OHP + split fresnel + tilted LCD = out of focus image

[macguver]

Hello,

this might be quite simple for the most of you, but apparently I don't understand the principles of basic optics.

So I split the fresnel on my OHP, and sandwiched the LCD between the fresnel lenses, tilting the LCD also by about 12'degrees. Then I placed all this in a wooden frame, and on top of my OHP.

Result is that I got more coverage of the LCD, but I can now only focus the picture so that the image is sharp on the center and top & bottom are out of focus.

Or I can focus it so that the top is sharp, but bottom is out of focus and so on...

Is there any easy way to fix this than repositioning the lens, which is not easy since it is attached to the OHP's arm.

If there is not, then I guess it is easiest to just level the lcd to 0'degrees.

Hope you can understand my explanation.

[sbisdabomb]

is the bottom\rear fresnel the same distance it was from the light before you split them, is the objection lens you have a varifocal lens, so you can't change it's height?

[macguver]

The bottom fresnel is a little higher than it's original position on the OHP. The OHP has and adjustable Lamp height, so I tried it but it did not seem to help.

And If I understood correctly, my Medium 10K OHP does not have a variofocal lens, so I focus the image by moving the lens up and down on the OHP's arm.

Here are some pictures of the setup:

Image 1

Image 2

[Rox]

tilting the lcd or triplet will not correct the keystone.

any mirror sistem or tilting lcd sistem is not a keystone correction.

just tilting the field fresnell works as keystone, and always in a rasonable angle.

[sbisdabomb]

assuming you can raise your objective lens high enough I don't think it should mattter that you've change the location of your top\feild fresnel. You might want to try and get your bottom\rear fresnel in the same location it was originally, becuase it's expecting the source of the light to be at it's focal length.

this may sound stupid, but also make sure you didn't swap the fresnels, and that the grooves are facing the LCD on both fresnels.

[18wheeler]

I don't know about tilting triplets, but tilting LCD can correct the keystone. It was what I did with a 10" lcd screen. the image is sharp and clear corner to corner. it worked better than the tilting fresnel in my particular case, because light path matches the optimal view angle of the LCD is from 6 o'clock direction.

Originally posted by Rox
tilting the lcd or triplet will not correct the keystone.

[Rox]

please would you post a pic of your tilting setup?

or try to describe it as precise as posible.

[macguver]

I didn't intent to correct keystone by tilting the lcd, I tried to get the optimum view angle for my Benq FP567S panel by tilting it 6'o clock a few degrees.

The out of focus effect is exactly the same if you would tilt the LCD-panel on top of an original unsplit fresnel. I mean uneven distance between the lens and the tilted LCD seems to cause problems here.

Would lowering the bottom fresnel closer to its original position affect the focus of the image?

[Rox]

no, boton fresnell does nothing to do with the focus of the lcd

anyway, if you don´t need keystone; then you have to place the lcd parallell to the lcd and paralell to the wall. Thats the only way.

tilting lcd=tilting triplet, so each object-triplet distance has a correspondent throw so each part of the lcd would need a different throw to the image being focused. This will give you a focused image somehow but it wont be square since the magnification on each part would be different.

[18wheeler]

I have to make an correction. That projection was done 20 months ago, and my memory is a little fuzzy.

Anyway, both tilting LCD and tilting fresnel correct key stone nicely.

tilting LCD gets a better image quality ( brightness, color etc.) because the optimal view angle achieved. but it has limited keystone adjust ability, i.e. you should not tilting LCD too much to compensate keystone, otherwise you get unevenly focused image. HOWEVER you can try tilting LCD and projection lens at same degree to solve the focusing problem. From a simulation I did, it (SEEMS) having better focusing. but I didn't try it in person

tilting fresnel will get keystone correction done nicely without focusing problem (within a certain limit), but it doesn't provide optimal view angle though.

Originally posted by Rox
please would you post a pic of your tilting setup?

or try to describe it as precise as posible.