What's the focal length of that lens???

[Ole]

I found an elegant way of measuring the focal length of any lens, including tele constructions:

In an otherwise unlit room I taped a sheet of white paper to the wall opposite of the window. I drew one vertical line on the paper. I then focused the image of the window on the paper, lined up one edge with the line, and drew a line down the other edge.

Having a few lenses of known length helps a lot, of course - any lens will do! 35mm, MF, LF, tele or retrofocus, close-up "filters" (+1 is 1m, +2 is 1/2m and so on); anything with known focal length.

Plot your known focal length vs. width of window image, connect with a line. You can then read off the approximate focal length of the unknown lenses.

Since walls and windows tend to stay put this is repeatable - until you move to a different house.

Half an hour's fumbling about with lenses and I had a neat list of 10 previously unknowns!

[JosBurke]

Are you referring to focal length or image circle?

[Ole]

It measures focal length by way of projection of a distant object of fixed size. It is possible to calculate the focal length just from the (known) size of the "distant object", the distance from the "film plane" (wall) to object (window) and the size of the projected image, but a table or even curve is much easier.

Note that the rest of my hallway is dark, so I see only the image of the window. I still don't know anything about the image circle!