My Method of Making Bellows Compensation Tables, including a math utility

[C. D. Keth]

This is a subject that has been beaten nearly to death but I do something a little different than most people so I thought I'd show it off for the few people it might appeal to. I like to precalculate compensation in 1/3-stop increments and tape a little table onto the lensboard of each lens. That way it's always there with no thinking of fumbling. I just need to measure (either a tape measure or my hand-span is 8 inches almost exactly) the bellows I have out and look it up on the table.

I use this graphing utility (https://www.desmos.com/calculator/ysidybea5i) to make the calculations faster.

f=focal length

b=bellows compensation in stops

The graph will read out a bellows measurement in inches. If you want bellows in mm, remove the "/25.4" from the equation. If you want bellows in cm, replace the "25.4" with "10."

When I get a new lens, I use that utility to create a little table in excel that will get taped to my lensboard. The formatting will be ugly but my table for my 305mm g-claron, for example, goes:

305 mm
1/3 13-1/2 in
2/3 15-1/4 in
1 stop 17 in
1-1/3 19 in
1-2/3 21-1/2 in
2 stop 24 in
2-1/3 27 in
2-2/3 30-1/4 in
3 stop 34 in
3-1/3 38 in
3-2/3 42-3/4 in
4 stop 48 in

[Jerry Bodine]

To add to the "beating", I reserve my lensboard space to corrections for ACTUAL shutter speeds (which change occasionally), since my Sinar lensboards are 140x140mm (5.5" square) and with large diameter shutters leave little room for taped-on data. Pre-calculated bellows extension values in 1/3-stop increments are kept in a small field notebook. I prefer to work only in metric units and use a tiny retracting Lufkin metal tape measure in mm units. To expedite bellows extension info -when I'm in a hurry- I prefer to use Alan Ross' method, which he describes as follows:

When a lens is focused at infinity, the distance from ground glass to lensboard is just about the same as the lens focal length. When the lens is focused on a closer subject, the distance from the lens (board) is increased, and the amount of light striking the film is diminished because the lens is farther away. Calculating the correct exposure compensation is beyond easy! All you need is a small tape measure and a meter showing f-stop numbers! Let's say you have a 150mm lens, and after you have focused on your subject, the lensboard measures 180mm from the ground glass. Just think: what is the difference between f15 and f18? It's about ⅔ of a stop! You need to give ⅔ stop more exposure to compensate for the bellows extension! Same with any other focal length/extension. A 210mm lens with bellows extension of 320mm would be f21 to f32 = just over 1 stop more exposure needed! If you want the actual formula, it is the Extension Squared divided by the Focal-length Squared.


FYI - This formula is applicable to symmetrical lens designs and it should be OK to just measure extension from groundglass to the lensboard. But since I have some unsymmetrical lenses I did separate tests by focusing the symmetrical lenses (3 Symmars) on infinity and measuring from the groundglass to the respective focal length to see where it fell on the lens. All Symmars fell at the front surface of the lensboard. Then I did the same with the unsymmetricals (3 Super Angulons) and found it fell at the plane of the aperture control ring on the lens barrel. So I'd advise checking any lens, especially a tele, in this manner. My field notebook extension data has built-in adjustments for unsymmetrical lenses that allow me to always measure from groundglass to front surface of lensboard in the field.

[C. D. Keth]

To add to the "beating", I reserve my lensboard space to corrections for ACTUAL shutter speeds (which change occasionally), since my Sinar lensboards are 140x140mm (5.5" square) and with large diameter shutters leave little room for taped-on data. Pre-calculated bellows extension values in 1/3-stop increments are kept in a small field notebook.

Maybe you can get into miniature bookbinding and attach a tiny accordion-fold reference to your lensboards.[emoji1787]


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[Ari]

I drew an inches scale directly onto the white side of my dark cloth; you can also draw a scale in cm, whatever works.
Not my idea, I think it came from Maris Rusis, and it's quite handy. Saves you from having to use yet another accessory while shooting.

[cowanw]

Nine inch lensboards are making a comeback!

[C. D. Keth]

Nine inch lensboards are making a comeback!

You could slap a little spiral notebook on the edge of those suckers!


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[Willie]

Basically a 12 inch lens - so figure F 12. Bellows extended to16 inches - figure f/16(one stop). Bellows extended to 22 inches - figure f/22 - and so on.

Pretty simple with most focal lengths you will use.

[C. D. Keth]

Basically a 12 inch lens - so figure F 12. Bellows extended to16 inches - figure f/16(one stop). Bellows extended to 22 inches - figure f/22 - and so on.

Pretty simple with most focal lengths you will use.

Yup, I’m aware of that method.


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[Drew Wiley]

I did the miniature metric tape rule route for awhile, then permanently converted to the little 2-pc plastic Calumet magnification scale.

[Drew Bedo]

When shooting outdors, I use a quick and dirfty approximation that has worked well in the past:

My most used focal lengths in the field are 150mm and 210mm.

I measure bellows extension from the GG to the lens board.

For the 150: Foe each inch beyond 6" I add 1/3 stop.

For the 210: Foe each inch beyond 8" I add 1/4 stop.

Not exact I know, vbut I can do this in my head with a cold wet wind blowing and my wife honking the horn because we are late to her mother's for dinner.

[Kiwi7475]

I’m lazier than everyone here, I just pull up an iPhone app, such as Reciprocity Timer.

[C. D. Keth]

I’m lazier than everyone here, I just pull up an iPhone app, such as Reciprocity Timer.

I do what I do because I'm lazy. :D

I do the work once so I can skip ahead thereafter.

[Jim Noel]

Why not get a good old Weston Master Meter. Such calculations are almost automatic if one can't make some simple mathematics computations in their head quickly and efficiently.

[cowanw]

Why not get a good old Weston Master Meter. Such calculations are almost automatic if one can't make some simple mathematics computations in their head quickly and efficiently.

How so, for bellows compensation?

[C. D. Keth]

How so, for bellows compensation?

I assume so I can use the method that likens the focal length and bellows measurements to f-stops to figure out the compensation. One would “count off” the steps in the meter dial, I guess.


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[cowanw]

Yes, I suppose so, but I wondered if there was something else to it.