What is the difference between using a simple single OHP projection lens vs triplet lens?
What are the pros and cons?

Thanks.
--dr_gsb

Most OHP lenses will be triplets, but some are doublets as well, and it's certainbly possible for a single lens.

The multi-element lenses correct for some of the undesirable properties of a single lens. Single lenses will deflect light of different colours unequally, and will have a slight distortion in the overall shape of the projected image. By using more lens elements, we can extend the capabilities of the lens (though when things start to drop off, they will do so more rapidly.)

Thanks.

I've seen that when I fix the distance between the fresnel and the single projection lens (say F ft), I get the image at a fixed distance (from the projection lens) (say D ft)and a fixed magnification (say M ft).
If I change the distance between the frenel and the projection lens (say F' ft), the image is formed at a new distance (D' ft) with a new magnification (M' ft).
There seems to be some relation between the image distance (D) and the magnification (M).
I algebraic terms:
D = f(F) and M = p(D)
where f and p are some functions.

How can I control the two things separately?
I mean I want to keep my pj at a very large distance, but don't want a very big image. Something like that.

Can a triplet lens help me achieve this?

Thanks in adv
--gsb

Then what you want is a long throw.

That means that you need to have a triplet (or whatever lens) with a very long FL, or you need a small LCD... Or both.

The larger your LCD, the shorter the throw. The shorter the FL of your lens, the shorter the throw.

Generally, the LCD size is less negotiable than the triplet size, since the LCD tends to be the single most expensive part of a DIY projector. Not always, since there are people building with cheap 7" DVD player screens, or even cheaper PSOne, or other video game screens, however most of these are limited in resolution, and are not recommended for larger screens. (50" or so for a typical PSOne LCD.)

So, if you have a certin LCD that you really want to use, you find a lens set that will get you what you want with it. If you have a lens that you really want to use, then you find an LCD that works. With a given triplet and LCD though, your throw will be what it is with those components.

Thanks. But my question here was whether I can control the image distance and its magnification separately.
E.g. Can I increase the distance without increasing the magnification?
AND
Can I decrease the distance without decreasing the magnification?
Something like a zoom lens maybe.

Does triplet lens give me that flexibility?
Or can some other combination of lenses be made to achieve this?

Thanks in adv.
--gsb