Measuring of Bellows for Exposure

[snommisbor]

So I am really trying to shoot much more LF and wanting to hone my craft. Using the app Reciprocity Timer it will give you a bellow extension and calculate the stop difference based on the length of bellows you have extended. My question is: Do you measure just the bellow or do you also add the depth of the camera back as well. So in my case an Ebony 4x5 which is probably an addition 1 1/2-2 inches to the film plain. So for example if the bellows are extended 4.9" with a 90mm lens would account for 1 stop according to my app, but adding an addition inch and a half or so of the actual camera it would add up to a stop. So which do you use? Just bellows, or bellows and camera? Thanks.

[Patrick13]

Isn't it technically from the nodal point of the lens to the film plane?
Using a 210mm lens as an example, infinity focus measured as above should be near 210mm and a 1:1 macro focusing would be 420mm measured? And that would yield a +2 stop exposure (or 4x the time).

[Jim Noel]

Although technically not accurate, measuring from the ground glass to the middle of the shutter will do the job well.

[Doremus Scudder]

As a rule of thumb, measure from the middle of the lens (usually the middle of the front standard unless using a recessed or extended lensboard) to the film plane, i.e., the ground glass. That is almost always close enough for just about any application. When in doubt, err on the side of overexposure.

I usually just measure from the front of the front standard to the ground glass, a little longer than the real extension, and figure from that. The tiny bit of overexposure is fine for negative film. If you're shooting transparencies, then measure more carefully.

Best,

Doremus

Oops, I see Jim beat me to it, and much more succinctly

[drew.saunders]

You should, in theory, measure to the film plane, but that's often difficult. I find it easier to focus the lens at infinity (or a building that's pretty far away, which is close enough to infinity focus) and then measure the distance between the front of the front standard and rear of the rear standard. For my 125mm lens, the distance between the standards happens to be 100mm when focused at infinity. If I was focusing on something fairly close such that the distance between the standards was 150mm, I'd know that I was 50mm above infinity focus, so I'd use 175mm for any bellows extension calculations. I suppose I could carefully measure and mark where the film goes, and then look up the flange focal distance for each lens, but it's easier to do the infinity focus once and use easy landmarks.

[climbabout]

Rob - the short answer to your original question is measure from the center of the lens to the film plane as others have already posted. Here's another way to calculate bellows factor that I learned many years ago before electronics, apps, and the internet. Use the focal length of the lens that you are using as a base measurement - i.e. a 210mm lens = about 8" - back in the day lens focal lengths were always expressed in inches with 25.4mm= to 1 inch. So if you are using and 8" lens, then 8" of bellows extension is focused at infinity and requires no exposure compensation(think of the 8" as F8). If you have 11" inches of bellows think of that as F11 - 1 stop of extra exposure needed. 16" of bellows = F16 - 2 stops extra exposure and so on. If you are using a 12" lens (300mm) the 12" of bellows is your starting point requiring no compensation. 16"= F16(approx) requiring 1 stop, 22" = F22 requiring 2 stops and so on. For field work and negative film, this method is plenty accurate. When I was doing tabletop work with reversal film and much closer focusing distances where accuracy was much more critical, I used a target and ruler sold by Calumet. The target was placed in the scene where the focus point was and you used a ruler to measure the size of the target on the groundglass and the ruler gave you the exposure compensation factor.
Tim

[paulbarden]

I use the Cooksey-Talbott calculator, which is simple and always accurate enough for my needs: Instructions page here and the actual calculator here.
You can save the URL to your smart phone (which is what I do) and have it available wherever you are.

[Jim Galli]

I've done tons of these up in my studio playing with old lenses on a big old studio camera with 36" of bellows available. I learned early on that we mostly overthink this. I measure from the middle of the lens to the ground glass, and then I measure the actual aperture size and divide. Period. No factors trying to regurgitate the lens assigned number to it's multiplier etc. Measure the hole, measure the length, divide. Easy peasy.

[Mark Sawyer]

I'm agreeing with Jim 100%. Even with a little human error, figuring the f/stop from scratch still has more accuracy than the speeds of most large format shutters, even the modern ones. And it's easier than the traditional "divide the square of this by the square of that to get a factor to multiply something else by" method. AND it keeps you in touch with what an f/stop actually is. Most photographers these days have no idea...

[Doremus Scudder]

I agree with Mark and Jim that calculating in the field is a PITA. However, I don't feel like measuring the aperture either and, for most shots I need a bellows extension factor for, I can't even easily do that.

I simply made a chart that has all the lenses I commonly use on it with bellows extension factors and time/f-stop adjustments up to the longest bellows I own. It's in my exposure record notebook along with reciprocity failure corrections for various films, filter factors, EI adjustments for various developments, etc., etc. If I need bellows extension correction, I simply measure and look it up on the chart.

FWIW, I think I posted my chart (as a pdf-file) somewhere here a while ago. Anyone interested can search.

Best,

Doremus