Someone had posted a formula for changing the focal length of a fresnel by stacking
them on top of each other. I can't find it anywhere. Can someone please post it again?
I've got 2 12.5" fl fresnels that I can't figure the combined focal length on. I think it's going
to be 6 1/8" but that's a guess. Thanks.

http://www.lumenlab.com/forums/index.php?s...ndpost&p=108270

Thanks man. I was close. It comes out to 6.25"

Without space i.e. stacked touching you can use 1/FL1+ 1/FL2 = 1/FLeffective