Rear Projection

OK, I don't know why I didn't think about this earlier. But in terms of correcting the throw problem we have with a 330mm projector lens, has anyone looked into using a short projector lens, say 240mm with a rear projection system?

My point is this: If you have a room that is say 27 feet long and you want to project an image that works out to say 80 inches, putting the projector at the other end of the room from the screen would require a very long projector lens. But using something shorter on rear projection would give you the results you want, size wise and still get the projector out of the way. ( I know somebody on this site had asked me about this problem. I just forget who.)

The reason I picked 240mm is because the rear fresnel is 220mm and is easily possible to reverse the fresnel lenses in the box to make the 220 your collector. Also, I know SurplusShed sells a 240mm copy lens that might work for this purpose. (You could also just buy two 220mm fresnels and shorten the overall length of your box by 4 inches.)

Using the LumenLab focal length calculator that means you could project an 80" image from ~60 inches.

It just seems to me that this is a fantastic and cheap solution with some interesting advantages besides the throw. The only thing I don't know is the screen material needed. I know Dazian sells rear screen projection material, but I haven't priced it.

Thanks,
Zendance

You could even reduce the overall distance you need for the throw.

So at 60" throw (80" screen), you really need 60" + 28" (length of box) = 84" of room behind the screen.

But, lets say you stand the projector up vertically so it projects towards the ceiling. Then you put a large mirror (say 18") at a 45 degree angle suspended above the projector. The mirror reflects the light from the projector onto the rear screen.

The 240mm projection lens will project a 16.78" image at 20" from the lens. That reduces the amount of room you need by 20". Since the box is now vertical, the amount of room you need is only 6" instead of 28".

So, 84 - 20 - 22 = 42 inches of room behind the screen to project an 80" image.

Thanks,
Zendance

Zen, funny you should bring this up because that's what I'm doing with the 7" projector. Slightly different system but a RPTV all the same.