I was wondering how far the front fresnel can be from the LCD, and what are the results of it being too far? I know that if it's too close to the LCD you will see the grooves. In my current design, my front fresnel is about an inch from the LCD so I have room for keystoning in both directions.

Hi tmproff,
The distance from the lcd to the collector fresnel is 20mm in a straight shot projector without keystoning. This distance will give you the best focus. However, for keystoning you need to allow room for the fresnel to tilt to be able to achieve the desired angle of correction. The only way to do this with great focus is to be able to move the fresnel forward and back. you need to be able to do this because you need the edge of the fresnel that leans tward the lcd to reach the 20mm mark. The fresnel, along with it's ability to focus the light tward the triplet will also magnify the image. The further away the fresnel is, the bigger the image is. This magnification with the fresnel at an angle gives us the keystone correction. If you simply place the fresnel at a distance from the lcd to allow room for it to swing to every angle and the trailing edge doesn't make the 20mm mark you will not have even focus in your projection on both the top and bottom of your image unless you tilt the projector to that one spot were the trailing edge hits 20mm. with everything I have tried I have found no way to auto correct for keystoning. While it is great to allow the fresnel to use gravity to be able to hang parallel to your projection screen, you will still need to move the lens forward or back to get it in focus.

tmproff,

This might help you out. This was my solution for keystone correction. It was inspired by Moose, and I'm very happy with the results. The distance of that last fresnel is very "forgiving" , just be sure you can move the triplet to adust for focus.

http://www.lumenlab.com/forums/index.php?s...opic=4631&st=60

dobber