Measuring bellows compensation factors

[Tom_3925]

I'm quite new to large format photography, so I spent some time looking into bellows compensation factors. I ended up designing rulers in Photoshop to measure the bellows compensation factor based on bellows extension. So far I have created designs for 90mm, 150mm, 180mm and 210mm lenses.

I have written a short description of the steps I took and was wondering if anyone would be able to check that I've got the maths right and have a look at one of the rulers. The URLs for the description and image are:

Description

Ruler for 180mm lens (TIFF image)

Thanks very much

[Michael E. Gordon]

We non-engineer (artistic?) types prefer easy-cheat methods like the QuickDisk or the Calumet Exposure Calculator

Mathematics hurt my head when I'm busy trying to be creative.

[James E Galvin]

I did the same thing, but: I put all of my scales side by side, folded the paper so that half of the scales are on each side, and covered it with transparent tape. Also I picked a place on the camera that measures easily (lens to film is not as convenient as between the standards) and cut the scale down to measure that. Always a reality check: focus on infinity, check that the scale indicates no compensation.

[Leonard Evens]

Tom,

Your analysis is correct.

I did more or less the same thing that Jim did. I can roll my scale up in something less than an inch in diameter, and it fits easily in my camera pack.

The only other bit of advice I have is that the extension is the distance from the film plane to the rear nodal plane of the lens. For most lenses, that is more or less at the lens board, but for telephoto or reverse telephoto lenses is may be some distance from the lens board. This can be dealt with by putting an appropriate offset on your tape. If you can't figure out what to do, let me know and I will describe it.

P.S. As Michelangelo proved, there is no contradiction between engineering and artistic mentalities. The quickdisk method is actually harder to use than a tape of this kind. (And it would be even harder to make one of your own from first principles as Tom did.) You have to put it in the scene, place something on your gg to read the f-stop change and then remember to remove the disk. The scale you just lay along the rail or bed without leaving the camera and read the f-stop change right off it.

[fishfish]

The easiest way to address bellows extension factors is with two small pieces of card or plastic. I use plastic as it is more durable, with the card-contents printed out from my computer and taped to them with clear packing tape.

One card/pastic piece is 4" long and 2" high, with a two inch black square in the middle, and two 1"x2" rectangles on each end (a pattern of white, black, white, so to speak). The second is the same size, and is marked off in 1" incriments. On this the appropriate exposure adjustments are writen (see below). The item is used as follows, with any lens, on any format:

Place the plastic with the black square at the focal point in the image (the black square with white edges makes it easy to focus on). Then on the ground glass, measure the size of the square with the second plastic piece. For life sized (the 2" mark) the card reads +2 (_2 stops for life size). At the 1" mark, it reads +1 (+1 stop for 50% life size). At the 4" mark, it reads +4 (+4 stops for 2x life size). All the settings for the other magnifications can easily be interpolated between the +1, +2 and +4 settings (or you can mark them, if you are so inclined).

I used this technique for years in a copy lab I ran, and it is easy to use, small to carry, and easy to replicate when you inevitably lose one part or another.

[Tom_3925]

Thank you very much for the replies and suggestions - sounds like I'm on the right track but need to check what points to measure from and to.

[Leonard Evens]

Eric,

To each his own; we all like what we are used to. But the quick disk method is definitely superior to other methods which use a square or rectangular target. It uses a circular target, so all you have to do is put it with one diameter in the plane of exact focus and then measure the largest dimension on the gg. It comes with a scale to use on the gg calibrated in terms of f-stop increments.

[steve simmons]

Here is how I figure the extra exposure needed.

I carry a cloth measuring tape like what is found at a sewing store. Then (and this works for any lens) for each 25% I increase the bellows extension I ad 1/2 stop of exposure. For a 50% increase I add one stop.

For example, with a 150mm/6" lens if the bellows is extended to 7.5" I add 1/2 stop of exposure. For a 210mm/8.25" lens I would add 1/2 stop if the bellows is extended to 10" and one stop if the bellows is extended to 12".

steve simmons

[Michael Mutmansky]

Steve's method is essentially what I do, and it certainly is accurate enough for normal shooting. If you are shooting copywork or happen to be concernd about accuracy for a specific reason (say, consistancy of negative densities among a group with different magnification ratios), a more accurate method would probably be preferred.

I use a small tape measure that I got at a trade show. If I cared to be really accurate, I could make a chart as you have done for each lens, and then laminate it and attach it to the tape. Then, you will have all the information necessary to do the adjustment in the field without making any calculations; simply a measurement, and a look-up off the chart.

---Michael

[Ernest Purdum]

Old editions of the "Kodak Master Photoguide" contain a convenient "Effective Aperture Computer". The one I use is dated 1965, but it works the same now as then.