever do with 3 semesters of
Calc. I didn't use it yet......
From: "Charles Sdunek"
>I was wondering how you figure your exposure using
> a hand held light meter when shooting through extension tubes. Does anyone
> know a formula for how many stops to increase exposure to compensate for
> distance extended?
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