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I actually made almost this exact thing years ago, except mine folded up accordian style. Do the math once, throw away the tape measure and calculator in the field...

2. Most interesting discussion. I found all of the replies of interest, and no, didn't scare me back to 35mm... my brain may be twisted though. It's interesting to see the various methods of solving the same problem, much as artists of all sorts do in their work be that music, painting, photography or any other form. Now, I just have to find one that works and fits my aptitude.

3. Not sure if anyone said this already, but bellows extension factor is pretty easy, once you realize that the inches of bellows draw are in the same relation as f-stops. All you need to know is the length of the lens in inches, and how far out the lens is racked.

For example, say you are using a 210mm lens, which is approx. 8 1/4 inches. Convert the inches to an f-stop, and round off to the nearest stop or 1/2 stop. In this case, your "base" exposure is f8. If your lens is racked out to 11", then you need to increase the exposure by one stop, the difference between f11 and f8. If you're racked out to 16", then it's two stops. For distances in between, approximate. Racked out to 13"? Well, that's f11 + a third of a stop or so. So you'd increase the exposure by about 1 and 1/3 stops.

Close enough for government work!

4. Ok, not to be contrary, but I want to do this completely differently.....

For me, since my mono-rail doesn't have a built-in ruler, I would find it much easier to just have a table for *subject distance*. I think I could accurately guesstimate subject distance better than bellows draw (image distance). Why does no one do this? Why is the sky blue?

Having a table divided by 1/3 stops would also be nice. So, if I have a row of desired exposure compensations (like 1.33, 1.66, 2, 2.66, etc... all the way up to 16 let's say, a.k.a 4 stops) and I know my focal length (180mm), I should be able to calculate for subject distance, right?

Can anyone do the algebra and concoct a singular formula? I was having to convert exposure compensation to magnfication and then to subject distance and at about 2 o'clock in the morning I was beginning to lose my patience.

5. ## Exposure Compensation Based on Focal Length and Subject Distance

You can easily generate the table you want with a calculator with the natural logarithmic function and the following information.

Let the number of additional f-stops of exposure required for bellows extension be x, f = focal length of the lens, and p = diaphragm-to-subject distance.

Then

x = 2*ln[p/(p – f)]/ln2

Example 1: f = 180mm, p = 3 meters = 3000mm.

Then

x = 2*ln[3000/(3000-180)]/ln2 = 0.18 stops.

Example 2: f = 180mm, p = 360mm

Then

x = 2*ln[360/(360 – 180)]/ln2 = 2 stops.

You can easily generate a table for each focal length you use. I find it more convenient to simply program the calculator with the generic formula and enter f, p, and execute. The result is the exact additional exposure needed in f stops. The calculator is easily carried in a camera bag or coat pocket.

6. That's good, but how about a formula where p=_____. I think I have a learning disability or something when it comes to algebra. If only they had told me in high school that I would need this stuff in 10 years to figure out my photography exposures, THEN I would've paid more attention.... maybe.

7. Originally Posted by holmburgers
Ok, not to be contrary, but I want to do this completely differently.....

For me, since my mono-rail doesn't have a built-in ruler, I would find it much easier to just have a table for *subject distance*. I think I could accurately guesstimate subject distance better than bellows draw (image distance). Why does no one do this? Why is the sky blue?
Because at those distances you can guesstimate, you don't need compensation.
At those where you do, guesstimate errors are big.

8. Yeah, but why is the sky blue?

No, but seriously... someone could just as easily measure the distance, but it seems no one uses subject distance to figure bellows-factor. That's what I was wondering "why does no one do this?", not estimating distance.

9. Originally Posted by holmburgers
Yeah, but why is the sky blue?

No, but seriously... someone could just as easily measure the distance, but it seems no one uses subject distance to figure bellows-factor. That's what I was wondering "why does no one do this?", not estimating distance.
Because it is much, much easier to measure bellows draw than subject distance.

10. Originally Posted by holmburgers
Yeah, but why is the sky blue?