Shortening collector focal length

[Gizmotech]

Could some one please explain to me why the focal length before the condensor is reduced and not after the collector in a unsplit fresnel configuration. You would think it should affect either side of the fresnel resulting in a shorter focal length on the collector side as well. How could Focal length after the collector fresnel be shortened apart from going for a shorter focal length Collector fresnel or by folding the optic patch?

[Rox]

i don´t know what are you asking at all .

when you have rear fresnell and front frsenell, you can consider like if you had 1 fresnell. The focal of this new fresnell deppends on the other two focals.

lets say you have 220/330 dual fresnell from a ÖHP, then it is lke if you had 132mm focal 1 frresnell.

you can place a bulb at diferent distances from this new equivalent lens, each distance will focus the light at each associated distance at other side.

allways is the same formulae; 1/F=1/d1 + 1/d2

d1, d2 distances and F the focal of the lens.

[Gizmotech]

So does that mean this theory is incorrect??

http://s6.invisionfree.com/diyprojec...p?showtopic=35

There it says that only before the unsplit fresnel lens it will shorten but not after the unsplit fresnel lens. Any comments???

[Rox]

sorry link does not work