Originally posted by Charles1 I just bought a set of 3 extension tubes to use with my *ist ds. I am using a Tamron 75-300mm zoom. How do I determine correct exposure ? Can I use an external light meter and them use a formula to determine correct exposure ? I tried a few by trial and error and got some very good results, but would like a reliable way to determine correct settings......

Thanks... Charles1

Here is a formula to calculate exposure increase factor for symmetrical lenses and nonmacro versions. This is the way I do it when using film. It is based on formulae in the Kodak book,

*Closeup Photography* (1984).

EI = (m + 1)2 note: that is squared, the 2 is supposed to be superscript

Where:

EI = exposure increase factor m = magnification Magnification can be calculated by: m = x/l

Where: m = magnification x = length of extension l = lens length Fore example when I use my 28.5 mm pentax tube,
On a 28mm lens I lose approx 2 stops (EI = 4 from above formula)

On the 50mm I lose just over 1 stop (EI = 2.46 from above formula)

On a 135mm lens you lose about 3/4 stop (EI = 1.47)

On a 200mm lens I lose about 2/3 stop (EI = 1.31)

Remember that exposure factor isn't the number of stops. You will determine that based on the fact that each stop lets in 2x the light the previous one did. Remember that you can compensate for this by the aperture, shutter speed or both.

Asymmetrical lenses require a more complicated formula because pupilary magnification of the lens has to be calculated.

This is the way I did it for bellows and tubes when I used film. I carried a cheat sheet.

Edit: I want to add that macro was the main motivation for me to go digital. I use the green button and histograms to get me where I need to be on digital. I used to shoot slides and would bracket. A good day for my scientific presentations were when I got 3 good slides out of a role of 24 or 4 out of 36 in the macro range.