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

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

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.

2. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

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...

3. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

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

4. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

Originally Posted by fecaleagle
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.

5. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

Originally Posted by Mark Sawyer
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.

6. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

as a questimate... if you take a half inch off the barrel diameter it gives a ball park for aperture.
Multiply x8 for a rapid rectilinear and x4 for a petzval
then wait for the surprise of finding out what it really is after it arrives

7. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

For Petzval lenses, here is a very rough guide using the lens' diameter:

1.50-2.00 inches = 1/4 plate which typically is a 6-7 inch focal length lens
2.00-2.75 inches = 1/2 plate which typically is a 8-9 inch focal length lens
3.00-3.75 inches = whole plate lens which typically is a 11-12 inch focal length lens

Dan

8. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

Nice. Very helpful everyone. Def cleared up some misconceptions and reaffirmed some general observations. THANKS!
I recently learned the trick with the window and measuring from a sharp image with a 48" lens.

9. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

Originally Posted by Mark Sawyer
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

10. ## Re: How to determine focal length of brass lenses from length and diameter measuremen

Originally Posted by Leigh
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!)

Originally Posted by Leigh
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?