I just saw the results from this guy: http://www.allinbox.com/aktarus/aktarus.htm

How is he getting such great lens shift and no pincushion on widescreen movies? So he must be shifting the top fresnel up but how much?

Well, Im not optics expert, butm it appears that you would need an oversized fresnel, and that it is shifted up 50% of the total height. The LCD would need to be shifted up as well.

Here is the image of his diagram



From just looking at that that image, I deduce that the fresnels are significantly oversized, and in fact would need to be 2x the height, since the entire LCD height needs to be in 1/2 the fresnel. This would make a 15" projector need about 24" tall fresnel

Also, you would waste ALOT of collected light.

yeah but he is using split so he can use 12inch high fresnels? he is just shifting up the forward fresnel i assume. he doesnt seem to have light issues.

I have attached an edited image of the one above. The yellow lines represent the other 1/2 of the front fresnel that he ISNT USING.

You can see that the LCD height in his drawing is 1/2 of the fresnel height. Since that is the case an ~24" tall fresnel would be needed. ( I belive a 15" LCD is ~12" tall)

Also at the bottom, where I drew the black squiggly. You can see that alot of collected light is wasted. In his box, you can see the peice of wood that he has blocking this portion.

I've done lens shifting using a similar setup on a 15" screen, although I had a fairly noticeable "bow" at the top of the screen, but not noticeable once I got my screen boarder up and dark wall color.

It appears to say (I don't read french very well) that he's shifting 2/3, but I think he's probably shifting 1/3 of the LCD height.

A 15" LCD is 12x9". Therefore if you were to shift the front fresnel up by 1/3 of 9", that would be 3"... you would need 9" + 3"*2 = 15" (381mm)... A 3dlens 330fl fresnel comes in 15.5x15.5" (which provides and additional 0.5" for mounting, etc.), which is probably similar to what he is using.

I wouldn't say he's "wasting" collected light, but if he's actually shifted his LCD up without moving the lamp or collector position, he certainly won't be in the "sweet spot" of the light being collected, which would make for a dimmer projection... but I guess that is a trade-off when trying to keep the bow at the top of the image from becoming too large.

FWIW, I tried 80mm (3.15") of lens shift by moving only the field fresnel and triplet, but the bow was too great... I settled for about 45mm (1.77") with an "acceptable" bow at the top of the image.

cheers,
gs

I did some experiments moving the fresnel lenses. I found that you could just move the bulb up and the triplet down (or bulb down and triplet up) I get a great picture. Just keep them on an axis that goes through the center of the fresnels and l LCD. The triplet and LCD size are the limiting factors for my setup. I would think that if you used a pro lens with a small monitor you could get lots of lens shift.

how much did th 45mm lens shift move the projection up? if the projection is magnified 5x(according to dazz focalcalc) does that mean the 45mm would translate into a 225mm upwards movement of the projection?

I don't remember the exact numbers... i'll have to dig through a pile of papers to find my notes when I was doing this... I made a post in the lens shift thread here showing my setup... I believe I was going to use 45mm, but ended up using only 40mm, with about 6° of keystone correction... I failed to mention that I used both lens shift and keystone correction to get the image where I wanted it without getting a large bow at the top of the image... (sorry about neglecting to say that in my post above)

gs