Hahaha, well I didn't expect a coherent answer to that question, but I'll take it!
I used to shoot a lot of table top shots that required the calculation of bellows extension. I'm dyslexic and can't do math well. Calumet had this device that I used a lot. It was basically two pieces of plastic. One piece was a 2"x2" plastic square where I put in my shot. The other piece of plastic was a ruler about 1"x 4 inches long. I just measured the magnification of the square on the ruler and calculated the bellows extension. It was a simple and slick little device that was effective.
Take a look at this link:
I'm familiar with that, but it seems cumbersome, at least for "field" stuff. (as if taking a monorail into the field isn't cumbersome enough)
My reasoning for wanting to have a table with subject distances is the ability to take some measurement-standard on your body. For instance, the "hang ten" sign with your hand; that distance won't change, and if you know what it is you could quite accurately measure distances in the field w/o a ruler or anything. The distance from your elbow to fingertips could be another measurement, and so on.
Come to think of it, I'm just gonna get a huge ruler tattooed down the length of my arm and a yardstick down my leg....
I don't understand why you want a complicated formula based on the subject distance when you could just velcro a measuring tape to your tripod and use your cell phone's calculator to figure the traditional bellows extension factor. It takes 30 seconds to get the right factor and then you can either divide the ISO by that factor to get a new ISO, or just have a table that gives the right aperture adjustment for each factor in third stops. You can even make it faster by printing a table of the square of each focal length you own so you can plug it in the calculator immediately without doing math.
Quick & Easy Bellows Compensation with a Calculator
The easy way is to program a pocket calculator and take it with you. If you have trouble programming it there are many high school or college students who’d do it for you in a minute. You likely know at least one such person who’d do it for you.
Once that’s done you carry a metric roll-up tape measure or a dressmaker’s tape (they have metric versions very cheap at any fabric shop). Here’s all you do.
1. Compose and focus
2. Measure the distance E from film plane to diaphragm in millimeters (this isn’t fussy)
3. Enter f, E, and press the execution key to display the answer in f stops
What could be simpler?
The equation is
x = 2*ln(E/f)/ln2
where x = number of additional f stops for the bellows extension, E = bellows extension measured from the film plane to the diaphragm, and f = focal length of your lens.
It isn’t rocket science and you really don’t need to know any math to use it.
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Thanks everyone, I know that you all have very good reasons for doing it the traditional way. It's simple and straightforward... I'll do it already!! However, I'm also wanting to take a fresh look at this and explore something different. Let's leave it at that. Besides, the more exposure mistakes I make, the more film I use and the more I'm feeding the market!
I really believe that having an intuitive grasp of how subject distance relates to bellows factor would result in near instantaneous exposure compensations, in your head. It's much easier to estimate distances on the scale of feet and inches rather than mm or cm.
Now, what was it that Robert Frost said?...
If your subject is at twice the distance from the lens than the focal length of that lens, it is a factor of 4, or 2-stop difference (e.g. 300mm distance using a 150mm lens). If the subject is 3 times the distance from the lens than the focal length, it is a factor of 2, or 1-stop adjustment. If the subject is at an infinite distance from the lens, the factor is 1, or 0-stop difference.
IMO Greg's got the easiest solution and is more or less what I use.
My method(?) goes like this:
With a 150mm/6" lens,
Halfway between gets half the correction 1/3 gets 1/3 the correction. Plus or minus depending which side you're measuring from.
With my hand, 9" is the spread from thumb to little finger whilst hanging ten.
Here is a simple way to calculate compensation requiring only an inch tape measure.
take the focal length of your lens in inches and the extension of the bellows in inches and using these numbers change the unit to Fstops.
What is the difference? that is the number of stops you must compensate.
example a 210mm is approximately an 8" lens and with a 16" extension you derive f8 and f16. What is the difference between F8 and F16? 2 stops!
You're done. BTW, wide angles are terrible for closeup due to excessive lateral chromatic aberration. Questions see RodKlukas.com
Should help you all. 75mm= 3"= F2.8 1/3, 90mm = 3.5"=F3.5(2.8 2/3), 135mm = 5.6"=F5.6(close enough) etc
Remember though that the extension in this methode is the full lens to film distance.
Originally Posted by RodKlukas
I mention that, because when SLRs are involved it is normal practice to use only the extra extension (the length of an extension tube, or bellows put between lens and camera) in calculations.
So when using the above method to calculate the compensation you need for an X" focal length lens with an Y" length tube, you have to add those X" to the length of the tube Y before 'converting' them into f-numbers.