bellows compensation

[putri]

i have a 360mm lens for my 8x10. how does one calculate the exposure

compensation for the close up shots with the bellows extension.

i hear that there;s a formula of some sort..

[ann_m]

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

 

...also check out the QuickDisc at the bottom of that page for a simple effective method.

[neil_poulsen1]

Calumet distributes an inexpensive tool that consists of a 2" square target and a special ruler. After placing the target in the scene being photographed, one "measures" the target on the ground glass using the special ruler. The ruler has a scale that automatically determines in stops the bellows correction that's needed.

 

As a statistician, I'm a math guy. But, why do math when one can have such a simple, intuitive tool that makes the math unnecessary?

 

It's called a Calumet Exposure Calcualtor, and it sells for about $8.

[leonard_evens]

Divide the bellows extension by the focal length. Then multiply the time indicated by your meter at the desired f-stop by the square of that factor.

 

Let's consider an example with your 360 mm lens. Suppose when focused on your subject, the lens to film distance is 420 mm. Then the factor is 420/360 = 7/6 ~ 1.17. Multiply the time by its square or 1.36.

 

Alternately, you can change the f-stop by opening up the appropriate number of (fractional) f-stops. The formula for this is

 

2 log(factor)/log(2)

 

In the above example, using a scientific calculator, I find you should open up by 0.44 stops, or a shade less than half a stop.

 

Doing this calculation can be awkward in the field, so there are various shortcuts for avoiding it, some of which are described in the previous responses. Another method which works if the factor is not much bigger than 1 is to multiply the fractional part by 3. In the above example, the fractional part of the factor is 0.17, and 3 times that is 0.51 or just about half a stop. This method slightly overestimates the fractional f-stop change. It won't work too well if the amount the factor exceeds 1 is too large. For example, if the factor is 1.4, this method gives 1.2 stops, while the correct answer is 1 stop.

 

One further remark. The bellows extension has to be measured from the ground glass to the rear node of the lens. For most normal lenses this is close to the lens board, but for telephoto and some wide angle lenses, it is located somewhere else. You can avoid that problem entirely by calculating the scale of reproduction M, which can be calculated as follows. Focus at infinity, i.e., on a sufficiently distant object, and note the position of the standard on the rail or bed. Focus at the desired distance and find the additional extension beyond the infinity focus. Divide that by the focal length to get M. Finally, add 1. In the above example, the additional extenstion will be 60 mm. So M = 60/360 = 1/6 and the factor is 1 + 1/6 = 7/6 ~ 1.17.

[brian_ellis3]

Look in the archives. This question comes up and is answered here about once a week.

[danny burk www.dannyburk.]

Forget the math and formulas - just use the Quick Disc. Easy, accurate, free, and takes no room in your pack.

[adrian_tyler]

quick disk dosen't work when you need to do really close up

[leonard_evens]

I understand that Russell was probably not concerned about just how to do it, and the methods such as use of a reference disk have big advantages. (Like all things they also have disadvantages.) But he did ask about a formula. He may very well prefer not to use it, but isn't it a bit presumptuous to tell him NOT to use it.

 

There may be some people who prefer to use the formula, or even if they don't, they may want to know the basic math which the disk method or other aids make use of. When you use such an aid, you are not avoiding math. You are letting someone else do the figuring for you and accepting the results. If the world consisted only people who avoided math in all circumstances, who would construct these convenient aids?

 

My favorite aid is a paper tape which is marked in different colors, one for each of my lenses, with the appropriate f-stop corrections for the bellows extensions as measured on the camera. I just place it along the rail and read off the f-stop correction, so I don't have to put anything in the scene and then retrieve it. It also rolls up quite compactly. It took me perhaps half an hour to prepare, and I can always make another one if need be.

[bill_taylor2]

I want to echo Neil Poulsen's answer. Get the Calumet exposure calculator. And to go one better, steal an idea from the Quick Disc too.

 

I went down to the hardware store and found a 2 inch diamater Stainless Steel washer, which just happens to be the same size as the Calumet focus target. Picked up some white and some orange spray paint too. Painted the washer, one color to each side.

 

Now I have the best of both worlds, the Calumet exposure ruler, and a nice round Quickdisc-like target that is easy to see on any background. Just lay it down in the scene, find the longest dimension with the ruler, and read off the exposure compensation.

 

The Quickdisc target is 2-7/8 inches. I looked for a washer that size, but couldn't find one. And I like Calumet's ruler better anyway, if only because it is durable plastic and not just paper, and it is a bit more compact.

 

The SS Washer alone was about $1. The Paint was about $9. If its worth doing, its worth Overdoing. ;-)

 

Bill