How to determine focal length of brass lenses from length and diameter measurements?

[Rob_M]

I have been researching/looking at brass lenses on ebay and have found that many give only the length and diameter measurements.
How do I find the focal length from these measurements?

Thanks.

[Mark Sawyer]

You don't. Physical size has nothing to do with focal length. The closest you could get would be if the maximum f/stop and diameter of the aperture were known. You could multiply one by the other to get the focal length. Mind you the aperture was often marked in systems other than f/stops.

Or, if you had the lens in hand, you could just measure from the aperture to where the lens focuses an image at infinity...

[fecaleagle]

You'll have to ask the seller (usually un-knowlegeable if they didn't give a focal length in the first place) to estimate it for you. Tell them to point the lens toward a window in a dark room and measure the distance from the middle of the lens to the wall when an in-focus image is projected.

Other than that, the diameter of the glass will only help you determine the speed of the lens, given the focal lenth, IF you know the lens design.

In my experience, very large diameters (over 3.5-4") are 10" FL or longer for petzvals, but I don't believe there are any surefire methods of divining this information without asking the seller to make an estimate for you.

-William

[Mark Sawyer]

Other than that, the diameter of the glass will only help you determine the speed of the lens, given the focal lenth, IF you know the lens design.

f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.

[fecaleagle]

f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.

Deferring to Mark. My bad...

[Leigh]

f/stop = Aperture/Focal Length, so if you know the aperture and f/stop, you can extrapolate: FL = Aperture x f/stop. For all designs.

However...

Focal length is the distance from the rear node to the film plane.

Unless you know the lens design, you don't know where the rear node is relative to the elements or surfaces.

Also, the "aperture" diameter used in the quoted calculation is the diameter of the entrance pupil, which may
differ substantially from the measured diameter of the diaphragm blade opening in some lens designs

- Leigh

[Mark Sawyer]

However...

Focal length is the distance from the rear node to the film plane.

Unless you know the lens design, you don't know where the rear node is relative to the elements or surfaces.

Oh no, nodal points! I must confess, I'm getting to the age where "close enough is good enough", and, (barring telephoto and retro-focus lenes), the aperture location is close enough to go by. So while Leigh is quite right, I've never heard of a photographer who knew exactly where his nodal points were for a particular lens, much less use them. (And just try explaining nodal points to a seller who doesn't even know focal lengths!)

Also, the "aperture" diameter used in the quoted calculation is the diameter of the entrance pupil, which may differ substantially from the measured diameter of the diaphragm blade opening in some lens designs

The aperture would be the diameter of the opening of the lens measured through the entrance pupil (front element). Which designs does this not hold true for?

[Leigh]

I've never heard of a photographer who knew exactly where his nodal points were for a particular lens, much less use them.

Absolutely correct*.

That's why lens data sheets show the Flange Focal Distance, which is from the lensboard to the film regardless of node location.

With some telephoto designs, the second (rear) node is in front of the front element by many centimeters.

The aperture would be the diameter of the opening of the lens measured through the entrance pupil (front element). Which designs does this not hold true for?

Correct again, for all lens designs.

I was clarifying that the dimension used was not the physical diameter of the diaphragm blades at an f-stop setting.

You can measure the aperture physical diameter with a pair of calipers, but that's not the value needed for the calculation.

- Leigh

*NB: One exception is panoramic photography using multiple images.
For this technique the vertical axis of rotation of the camera must pass through the front (first) node.

[Mark Sawyer]

I was clarifying that the dimension used was not the physical diameter of the diaphragm blades at an f-stop setting...

Actually, you could if you used that f/stop value in the equation. Say you have a lens closed down to f/16, and you measure the opening through the front element to be a half-inch. 16 x 0.5" = an 8 inch lens.

[Leigh]

...and you measure the opening through the front element... [emphasis mine - Leigh]

The apparent diameter measured through the front element is the diameter of the "entrance pupil".

This can be very different from the physical diameter of the aperture blade opening in the shutter with no lens elements mounted.

This is the distinction I was trying to make.

The formula is: N = f / D
where N is the f-number, f is the focal length, and D is the diameter of the entrance pupil.

The entrance pupil diameter required for a given f-stop is: D = f / N.

- Leigh