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Keystone Correction - How Much ? (Newbie Warning)

Discussion in 'Projectors, Screens & Video Processors' started by chrisconnew, Nov 12, 2002.

  1. chrisconnew

    chrisconnew
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    just a quick question from a new forum member - read newbie

    i'm looking into the possibilty of getting a projector (sony hs1 or hs2) for watching dvds at home on a big screen - and although i've been reading up on the pros and cons i'm a little confused about keystone correction.

    I'm in a situation where i'm limited on options for where to put the projector, and ideally it would end up on a shelf in a corner of the room. To fill out the detail a little - imagine a 4m square room, 2.2m ceilings, with a 2 seater couch (see dodgy ascii drawing below.

    = - Screen
    # - Couch
    X - Projector
    o - floor (had to pad it out)
    ----------------------------
    |oooo=========oooo|
    |oooooooooooooooooo|
    |oooooooooooooooooo|
    |oooooooooooooooooo|
    |oooooooooooooooooo|
    |oooooooooooooooooo|
    |ooooo######ooooo|
    |ooooo######oooXo |
    |oooooooooooooooooo|
    ----------------------------


    What i'd like to be able to do ideall is to project the image to the screen as shown above, with the projector on a shelf up by the ceiling - so about 2m high.


    Although i understand that keystoning adjusts the projected image, what are the limits in doing this? This is probably a simple question where i am misreading the websites i have looked at , or is listed on the specsheets for various projectors.


    I'd really appreciate some info/advice here - from what i have seen so far people have their projectors mount directly infront of the screen, suspended from the ceiling in various ways, but i am fairly limited in space and how obtrusive i can be in the room.

    Thanks in advance

    Chris
    chris@REMOVEthisBITconnew.net
     
  2. Chris Frost

    Chris Frost
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    Keystone correction comes in two flavours - the most common is Electronic, the second option is Optical. They both achieve the same end result but it different ways. We will deal with Electronic Keystone Correction first.

    Electronic works by distorting the picture before it is written on the LCD panels which generate the image. There are two side effect of this. First, not all the pixels are used to create a picture so there is a loss of resolution. To combat this some manufacturers are using smarter electronics to reduce the effect, which partly explains why the images from projectors with the same spec on paper can look so different in the flesh. Second, the unused pixels still allow a little light through, so they cast a faint grey shadow on the wall around the image. The more keystone used the bigger this shadow becomes. Sideshot type systems tend to exagerate the effect. There is very little that can be done to avoid this other than not use keystone or have a wall covering that absorbs the stray light.

    Optical solves both the above problems. It uses lenses to distort the image after it is written on the LCD panel, so there's no loss of resolution and no grey shadow.

    "Great" you're thinking... so why don't all manufacturers use an optical system. The answer is cost ofcourse.

    To work well and avoid problems with focus and brightness varying across the screen requires a manufacturer to use really expensive lenses. NEC have come closest to a budget solution with the VT45. However, their system only works across a limited range.

    The best solution with any single lens projector is to position it so that you don't need to use keystone at all. There are projectors which can be positioned high up at the back of the room to give a cinema layout effect with using keystone.

    However, if your only location isn't ideal then you'll have to look at projectors with sideshot and judge for yourself if the compromise is worth it.

    Regards
     
  3. MAW

    MAW
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    Just to add, side keystone is not allowed for by all PJ's, just in case you hadn't noticed!!!! Also, you can often much reduce vertical keystone by inverted mounting. Most Pj's still basically designed to sit on a desk, and 'throw' the image upwards, so if it's on the ceiling, turn it upside down, and invert the image also.
     
  4. chrisconnew

    chrisconnew
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    thanks for the replies. most of what you've pointed out i think i understood before, but it clarifies things a bit.
    The main thing i am wondering about is to what extent can you use the keystoning. The sony brochures talk about being 2.3m from the side of the screen - but how do you work out the limits of how much you can practically adjust the screen? in my case being 3-4m from the screen, and as much as 2m to the right, and 1.5m up from where a screen will be mounted - is it certain i can keystone correct the image this much? or do i need to work something out to narrow down my range of projector choices.


    Chris
     
  5. MAW

    MAW
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    You need to check the manual, then how is your trigonometry? It's the only way. Manual will probably give you angles, but you might be lucky and get a diagram. Some mfrs. have the manuals in .pdf on their sites, if your dealer can't help. Don't know about Sony. Your problem sounds fairly extreme, offhand I'd say you might not get the best results with that layout.

    I take it you have a doorway or something in the middle of the back wall. Couldn't you ceiling mount the PJ centrally, invert it, to minimise keystone, and compromise on screen size if necessary? I think you'd be better pleased. I have about 5 degrees keystone correction in my setup, and I wish I didn't.
     

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