I have an elmo hp-L3550s overhead projector. The lens is around 3.5" (appr. 85 mm, but not sure on exact measurement). I was wondering if I would be able to swap out that lens with a triplet lens or not. I really don't know anything about focal lengths in regards to the fresnel lens or anything. Thanks everybody. This is my first forum post by the way.

Why do you want to do this?

on the old DIY projector site they talked about making overheads better by changing the light source as well as changing out the lens. For a $30 lens, if it makes a difference I thought it might be worth it. Thanks
Heres that page - http://www.diyprojectorcompany.com/other/a...hp_retrofit.htm

oh I completely missed where you said it was an OHP.. I was thinking small commercial projector. You can definitely swap it out for a triplet lens, but it depends on how close you can get the focusing assembly to the fresnels, and the focal length of your field fresnel. any idea on these two numbers?

The distance from the top of the fresnel to the bottom of the lens curvature can be between 13" and 21.5" approximately. I actually rigged the assembly to be able to go a little lower for a larger screen sized based on my throw distance. I measured the assembly range as accurately as I could, but I may be off by 1/4" or so for the 13" measurement. The focal length is something that I'm not sure about. In the middle of this reply I called ELMO and asked about it and the tech guy said that my lens was "triple coated" and in his opinion it wouldn't make much difference compared to a triplet. Any thoughts on that? Thanks.

well thats a load of hog wash.. it won't make a difference if you're projecting diagrams for a 3rd grade english class, but it will when projecting higly detailed images from an LCD

The range of motion on your lens assembly is adequate to be used with the standard 320mm LL triplet. For image sizes of 80" to 150" diagonal using 15" LCD, you need a range of ~14" to ~ 15".

You will need to use a 317 or 330mm field fresnel with this lens. might require you to remove the OHP fresnels, or at least cut them apart if they're unsplit and and ditch the top fresnel for a 317/330.

There is only a single fresnel, unless its 2 made into one or something, but I doubt it. I know very little about all of this unfortunately. is there a way to determine the focal length?

I'm assuming its two fresnels stuck together. This is typically the case with OHPs. You can split it apart with an exacto knife or cut a small amount off around the edge of both to free them.

to check the focal length, remove the projection lens assembly from the OHP so it doesn't get in the way. Take a piece of cardboard and hold it above the fresnels. move it up and down until you see a focused dot on the cardboard. The distance from fresnel to cardboard will be the fresnel's FL.

Yes i have used the Lumenlab Triplet inplace of the lens in an Apollo OHP and used a QA-2500 lcd panel (1024x768) with Windows Media Center.