finding out the proper lens

[stan kern]

I have a friend who has a 40 watt desktop laser and he would like to re-place mirrors and lens, mirrors are easy just measure
them but what about the lens, to get the right focal length
Can anyone suggest what to do?
Thanks

[Gary Hair]

You get what you need, no such thing as the "right focal length" - unless the laser is fixed focus and no z movement possible. If you want to guesstimate the focal length of the existing lens do a trick that Kev posted a long time ago - hold the lens over some text and measure the height above the text when it's in focus looking through the lens. This won't be perfect but will get you a close enough measurement to know what it is - 1.5", 2", 2.5", 3", 4", are typical.

[stan kern]

thanks a lot .will do