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Keystone: good or bad?

Discussion in 'Projectors, Screens & Video Processors' started by HTnewb, Jan 14, 2005.

  1. HTnewb

    HTnewb
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    Hello,

    I've heard that the keystone feature on projectors are not good, meaning that if activated it will reduce the picture quality. :confused:

    Does anyone know if this is true? Please post reply if you have info on this.

    TIA
    HTnewb
     
  2. Warpaint

    Warpaint
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    If you use large amounts of keystone adjustment then yes you will lose picture quality.
    The picture will be electronically adjusted to fit into less pixels at either the top or bottom of the picture with vertical keystone adjustment. This means you will be losing resolution and probably lose detail. If you also have horizontal keystone and use it at the same time then the problem is even greater.
    Having said this, if you only use a slight amount of keystone then I doubt that you will see any problem.
    I would try to avoid using any keystone adjustment at all and overscan the picture slightly to maintain the correct picture shape.
     
  3. HTnewb

    HTnewb
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    Thanks for the reply Warpaint!
    When you mention "overscan the picture slightly to maintain the correct picture shape", what does this mean? I have the BenQ PB6100 series PJ, and not sure if that is able to "overscan".

    TIA
    Phil
     
  4. Warpaint

    Warpaint
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    Nothing too technical here.
    Just use the zoom on the lens to make the picture slightly larger then the white area on your screen. The picture then overlaps (overscans) the black border at the edge of the screen. This gives a nice sharp edge to the picture.

    Hope this helps.
     
  5. LV426

    LV426
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    Yes, but, if the projector isn't mounted in the correct position in relation to the screen, although by this method, the edges will be clean and rectangular, the actual image contents will still be distorted. Vertical lines (say) will be slanted if the are towards the edges of the screen.

    The answer to the question is, that it is always best to site the projector in relation to the screen such that the image projected is the correct shape, without having to resort to digital keystone correction. This positioning is flexible to an extent where the projector has lens shift, but is inflexible when it doesn't.

    However, if the layout of the room (etc) means it is completely impossible to achieve perfect positioning, then using keystone is probably better than watching a mis-shaped image (clean edges or no). Better in such cases, though, to use lens shift where provided.
     
  6. RTFM

    RTFM
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    From my experience even small amounts of digital keystone correction should be avoided.
    There was a thread on this subject on the AVSForum a while back and someone posted a test page which showed this to good effect.
    I'll post a link if I can find it.

    Jeff :rtfm:
     
  7. Nick_UK

    Nick_UK
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    I don't watch test pages on my pj - I watch movies. I use a small amount of keystone correction, and don't notice any untoward effects in normal use :)
     
  8. LV426

    LV426
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    Agreed. Better to have a properly rectangular image. Better to have this by proper positioning, but if that's simply not feasible, keystone correction is better than a trapezoid image.
     

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