I have a DF fresnel. I cannot split them and dont want to

i need to know what the focal length of each side is

how do i do this?

If it's unsplit it should have a sticker that says 'up'. The up side is the 330mm FL.

yeah, the thing is, this isnt the Fresnel from lumenlab. its one i ripped out of an overhead projector.

I already know which side is the condenser and which side is the field lens

im just trying to figure out how far to put it from the bulb


could i put a piece of paper where the lens would be and pull the paper back until the light focused into a small dot? im guessing that would give me the focal length of the field side.(thats just a guess )

how would i judge the focal length of the condeser? can i stare through the lense, and move it until the bulb is in focus?

If you still have the overhead projector you can simply measure the distance to the lamp. If you don't you will have to set up a point light source to check it. It is most likely the standard OHP fresnel which has two identical lenses with a focal length of around 325mm.

I still have the projector. I bought it 5 years ago
here it is
Overhead 3M1706

the frez is 10.5" by 10.5" not including the border

the lamp head is adjustable up and down for focus

should i move it all the way up? or all the way down?

just realized you said the distance to the lamp not the lens

the lamp is set up to reflect off a mirror at a 90 degree angle

Just measure the distance from the lamp in the OHP to the stage. That will tell you the condenser's FL.

Measure from lamp to mirror and mirror to stage. Add together.