Hey guys, ive been reading on keystoning for a while now and this is what I came up with. I have a feeling something is not right though, what do you guys think?

This would be a solution for a floor mounted PJ in which the angle of the projector is 10° from the floor... basically the field fresnel will be parallel with the screen.

Is this what you have in mind ?

cheers,
gs

The triplet wouldn't move with normal keysoning. It would remain in a direct horizontal line with the light source.
What you're showing is some sort of lens shifting which could only be achieved by either moving the light source up or by lowering the the front fresnel.

smalls... you make a good point there... doh...

gs

Ehm if you say so. I thought normal Keystoning was about tilting the 2nd fressnel a bit.

You appear to have combined both keystoning (which is as you say... tilting the 2nd, field, fresnel) and lens shift. With only keystoning the location of the triplet does not change, it remains on the blue centerline.

By tilting the 2nd fressnel you change the way to light goes right? Or will the light stell got like a straight line?

How about this?

nono.. everything needs to remain perpendicular to the line from the bulb to the center of the screen, and parallel to each other except the collimating fresnel. the fresnel needs to be perpendicular to the ground. so in that last image, just tilt the triplet back to perpendicular to the blue line and you're set

Good! This would be a ceiling mounted PJ pointing down. Just remember that in order to acheive a square image, the front fresnel must be parallel to the screen... thought of course there are a few other ways to increase your keystoning angle if need be..

Edit... Sorry, didnt see the tilted triplet

Hm but how does the screen change then?

Here's a nother image I made..



Whoops, triplet is wrong, let me see...

the screen changes because of the tilted fresnel.

And because of the tilted Triplet right?
So this is more accurate?



But beacuse of the tilted second fresnel the lowest corners stay sharp, right?

Oh Yeah I had a question about the triplet, how do I know in what angles the light goes?

yes that image is correct. you don't tilt the triplet because you want the center of the triplet to point to the center of the screen.

oke, well I think I have everything I need to figure it out except for one thing.

How do I know how my triplet "shines"?

Example,


Example2,


I need to know this so I can see how high I can hang my PJ from the floor without hitting the roof.

Example3,
As you can see im hitting the ceiling now, how do I know how it projects?

You should aim the centerline of the projector at the center of the screen... everything should be good from there. You can use the focal length calculator II in the build and calibration tools section to give you an idea of how big the projection will be for a given throw.

I will also point out that there were some people who did tilt the triplet slightly to improve focus. Focus is the biggest problem with keystoning as it's hard/impossible to get perfect corner to corner focus with a split fresnel setup. Tilting the screen slightly is also a way to reduce the keystoning angle and improve focus. I used a combination of lens shift and keystone correction with a 15" LCD with decent results... this is when my PJ was floor mounted... I used about 45mm of lens shift and 6-7° of keystone correction. Focus was good, but there was a slight bow to my image (16:9) on the top, but I was able to live with it... Just thought i'd mention this...

gs

Hm okay, I thought that you would have focus problem if you would go over the 15 degrees keystoning.. But 1-14 was fine..

everyone has a certain degree of "soft" focus they can accept... for me this was around 10-12° on a 16:9 screen. yes, I believe the accepted value is around 12-15°...

what LCD size do you plan to use, what triplet and what screen size are you planing to use ? these factors will determine you keystone angle... (if you hadn't figured it out already)

gs