why do the two fresnels have different focal lengths? really i guess i'm asking if there is a significant disadvantage to using two fesnels of the same focal length. i think i'm going to try to build a projector and i'd like to use some FS mirrors to bend the light path to keep the enclosure as small as possible and having two lenses with a 317mm focal length will actually fit better.

You can use two 317mm fresnels, although the projection may not be as bright. Three reasons I can think of that people use a 220mm rear fresnel: 1) You can design a somewhat smaller projector. 2) You get a brighter image. 3) Sometimes hobbyists use parts salvaged from overhead projectors. Many older overhead projecotrs used a 220mm fresnel and a 330mm fresnel. Hope this info helps.

the reason for using a 220mm collimating fresnel is because it uses a greater cross section of light from the bulb. see pic bellow.. it's highly exaggerated but you can clearly see why a shorter FL fresnel will give you a brighter projection by using more light from the bulb