How to determine focal length of unmarked lens?

[Account8600]

For an old unmarked lens, how does one determine its focal length? Can it be done without putting it in a camera?

[vinny]

Yes. Hold it up near a wall and find the spot where the image from a window comes into focus. Measure that, you'll be close. I just made that up but I think it works.

[Alan Gales]

I could pretend that I'm smart here but being honest you got me curious so I looked it up on the internet. Vinny is correct. What I found is that for a quick estimate you can focus the lens onto the floor from an overhead light source and measure the distance between the lens and the floor. A window and a wall should work just as well unless of course it's dark outside.

Remember that it's an estimate.

[Leigh]

The distance from the back of the shutter* to the image when focused at infinity** gives you
the "flange focal distance" or "flange focal length" of the lens.

Depending on the design of the lens, that may or may not be close to the optical focal length.

The optical focal length is the distance from the rear node of the lens to the image at infinity focus.
Since you have no idea where the rear node is, you have no way of determining the true FL.***

- Leigh

*NB: If the lens has no shutter, you would measure the distance from the diaphragm to the image.

**Def: Infinity >= 100 times the (assumed) optical focal length of the lens. Farther is better.

***Note: If you really want to measure the optical focal length, and have the time and equipment...

1) Mount the lens on a rail so it can move. Mount a flat piece of white paper (the "film") behind it.
2) Focus an image at infinity on the paper. Mark the lens position as the "zero" reference.
3) Put a flat object slightly shorter than the height of the paper in front of the lens.
4) Repetitively move the object and the lens until you have a focused image exactly the size of the object.
5) The distance the lens has moved from its "zero" point equals its optical focal length.

The accuracy of your result depends on the accuracy of your measurements, which should be 1mm or less.
Use a good magnifier to evaluate image focus.

[Alan Gales]

I agree with you, Leigh. Without knowing the rear node point you can't get an accurate focal length. What I read on the internet it said it would give you an estimate. They suggested measuring from the front of the lens to the image on the floor. Of course the actual distance of the node point to the front of the lens varies with different lenses.

I'm wondering how accurate the estimate could be?

[Leigh]

Hi Alan,

Please review my previous post. I was editing it while you were composing yours.
I added an accurate method for determining the optical focal length.

Of course the actual distance of the node point to the front of the lens varies with different lenses.
I'm wondering how accurate the estimate could be?

The position of the rear node is all over the map, from behind the rear element on short FL lenses to
hundreds of millimeters in front of the front element on telephoto designs.

You really must know both the design of the lens and its intended area of coverage to determine the rear node.
For example, a 200mm lens for a 35mm camera is very different from the same FL for 4x5.

But if you knew that, you would already know its focal length.

- Leigh

[Alan Gales]

Thanks, Leigh. What you posted makes a lot of sense to me.

[Emmanuel BIGLER]

Hello from France !

The following diagram is a ray tracing corresponding to Leigh's method
(sorry, the text is in French ; I hope that the ray tracing is self explanatory)
http://www.galerie-photo.com/images/...05-08_h013.png

I have, on purpose, chosen an "exotic" case where the principal points (or nodal points) H (=N) and H' (=N') are "crossed" i.e. a situation where H' is ahead of H. This sometimes occur, but has no influence on the proposed method.
The method is very interesting in the sense that you do not have to care for the position of the nodal (or principal) points.

Once an approximate value for the focal length is found, you can refine the measurement by comparison of the image size delivered by another lens of known focal length, for a far-distant pair of objects. But for this you need to mount both lenses on something solid .. like a camera on a tripod

[Mark Sawyer]

Seriously, just point the damned lens at something damned far away, and hold a damned piece of cardboard or something behind it and see where it damned well focuses, damn it. Measure from the damned cardboard to the damned aperture control, or where the damned aperture control would be if there damned well wuz one.

Geez, we're not large format lawyers...

[Leigh]

Geez, we're not large format lawyers...

Well, Mark...

Using your technique on a Nikkor T 12/800mm yields a measurement of 527.4mm (if your results are accurate).

The error is 272.6mm.

That's a damned big error.

- Leigh