Dino,


If you shoot a normal LCD projector straight at a screen/wall it will be a rectangular image with no sizing distortion. If you aim the projector up, the top of the image will be wider than the bottom (like a keystone).


So Keystone correction compensates for the natural distortion caused when the projector is above or below the center of the screen.


There is electronic keystone correction which alters the image on the panels, and there is optical keystone correction where the lens/mirror/prism corrects the picture distortion.


Different projectors use different keystone correction methods, so you need to check with the dealer, manufacturer, or manual to see how it is done with your particular model.


-Dean.

 

Dino,


I'm afraid your question is too broadly worded. I don't mean to put you off, but projectors differ greatly in design, and it's impossible to generalize about where a projector will be situated in relation to the screen without knowing something about the specific projector, your room dimensions, floor/ceiling mount, screen size, etc. etc. I would suggest the following approach.


1. Select one or more specific projectors that seem to meet your needs and budget.

2. Pick a target screen size which is appropriate to the dimensions of your room and the projector you are leaning towards.

3. Check out the manufacturer's web site for info on throw distance and projector positioning relative to the screen. Sony, for instance, has an excellent program called PJCalc which will give you exact placement info, right down to the inch, based on screen size and projector model.

4. In doing this research you will doubtless eliminate some projectors as being unsuited to your needs, but will understand the issues sufficiently well to direct more specific questions to this forum. But ... first do a search on the topic that interests you. You will probably find that your question has been asked and answered many times before.


Good luck,


Jeff


QUOTE]Originally posted by dinodog:
Hi all,

I have a question about the relaionship between projector & screen.When mounting the screen,what factors do i need to think about? For example, does the top of the screen have to be at the same height as the lens on an LCD projector? Can i angle the picture down or does the projector have to be mounted in a flat position. & what is KEYSTONE correction exactly?IS this a vertical or horizontal adjustment technique? Hope someone can help.

cheers.[/quote]

 

Thanks Dean for your input, this info will help. The projector i'm installing is a mitsubishi lvp-s250u, & this unit comes with a 15 degree digital keystone correcetion feature, so from your information i assume that when mounted in the inverted position(ie ceiling mounted) the top of the screen can be mounted at an angle of 15 degrees below the projector height,measured from the lens.ie at a distance of 4m from screen to lens,calculations being tan15= X/4, THEN X = 1.07M approx, meaning the top of the screen can be 1m below the height of the projector without adverse affects.Hope all this sounds right,i suppose i will find out soon enough,it's going in on friday, & really i was only after 20cm or so of drop anyway,so maybe everything will be ok.we'll see.

dino

 

I have just thought of something while reading the replies. Are all projections based on the middle of the screen or to the top/bottom of screen? I suppose i will find all this out shortly.

dino

 

Dino,


The lens of my Sony 400Q is lined up with the bottom of the screen when projecting from, or near,the floor. If mounted on a high shelf,or on the ceiling, it must be inverted and the image is digitally inverted -- switching the picture back to right side up. Therefore, when mounted high, the lens is roughly aligned with the top edge of the image. The 400Q does not do digital keystone correction, therefore the exact height of the projector is critical to determining the exact height of the image on the screen. However, minor keystone can be soaked up by the black border of the screen -- if it has a black border, therefore some small adjustments can be made by canting the projector up or down ever so slightly -- the less the better. Even with a projector which supports digital keystone correction you want to get the image as even as possible without resorting to correction.


I'd suggest you project on a blank wall or some temporary surface before ordering, or mounting, the screen.


Jeff

Quote:

Originally posted by dinodog: I have just thought of something while reading the replies. Are all projections based on the middle of the screen or to the top/bottom of screen? I suppose i will find all this out shortly. dino

 

jeff,

thanks for the reply & the input. The mitsubishi i have also does it's aligning from the top or bottom of the screen,depending on where the projector is mounted.So i think i have my answer, but i will wait untill i mount the unit before putting up the screen, & project onto the wall first to check.Once again thanks. I will return for an update once everything is in place.

cheers

dino

 

I am curious about digital keystone correction. If it is performed on the chip, doesn't this imply an, albeit small, reduction in the resolution along the sides? Is an optical keystone correction better or worse?

Thanks.

 

Dino,


Yes, my D-ILA FPTV also has built-in optical keystone correction, so you have to have it placed parallel to the bottom or the top of the screen.


Man E,


Yes, there is a slight loss in resolution with digital keystone correction, so optical keystone correction is generally considered a bit better. But it's not a big difference.


-Dean.

 

hi everyone,

have just installed the projector & it looks like i don't have to use any keystone correction.Everything is as it should be,so thanks all for your input. I will post some sort of message in the projector or screen section if anyone wants to know about the setup i have just installed.cheers

dino