# 7" projector measurments and bulbs

#### mindless_fool

##### Member
anyone have measurements between the frensels lenes and the lcd and from each frensel to the bulb and lens..im not sure what distance i should be putting my stuff

also what is the best bulb system to go with? 250W metal halide double ended? whats the cheapest?

thanks for any info guys

#### Megaman

##### Member
The distances all depends on the focal lenghts of your lenses and the distance to you screen (throw).

People usually put the bulb at the focal length of the first fresnel. (A little closer when using a condesor lens).
The distance between the lcd and the projection lens is calculated using 1/FL = 1/Dist + 1/Throw where FL is the focal length of the projection lens and Dist is the distance between projection lens and lcd.
The second fresnel should now be placed its focal length before the projection lens. (Be it before or after the lcd, but after the first fresnel).
If both fresnels are placed before the lcd you can have them glued together.

You would want a few cm between the lcd and the fresnels in order to not see the fresnel rings in the final image.