# Exposure with extension tubes or teleconverter?

Discussion in 'Medium Format Cameras and Accessories' started by stradibarrius, Feb 3, 2011.

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how do I calculate exposure when I attach an extension tube or a teleconverter?
My TTL bodies will give me the exposure through the lens, but my RB67 does not have a TTL meter and I use a hand held meter.

Is it a factor that is added based on the length of the extension tube???
The same for a teleconverter???

2. ### wiltwSubscriber

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A teleconvertor is simple...1.4x converter = -1EV of lens speed (f/4 indicated is really f/5.6); 2x converter = -2EV of lens speed (f/4 indicated is really f/8)

Tubes are more complex. It is determined by Extension/FL. Rules of thumb...
1/10 - 1/5 = add 1/2EV more exposure
1/4 - 1/3 = add 1EV more exposure
1/2 = add 1.5EV more exposure
1 = add 2EV more exposure
2 = add 3EV more exposure

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Thanks for a straight simple answer!

4. ### David A. GoldfarbModeratorStaff MemberModerator

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I do it by magnification. If you hunt around, I've posted a table that converts magnification to exposure factor as a DOC file, which you can tape to the back of your camera or to your light meter as a handy reference. It works for any format, and you don't need to know the focal length of the lens or how much extension you are using.

With MF or 35mm, you can estimate magnification by comparing the size of the subject to the size of the image on the groundglass, since you know the dimensions of the groundglass, or you could put a ruler in the scene at the subject position to measure it more accurately as needed.

5. ### CGWMember

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Assuming you don't have the manual, go the butkus site for the RB67 Pro S manual where there's a chart that explains all.

6. ### Alastair_IMember

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There's a table at the back of the RB67 manual (downloadable from several sources - it's page 27 of the Pro S manual) that gives the exposure compensation for each of the available lenses with either the no. 1, no.2 or both extension tubes.

7. ### Q.G.Inactive

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Halfway this page there is a short calculator procedure that takes as input the focal length of the lens used and the amount of extension added, and with pushing a few buttons produces the compensation required, in stops.
5 steps involved. Short and simple enough to even memorize.

8. ### paul ronMember

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And while you are reading the manual notice the scale on the side of the focusing rack, it has the bellows extension compensation on it as well for the different lenses.