Newbie Question

Hello Hello A-Marters!

This is a very rudimentary question...that I can't answer. ;-).
How do you find the focal length of a lens? I found a piece of surplus material at a optical shop that's selling very cheap. Only problem is, the owner doesn't know what the focal length is. Any help please?

-Adam Back

Well, there are a couple of possibilities:

1) Inspect the lens cell (if there is one) closely; most are marked with the aperture and the focal length. If the lens is unmounted, on rare occasions you can find that info printed on the side.

2) Take the lens outside during the day, and hold it up by the wall of your house. Point it at the Sun (DON'T LOOK THROUGH IT). Walk back and forth until the light passing through the lens focuses at a point on the wall. Measure the distance, and that should roughly give the focal length.

BTW, surplus optics need to be treated with caution. Only properly-matched doublets or triplets are suitable for astronomic use. There's lots of old military glass out there which was designed for photography, and it's not really suitable for telescopes.


Jeff