Hi there,

Im using the focalcalc to try and get dimensions that will allow my entire da-lite screen to be filled. The only way I seem to be bale to do it is to use a larger screen or increase my fresnel gap to 70mm. Since Ill be using LEDs for my lighting whats the maximum Fresnel gap between the LCD and front Fresnel ? What happens to the image as you increase that gap, and when is it too much ?

Thanks
C.

Light will diffuse as it travels. It is optimum for the rays to be parallel (more or less) when going through the LCD as this is the type of light they are designed for. It is also optimum for them to be parallel when hitting F2 as this is what the fresnel is designed for and will allow the light to focus on the correct point (the triplet). The further away F2 is from the F1 (or in this case the LEDs) the more chance the light has to diffuse and thus change the angle. This will result in a less sharp picutre (i beleive but may be wrong...).

However, there is some rule of thumb here as if f2 is too close to the LCD then you will get fresnel rings in focus and therefore they will show up on the projected image. In my opinion 70mm is too large a gap, but I have no experience of using LEDs so it may be ok. Also remember that ideally F2 should be slightly less than its FL away from the triplet, so it is focusing it at the correct point.

The best way to increase you projected image is to change your triplet for one with a different FL. However the focal length of the triplet should be around that of F2, so it may well involve changing both.

Matt

You can fudge a bit if you can move your lamp / lcd/ fresnels etc. I once had a gap of over 4" in my PJ.