Hello,
Building 1st small PJ using PSOne lcd and using old 3M OHP with FXL lamp source. Purchased varifocal triplet lens from LL.
Thought I read somewhere on this site about using (2) regular office supply fresnels for a decent light spread.
How can I tell what the FL would be for an unsplit design when I don't have a spec. sheet for the fresnels?
Should the (2) unsplit fresnels be up against the (lamp side) lcd or maybe a gap for air flow?

I'll use the split design & better grade fresnels on 15" lcd project later-on. (for that project I'll want keystone correction).

Thanks!

The focal length is (according to vonneuton) 317mm (for office supply fresnels), but you can measure them by shining a light through them and measuring the distance to the floor when there is the smallest point of light.

No, not right up against the LCD - that will cause moire patterns (strange concentric circles on your projection). According to the free pdf from LL, it's about 15mm away from the LCD and the focal length away from the bulb.

Thanks Again!

There is definetly a "side-by-side" difference from the office supply fresnels vs. the 3M OHP fresnels. The OHP has a much better sharpness & clarity. I'm going to try to salvage(cut) the OHP fresnels.

If I were to use the Office Supply fresnels: Since both fresnels have the same Fl. and using them in an unsplit design, Would the distance from lamp to the 1st fresnel be approx the same as 2nd fresnel to lens?

Thanks!

No, the lens will be closer than the bulb. the center of the bulb should be placed at the focal length of the rear fresnel. The triplet will be placed closer than the focal length of the front fresnel so the light 'cone' fills the triplet. If you know the focal length of the fresnel (go with 317mm, but I would still test it.. more below), you can use Focal Calc 2 to determine the distance from LCD to triplet, and just place the field fresnel ~15mm further back.

To test the focal length of the fresnel, you need to go outside on a sunny day or stand below a bright light. Using a ruler, position the fresnel ~317mm from the ground. Move it up and down until you get a 'focused' dot on the ground. The distance from fresnel to ground will be the approx focal length of the fresnel.