Empirically measure focal length on an old folder?

[Michel Hardy-Vallée]

I getting more and more fond of my old 6x9 folder, but because it was a consumer item, it does not have the standard f/stops increments, nor does it have any focal length information. I'm interested in measuring those mostly to get my light metering estimations right and for the entertainment.

Now I know that f/stop is a relationship between aperture and focal length, and I'm sure I can find the formula somewhere on the net. But regarding focal length, what should I measure? The camera covers 6x9, and focuses on two fixed positions, "near" and "far". Should I just measure the distance between the lens and the film plane? It's not even a triplet, it's barely a single piece of converging glass.

 

[Ole]

F-stop is the relation between aperture and focal length, expressed in the same units. So with an aperture opening of 12.5mm and a 100mm focal length, the f-stop would be 12.5/100 or 1/8. This is usually expressed as f:8.

Note that the aperture diameter must be measured through the front of the lens, since it is the light-gathering opening that is important.

A simple lens will have it's optical center near the center of the lens, so meauring that distance would be close enough. For better precision you can measure the size of the projected image compared to other lenses with known foal lengths. This can also be done as a quick way of finding focal lengths - I made a chart of the size of the image of a window projected on a (paper taped to ) a wall in my house, and got a nice little table of the fcal lengths of all my old odd unmarked lenses.

 

[Surly]

Ole is right on the money. Simple math.
BTW the F stands for factor.

 

[BradS]

I would just clarify what Ole said a bit. The focal length of a lens is defined as the distance from the rear nodal plane to the plane of focus when the lens is focussed at infinity. For practical purposes, (assuming the design of your lens is "normal", that is, assuming it is not a telephoto or "wide angle" design), the rear nodal plane on your folder can be taken to be approximately coincident with the plane of the lensboard.

 

[Dan Fromm]

6x9 folder? Lens no shorter than 100 mm, no longer than 105. That's how they are. If you need a more precise estimate, Ole's suggestion is better than Brad's because a meniscus lens' rear node isn't inside the lens.

 

[Ole]

BTW the F stands for factor.

This is one of the eternal discussions...

But the F stands for Focal length. Old lenses will be marked with something like "F=180mm 1:6.8" (copied straight from my "C P Goerz Berlin DOPP.-ANASTIMAT Serie III DAGOR"). the F is focal length, and the aperture is a fraction (incidentally the aperture ring of that lens is marked in Stolze numbers, not f-stops!).
Other makers used the "f:6.8" or "f/6.8" notation for the same aperture - the "f" equates to focal length in the basic formula.

T-stops are a different matter, and most often found on very expensive movie taking lenses. It would also be useful to have them on many modern zoom lenses - 20 plus lens elements absorb a significant amount of light!

 

[Michel Hardy-Vallée]

You guys will always know everything, thanks to all for that! I'll go make measurements and try Ole's technique.