1. ## Re: Bellow compensation.

Originally Posted by Mark Sawyer
Just divide the bellows extension by the aperture diameter and you have your f/stop value...
Originally Posted by Vaughn
But first you have to do the math (using the focal length and f/stop number of the lens) to get the aperture diameter, correct?
Yes, there's one simple division problem to do. And you have to make two measurements with a tape measure.

But if you're reeeeally lazy, set your f/stop to one inch and measure the bellows extension in inches. Or set it to one centimeter and measure the bellows extension in centimeters. Then you're just dividing by one; you bellows extension is your f/stop.

See this lens currently on ebay, #330714904774:

Notice it's got a big white mark at about f/11? Let's check my old Velostigmat and see what that measures to be...

Hmmm... about an inch. (Remember, there can be a little slop in the iris accuracy, just like on shutter times.) There once was an old photographer who had the option of measuring his bellows extension, and that was his f/stop, period. 18 inches of extension? f/18. 14 inches? f/14. 26 inches? f/26.

I can't swear that this is what was going on, but I've seen a few shutters marked this way, and it's always one inch, sometimes also at half an inch (easy math, cut your extension in half!), or one centimeter. If you aren't fussy about your taking aperture, it can make life easy.

Today's photo-tip for lazy math-phobes!

2. ## Re: Bellow compensation.

But what if with your shown lens, and 1 inch iris setting, you focus and your bellows measures 11 inches? You have F11, that easy?! Uncanny! Unheard of! What will they think of next?!

3. ## Re: Bellow compensation.

Yup, although technically, at 11 inches extension with a twelve inch lens, you've pulled a Buzz Lightyear, focusing "at infinity, and beyond!!!"

And technically, a 12" lens should have its 1" aperture at f/12, but as I said, there's often a little slop...

BTW, I thought it was a neat little trick, and one I'd never heard talked about before, so I gave it it's own little thread under the Lenses & Lens Accessories forum, (though in retrospect, I guess it should have gone in the Style & Technique forum...)

4. ## Re: Bellow compensation.

Lynn's right. Divide the FL by the Bellows Draw first (always gives you a number less than one) and square that, getting an even smaller number. That's fraction or percentage of light the film is getting. The reciprocal, a number greater than one, is how much you multiply the old exposure time to get the new, longer time.

The same math, a little simpler is: new exposure equals ( BD / FL ) squared, then multiplied times old exposure in seconds. If Bellows Draw is divided by Focal Length, that will always be greater than one, and squared it is greater still.

In Excel I think that formula would be NEW EXPOSURE IN SECONDS =((BD/FL)^2)*OE (OE is Old Exposure in seconds, what the meter says. Bellows Draw and Focal Length are both in centimeters, or whatever.)

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