I was hoping to mount my LLPJ on the ceiling but there is a ceiling fan currently in the way. I do not want to remove the fan as we use it frequently.

Q: Is is possible/practical to design a box with a longer throw to the screen so I can move the PJ so it is behind the fan, farther from the screen?
maybe by using fresnels wih different focal lengths etc.
maybe by using the pro tripllet lens with a focal length of 500mm edit: I just saw the price of this thing....trying to keep costs down, the diff between a $30 and a $200 triplet is prohibitive.

Q: Will this decrease the image quality or brightness?

Q: Has anyone done this?

Instead of the Pro lens you can use an 18" Beseler lens off of Ebay. You should be able to get one for about $60. Then just buy a 550mm fresnel from 3d lens.com and you will have a much longer throw.

Or consider either a floor coffee table mount or possibly a vertical configuration. Or, even nicer, a vertical mount in a nice piece of furniture (end table between chairs) with a retractable mirror under the lid?

longer throw will always decrease brightness if all other factors remain constant (bulb, lcd, ambient light, etc). this is a fact of physics and cannot be avoided. however, 95% of the PJs here arn't bright enough to be used with ambient light anyway so this isn't a big deal

The only ways to increase throw while keeping screen size are to move to a longer FL triplet, or changing the size of the LCD. i.e. 17" LCD will produce a larger image from the same throw as a 15" LCD when all other components are similar.

18" lens with 550mm collimating fresnel lenses is a good combo for longer throw without having to sell a kidney for the pro triplet.

I suggest you use FocalCalc2, found in the calibration and tools forum to mess around with distances and find a combo to give you the appropriate screen size from required throw distance. remember you're going to want to use a split fresnel setup since you want to mount the PJ on the ceiling so you can keystone the image. otherwise the projection will look like a trapezoid.

which fresnel do you use lens side of the LCD with a 18" Triplet? 220mm?

18" = 457mm, so I would say use the 550 fresnel... depending on your setup, the LL 650 (measured more closely to 600mmFL) might be a better choice, but I think most with the 18" lens use the 550 from 3dlens.

cheers,
gs

For the colminating and the fresnel on the lens side ? both 550mm?

I say 550mm on the lense side, 220mm on the bulb side.

thanks.

you specifically asked about the "lens side" which I took to mean the field fresnel (the one closest to the triplet)... this should be a 550mm. (... or the 650 depending on your setup)

As for the collimating lens (the one closest to the lamp), the standard is 220mmFL... hence tameone's suggestion of 220/550...

If you plan on a precondenser setup, you may choose a 330 over the 220... but that is a different topic...

Of course going to the LL Pro Lens arrangement is even better...

gs

why do I always type lense instead of lens

This is the setup that I'm using and it's plenty bright and sharp. Take a look at my projection here.
With this setup I have a 7ft wide projection at a 11ft throw.

lol I do the same thing, Tame.

Longer throws don't necessarily decrease brightness. All of the light leaving the lens will end up on the screen. The brightness of the screen is relative to the screen area (same energy spread over larger or smaller surfaces). With the same lens system a longer throw will increase the area that the light spreads over, decreasing average brightness. However, if you change the optics to get the same screen area at a longer throw you should get the same average brightness. Better yet, it will be more even fom middle to edge with the longer throw.

In short, it is not the length of the throw, it is the area of the screen image that determined brightness.