LONG THROW Vs Keystone Correction

The way I understand it...

Using a standard 80mm triplet will give you an image that is to large to put the projector behind the view without the viewer being right up on the screen. A long throw lens will allow the projector to sit much further back for the same image size. However it seems like folks are having problems using say a 135 mm long throw triplet with 15" or greater sized LCDs.

Instead of trying to use a long throw lens... why not just use the 80mm lens, and sit the projector on the floor/ceiling, closer to the screen, tilt it, and use the front fresnel to adjust for keystone?


2005-03-14 12:47 am
You're putting the fresnel lens in the image path, they are usually intended for concentrating a light source rather than sitting between the image source and projection optics. You will see some amount of blurring.

However: this might not bother you. In fact, some people intentionally defocus their projectors a tiny amount, because they find the sharp edges of pixels (screendoor effect) can be distracting. My preference is sharp pixel edges, especially for computer use rather than video playback.