one or two Fresnels?

[wok]

I have seen various designs posted on the net - some with 2 fresnels sandwiching the lcd and then some with only one fresnel between the light source and the lcd.
Does anyone have a decent explanation or opinion on which is better?

What is the relationship between the focal length of the fresnel and the projection lense at the front of the projector? do they have to be the same?

Also, i have seen on an auction site an oHP for sale with a glass fresnel, and a variable focus 3 element lense on it. Would it be preferable to use the fresnel and lense out of the same ohp - i am assuming they would be in the same focal range.

I am a newbie so any explanations would be greatly appreciated!!!

Thanks everyone!!!!!!

 

[ScottK]

The projection lens focusses on the LCD.

The field frensel focusses just beyond the projection lens, such that the beam is entirely captured by the lens.

 

[ELiTeHaYDeN]

so this means you can use a lens that would normally only work with a 12" LCD (without front fresnall) with a 15" with the front fresnall? is this correct?

 

[ELiTeHaYDeN]

from what I have read and seen this is how it should be (if and only if, your projection lens is too small or doesnt have the correct focal points, OR you want shorter focal points(so to speak) etc), however im by no means an expert...

 

[The_Mazta]

Hey, theres loads of different ways, none are right none are wrong, when i was builind mine i tried them in all areas, seemed the same to me, i used in the end, the 2 frez's together back to back,

check out my results

 

[ScottK]

So do fresnels together give a clearer picture?

Do the fresnels have to be larger than the LCD?

What causes the picture to lose focus away from the centre?
(Would a larger projection lens help?)

 

[The_Mazta]

mybe if your Frez's are not straight, or they've warped, i was told by ACE the other day, best thing to do is put glass either side of them, just thin 2-3mm glass and sandwhich them in, that way they cannot warp,