Question About lens size

[Smoke Eater]

I have a question concerning the size of lens and its focal length in regard to projecting a 5" lcd screen. If I have a 3.5" in diameter plano-convex lens with a focal length of 12 inches, will I be able to capture all of the 5" screen and project it? The reason I'm asking, I'm experimenting with a simple 150w spotlight, fresnels, and this plano-convex lens and I'm using a transparency picture (I printed it on a transparency sheet with my inkjet printer, a good cheap subsitute for experimenting with instead of an lcd) of approximately the same size as an 5 inch lcd screen. However, it seems to only project a "circle" of the center of the picture and not the whole thing. I'm thinking this is just because of the low-wattage, round spot light bulb and will be cleared up when I go with a larger light and proper reflector, but I would like to know if I need to find a larger in diameter lens or a different focal length to "capture" the whole image?

 

[tech head]

Projection

The problem is probably with the light. The general rule is that the focal length of the lens must be equal to or greater than the diagnal of the object being projected.
since your lens had a 12" focal length you should have no problem projecting the whole image.

 

[Smoke Eater]

Thanks Tech head, I knew that OHP's use a similar diameter lens and that worked ok, but I wasn't sure if the focal length might be causing me problems. But with your general rule of thumb, I should be ok and now it's on to the light and reflector