How do i get a longer throw from an ohp?


I am in the process of perfecting my first ohp monitor project.
I have a samsung 151s and a 3m 9700 ohp.
I planned on mounting the projector on my wall and project the image across my small bedroom to the wall on the opposite side.

On my test i noticed that the image would be far too big from that distance with the factory triplet on the ohp.

My question is:

Is there a way i can adjust the screen size so that i can mount the projector across the room with the desired size.

Or is there triplets for different throw distances.

My desired throw would be 13'
with a maximum screen size of 5' x 5'

Any ideas would be greatly appreciated.



2004-12-30 5:35 am
The samsung 151s is a 15" right? Going by that with the above mentioned lens with a focal of 450mm your image would be 7.8' by 5.9'

In order to get an image which fit within your 5' by 5' limitation with a throw of 13' you would need a lens with a focal of 661mm (~26") or greater (661mm focal would give you a 4.99' by 3.7' image)

The longest focal triplet that I've seen was an old brass triplet pulled from a vintage opaque projector with a focal of 22".

You might be able to find a duplet, or possibly make your own, which would satisfy that focal, but I'm not sure where, hopefully some others members could help you out there.

Essentially that 5' by 5' limit is really hurting your options. I can't imagine a room with only a 5' width or height. Why the 5' by 5' limit?
DIY duplet

I think you have two choices:

You may be able to find a telescope objective lens that has a focal length of 700 mm. I saw one on eBay months ago, that went for <$30 US.

Or you could get two +0.75 Diopter uncut eyeglass lenses (as large diameter as you can find). Put them in a tube so the concave curves are both facing each other, about 20 mm apart.

Neither one of these solutions will be as good as a triplet, but either one will give you an image good enough for watching video in your 5' by 5' screen area.
i have given up on the long throw for now. I am having trouble with other issues now. I will put off the long throw for now.

By the way i live in a historical apartment that has small walls that are broken up with windows and doorways. And that is the largest wall i have.

But thank you very much for the help. I really appreciate that.
no large wall for screen

Lots of people have that problem, so they use a screen. You can make a lightweight wooden frame and stretch blackout cloth over it, or you can get a long pull-down window shade to mount up near the ceiling. Either of these will fit in any budget.

Or for $0.00: You iron a white sheet, and then hang it on the wall. Just don't watch porn on it, since it will show through the window! I recall a fraternity getting busted by the campus police for using a sheet hung over the front window for a stag party: Quite a crowd gathered out in the street. :eek: