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Help calculating exposure with extension tubes?


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#1 ryan laing

ryan laing

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Posted 29 November 2008 - 02:21 PM

Hello.

I cant remember the formula for calculating exposure compensation once extension tubes are added. Can anyone here help or give some tips on this?
I'll be shooting on a Bolex with a 75mm f1.9 lens and a combination of extension tubes anywhere from 5mm to 75mm if it helps.

Thanks.
-Ryan
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#2 David Regan

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Posted 29 November 2008 - 05:52 PM

Just got this outta the handy ASC manual:

Distance from film plane squared, divided by the focal length squared = the exposure factor.

So if you are using a 75mm lens and say a 50mm extension tube that would be [(75mm + 50mm)^2]/75^2

Which yields 15625/5625 = 2.78. Which is your exposure factor, which yields an exposure increase of between 1+1/3 - 1+2/3 stops.
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