Just have a question. I know the distances i need to put the triplet from the lcd but would like to know why it is this distance.
i am using a split design and a 17" monitor so i have to put the triplet 572mm from the field frensel.
i don't really understand this as it would only put the triplet 552mm from the front frensel that has a focal length of 650mm. won't this mean the frensel focuses the image well past the centre of the triplet? and why doesn't having the triplet 650mm away work?

thanks

If you put an object AT the triplet focal length it focuses on infinity. So, the closer you want the image to focus the closer you move the object.

For a split configuration the front fresnel magnifies the LCD.

Think of the fresnels more as "focusing" or redirecting the light into the triplet and not "focusing" the LCD.

The fresnels and lamp are the light engine. The LCD and triplet are the projection engine. Separate but working in harmony.