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Exposure compensation for 21mm extension tube


New Member
I just acquired a used 21mm extension tube for my 80mm Planar f/2.8. This did not come with instructions, although its operation seems to be straight forward. However, if I remember correctly such a tube may require a slight exposure modification--is that the case?
> According to the HASSELBLAD close up calculator you should open the > aperture for a 1/2 stop This is also valid for other focal length lenses .This value is of course not a real "calculated" value , but as the ZEISS LENSES can only operate in 1/2 stops , go ahead and do so .

Magnification = total amount of extension / focal length of lens used.

Diaphragm compensation factor = 1 / (Magnification + 1)

Shutterspeed compensation factor = (Magnification + 1)^2

Compensation in stops (EV) = log(Shutterspeed comp. factor) / log(2)

The 80 mm lens has approx. 10 mm of extension in the focussing mount (difference between infinity focus setting and close focus limit setting).

So minimum compensation (lens to infinity):

Magnification = 21 / 80 = 0.26

Diaphragm compensation factor = 1 / 1.26 = 0.79

Shutterspeed compensation factor = (1.26)^2 = 1.59

Compensation in stops (EV) = log(1.59) / log(2) = 0.67

The same with the focussing ring of the lens set to the close focus limit:

Magnification = 31 / 80 = 0.39

Diaphragm compensation factor = 1 / 1.39 = 0.72

Shutterspeed compensation factor = (1.39)^2 = 1.93

Compensation in stops (EV) = log(1.93) / log(2) = 0.95


You'll see that there is no single amount valid for a specific tube combined with a range of focal lengths.
Even when used with one single focal length lens the amount needed varies enough that using one single value is not a good idea.
> Yes Qnu , you are right with what you state . But as we only have the possibility of extending the exposure in 1/2 stops , and not 1/3 as in large format , all values we can use are approximate values . So i look at it from the practical side , and if in doubt , i use an extension of 1/2 stop and for a second exposure a full stop . (always using the HASSELBLAD closeup calculator and regarding the type of lens and tubesize i am going to use) .

- 80 mm lens, 21 mm tube, no lens extension, exposure compensation 0.67, say 0.7 stops.

I'd give it a correction of 0.5 stops, only 0.2 stops off.

- 80 mm lens, 21 mm tube, full lens extension, exposure compensation 0.95 say 1 stops.

I'd give that a correction of 1 stops, only 0.05 stops off.
Would i use the same 0.5 stop, exposure would be off by almost exactly that amount.

So even when limited to 0.5 stop increments, this simple ex&le shows that "pauschal" answers do not produce the best results.

Doing the math once, producing look up tables for future reference, will obviate the need for bracketing.

By the way, the formulae given do not account for lens assymetry (which can make a huge difference), so when compiling look up tables, its best to look up the data (diameter of the pupils for assymetry, exact focal length of the lens, amount of extension built-in the mount) about each specific lens, and include that in the math as well.
Thanks for the information. I tried Onu's suggestion yesterday in some close-ups, but haven't had the film developed as yet.
> To Qnu I appreciate your precise comments and regarding this theme i agree to all you say . But thats (for me) scientific photography and i prefer the practical way . So i dont care if i have a bracket of 2 negatives or not . For me the outcome is important . But i will give you an other reason to see why precise calculating to for ex&le exposure extension of 0.67 stops is really not important : take for ex&le three lenses of the same type CF2,8/80mm and if you measure the times from the shutter (using an oscilloscope , and i have done so in the past) you will get slightly different times for all the three lenses . the same is valid for the opening of the aperture . You will experience that there are tolerances which make an exact calculation useless . Regards Jurgen (i will be a reading member from now on)
Unfortunately, the Hasselblad Closeup.pdf document doesn't cover the old tube lengths. The link has a general chart which includes the 21 and 55 tubes in addition to the 8, 16, and 32 tubes. Also, the older version of Hasseblad's Closeup booklets (printed in the '70s) has charts for the older tubes and bellows.


We use expensive Zeiss/Hasselblad gear to be sure we do not get enough "spread" to worry about.
And i really doubt the effects of that will cancel out the differences we're talking about.

In fact, when we find tolerances so big that they make calculations useless, we either need to get our equipment seen too, or switch to a cheaper brand.

So i beg to differ!

Yes, you could indeed call it "scientific" (not that it is), but the "scientific" effort is needed only once.
The little scrap of paper in my bag showing the minimum and maximum compensation for each lens i have in combination with whatever extension tube is really easier to use than that Hasselblad Close-Up Calculator you mentioned (also "scientific"?).

And one quick glance at that piece of paper (and you'll soon memorize the values for the most used combinations) means i never have to bracket, and get good exposure.

As the saying goes: "it's not rocket science".