Exposure Compensation for 210mm

[Ken Grooms]

What is the exposure compensation when racking out the bellows on a 210mm (8inch?) lens? Is it +1/3 per inch? Thanks.
Ken

[Brian Ellis]

I follow the old rule of thumb that you don't need to worry about adjusting exposure for bellows extension unless the object on which you're focusing is closer to the lens than 10 times its focal length. In the case of the 210 that means closer than 2100 mm or about 7 feet. If something is closer than that then how much you adjust depends on how far you're racking out (i.e. on how close the object is). I usually open up a half stop once I get within the 7 foot range (when using a 210mm lens) and a full stop if I get really close.

I realize that isn't terribly precise but it's as precise as you need to get if you use negative film, with which being off by even a full stop, especially on the over-exposure side, rarely makes any significant difference in the printability of the negative (and if everything else is right you won't be off by a stop following this rough guideline). If you use slide film, where even thirds of stops make a difference, you would want to be more precise. There are lots of formulas and calculators around to give you more accuracy than you need with negative film. If you don't get a quick answer here just Google on all the words "bellows compensation exposure lens" or something like that, I'm sure a formula will pop up quickly.

[Capocheny]

Ken,

I don't know of any general "rule of thumb" other than the one that Brian mentioned.

Are you talking about taking close-up pictures of something? This would require that the bellows be extended to some degree.

For a 210 lens, to get 1:1 magnification, you'll need 420 mm of bellows length. In that case, you'll need to increase your exposure by at least a stop, or more.

Since I do a bit of close-up work, I bought a bellows factor calculator gizmo from Calumet a few years ago. With this device, you would put one part of it into the picture, then measure the size of that item on the ground glass with a ruler type device. This, then gives you the bellows factor that you need to calculate into your exposure.

One of the fine forum members, Ralph Lambrecht, has such a device on his website, at:

http://www.darkroomagic.com

Go to the "Library" link.

Click on "Bellows Target & Ruler."

I'm sure you'll find this device quite useful...

Cheers

[Ole Tjugen]

At 1:1, the extension is 2x focal length, and the exposure compensation is 2 stops.

With 31 cm extension and a 210mm lens, you need one stop compensation. Think of the aperture scale: 22, 32, 45... The 210mm focal length is 21cm, think of "a tough below 22". Rack it out to a bit below 32cm, and that's one stop. A bit short of 45cm is two stops.

[Capocheny]

At 1:1, the extension is 2x focal length, and the exposure compensation is 2 stops.

With 31 cm extension and a 210mm lens, you need one stop compensation. Think of the aperture scale: 22, 32, 45... The 210mm focal length is 21cm, think of "a tough below 22". Rack it out to a bit below 32cm, and that's one stop. A bit short of 45cm is two stops.

Hi Ole,

Gads, I think I'm short on caffeine (or sleep!)... of course you're correct on the 2 stops!

Thanks for the correction.

Cheers

[Geert]

Because I'm not that much into math, I can't recommend anything better than the Quickdisc:

http://www.salzgeber.at/disc/index.html

Just don't forget to take it out of the scene before exposure ;-)

Greetings,
Geert

[Kevin Klazek]

The rule of thumb I use is 1/2 a stop compensation for every 25% of the lens focal length that you have racked out. If you rack out 50 mm with your 210, add 1/2 stop. If you rack out 105 mm add 1 stop, etc.. I do not do a lot of closeup work, but this simple approach has worked for me.

[steve simmons]

It is 1/2 stop for every 25% beyond the infinity extension. So, a 210mm/8.25" lens would require an additional 1/2 stop when the bellows is extended to 10.25", 1 stop when the bellows is extended to 12.5" etc., etc.

steve simmons

[darr]

There is an excellent article in the September/October 2006 View Camera magazine titled: "How to Optimize the Sharpness of Your Photographic Prints", by Robert B. Hallock (part II). If you turn to the last section of the article on page 60, and read under the heading Bellows Extension Factors, you may appreciate what the author has presented. I shoot a lot of macro work and have been using Bob Wheeler's VadeMecum in an HP48GX for years with much success as an aid when computing the math involved. However, the info in this article along with the graphs are IMO very well presented. For example, the last graph of Percent Shift from Infinity Focus Position I have copied and keep close by when shooting macro as it has kept my calculator in the toolbox for bellows extension.

[Leonard Evens]

No rule like 1/3 of a stop for each inch extension is going to work. But, if you look at the ratio obtained by dividing the bellows extension by the focal lenth, then something like that does work. Namely if you multiply the amount the ratio exceeds 1.0 by 3, you will get an approximation to the number of stops difference. This works provided the ratio does not exceed 1.0 by too much. For example, with a 210 mm lens, suppose you extended the bellows to 240 mm. The ratio would be 240/210 which is about 1.14. The increment beyond 1.0 is 0.14 and multiplying that by 3 yields 0.42, which is the approximate change in stops. So you would open up somewhere between 1/3 and 1/2 stop. But this fails badly if the increment beyond 1.0 is large. For example, if the ratio is 2.0, the case of 1:1 magnification, the increment is 1.0, and the above rule of thumb would suggest a shift of 3 stops. But the correct shift is 2 stops. The rule does work reasonably well for ratios up to about 1.3 but not too well beyond that.

The correct number of stops can be calculated as follows. Find the ratio of the bellows extension to the focal length, find its logarithm, multiply that by 2, and finally divide the result by the logarthm of 2. In symbols,

2* log(ratio)/log(2)

Here is a list of ratios followed by the appropriate increase in number of stops as calculated using this formula. You can put it in a table and carry it with you. In the list, the first figure is the ratio of bellows extension to focal length, and the second is the resulting number of stops given to two decimal places.

1, 0; 1.1, 0.28; 1.2, 0.53; 1.3, 0.76; 1.4, 0.97; 1.5, 1.17; 1.7, 1.53; 2.0, 2

That will take you to the ratio for twice the focal length which correpsonds to a 1:1 magnification ratio.

Of course, what Brian said is true. For negative film, if you err in the direction of slight overesposure, you are not going to get in trouble. So the simple rule he suggests should be enough. But for transparency film it is another matter. Then getting the proper exposure to within 1/3 stop may be important. Of course, with such film you are likely to bracket anyway, but you still need some basic starting point about which to bracket. You can use the above table