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

[Slade Zumhofe]

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.

I "think" what I am about to say is what Ole said--only in American terms. A 210mm lens is 8.25 inches--call it 8 inches. Think of this as f/8. If you extend your bellows to 11 inches--call it f/11, 16 inches = f/16 and so on. The difference between f/8 and f/....whatever your extension equals in stop is the amount of exposure increase you will need. Just convert all your lenses to inches--and take the closest f/stop. Remember 25mm is about 1 inch.

Sounds very elementary but it really works. I have tested it on every lens just short of 1:1---can't say for sure if it works for that or not.

Hope this helps.

[Eric Woodbury]

Slade

This is the rule I use and have for 20 years. It always works, 1:1 or otherwise. Maybe not close enough for chromes, but I'm not a chrome.

[Ken Grooms]

I just got my camera about an hour ago and I already love it! :)

The accessories will arrive Thursday (from Calumet), but I'm already practising. I placed a subject close-up - about 4 feet away. I measured the length of the camera with bellows closed - about 8 inches. Then I measured after focussing with the bellows racked out - about 12 inches. So, according to what everyone is saying, I'll either open up one stop or double the exposure time. Am I right?:o

(As a side issue - what is the optimal aperture for the Rodenstock 210mm f/5.6 Sironar-N MC - f/22? Thanks.)

P.S. I can't believed the condition my camera and lens is in - they look practically new!:)

[Andrew O'Neill]

Isn't the formula: Bellows Extension Squared divided by Lens Focal Length Squared?? I've always used this to determine exposure compensation...

[Slade Zumhofe]

You are correct Andrew--some say (bellows extension/focal lenght)squared--all the same actually.

The method described above has some math behind it but it does work and is used to get rid of the math.

This is a great article I found regarding this principle--has a connection to this forum as well.

http://www.largeformatphotography.info/bellows-factor.html

[Alan Davenport]

I measured the length of the camera with bellows closed - about 8 inches. Then I measured after focussing with the bellows racked out - about 12 inches. So, according to what everyone is saying, I'll either open up one stop or double the exposure time. Am I right?

Perhaps not. The baseline for bellows calculations is with the lens focused at infinity, and it's not at all clear that you started there. At infinity focus, by definition, the bellows extension is equal to the focal length of the lens. You then calculate exposure compensation based on how much MORE the bellows are extended from the infinity position.

[Slade Zumhofe]

Since he was originally talking about a 210mm lens I assume he meant focused at infinity when he said "closed is 8in.". "Closed" probably wasn't the best choice of words but I think I knew what he meant.

To answer your question--yes. If you use the f/stop--f/8 to f/11(ish)--method that would be 1 stop. If you want to do the calculations: (12/8)squared....or 2.25. This gives you an exposure factor of 2.25 rounded to 2x or 1 stop.

Hope this helps.

[Mtang]

These tips on using the fstop scale as a guide is great, thanks.
One thing that's unclear to me is - are you suppose to be adding the fstop adjustment (say it's 1 stop, adding from f/8 to f/11 keeping the same exposure time)? If so, it seems counter-intuitive to me, since you need more light to compensate for the bellows extension, wouldn't you open the aperture wider or subtract the fstop
(say to f/5.6)?
Thanks,
Mylinh Tang

[Slade Zumhofe]

That is correct. I can see how you could misread this into thinking that is what you need to do with exposure--go from f/8 to f/11. The method only helps you determine how much extra exposure is needed to compensate for the bellows extension. You may be at f/32 or f/22--doesn't matter. f/8 represents an 8 inch (210mm) lens and f/11 represents the length of the bellows when focused at a close object or 11inches of bellows extension. If your settings were at f/32 you could open up one stop or...increase your length of time (shutter speed) by one stop. Either way--with this example you are correct--you need 1 stop MORE exposure.

[Mtang]

Got it. Thanks.