Question on Bellows extension

[ndwgolf]

I have the App "Reciprocity" which will give you a compensated f stop for bellows factor. My question is how do you guys measure the actual extension in the field....is it just a case of carrying a small tape measure in your pocket and checking it each time before you take the shot or what??
I use a Wisner Technical 4x5

Thanks

Neil

[Dan Fromm]

I use a tape measure. High precision isn't needed.

[ndwgolf]

I use a tape measure. High precision isn't needed.
Am I right in saying when using a 360mm lens the distance from the lens to the film is also 360mm and a 210mm lens will also be 210mm when focused at infinity??

[Dan Fromm]

With most LF lenses, the diaphragm-to-film distance at infinity is approximately the lens' focal length.

In other words, the rear node -- that's where you measure from when calculating exposure compensation close up -- is approximately at the diaphragm. As I said, high precision isn't necessary. If you do the calculations you'll find that a 10% error won't kill you.

If it is easier for you, mark the infinity position. After you've set up for a closeup shot, measure how far the standard is in front of the mark. For most LF lenses, adding focal length to that distance will give the same total distance as measuring from diaphragm to film plane.

[xkaes]

The optical focal length of a lens and the flange focal length are frequently so close as not to be significantly different. But for some lenses they are very different and will mess things up if you don't use them correctly. Typically, the shorter the optical focal length, the longer the flange focal length will be. For example, my Fujinon 75mm has a flange focal length of 85mm. And with true telephoto lenses, the optical focal length is much longer than the flange focal length. For example, my Fujinon 600mm has a flange focal length of 384mm. So with most lenses, a tape measure will do the trick -- if you have a calculator, computer, formula, etc. But all that takes time -- especially in the field when you are thinking about other things. What I did was to create my own, personalized tape measure / exposure ruler. I know how long my bellows extends, so that is how long my tape measure is. Mine is simply paper -- 1" x 14" -- laminated for strength. I ran calulations for each of my lenses with the number of f-stops/shutter speeds I need to adjust for each lens and bellows extension -- one line for each lens with 1/3 f-stop increments. In the field, I simply place the end of my ruler at the film plane and extend it out to the front standard -- after focusing, of course. I look at the ruler and it tells me how much I need to adjust the exposure for that lens. These have been, and maybe still are, commercially made, but it's more accurate it you take the time to do it yourself -- because for lenses with optical focal lengths significantly different from the flange focal length, the standard exposure calculations will not be accurate since you have to start out at a shorter or longer infinity point.

[Luis-F-S]

Am I right in saying when using a 360mm lens the distance from the lens to the film is also 360mm and a 210mm lens will also be 210mm when focused at infinity??

Close enough!

[Doremus Scudder]

I carry a tiny tape measure. Some people I know have a seamstress' tape sewn along one edge of their darkcloths. Whatever works.

Just make sure to have a measure in the units you need, inches or mm.

Best,

Doremus

[xkaes]

Close enough!

The flange focal lengths of Fujinon's 210mm lenses range from 189mm to 210mm -- up to a 20% difference.

The flange focal lengths of Fujinon's 360mm lenses range from 352mm to 359mm -- up to a 2% difference.

2%? I could blow off. 20%? No way! On my Fujinon 600mm, there is a 36% difference. That's not walking on thin ice, that's trying to walk on water! But the decision is up to you!