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My Mate is Confused & I can't really answer him - Can You?

Discussion in 'Projectors, Screens & Video Processors' started by Tempest, Oct 8, 2004.

  1. Tempest

    Tempest
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    I've been trying to explain to my mate about projectors (LCD)
    I've explained most of it but I'm not sure about this last bit.

    He thought the whole image would be totally central to the projector lens in every direction.
    I explained (with projector right way up) most of the image is ABOVE the centre line of the lens.

    I showed him this photo so he could see what I meant:

    [​IMG]

    And explained it's good to keep it all square, else you need to use keystone correction.

    Everything ok so I thought, but then he comes back with this:

    Ok, so the lens is actually pointing upwards even though the projector is flat? Even so how can it be square when the centre line of the screen is not in line horizontally with the lens? Must be some projectory jiggery pokery going on....

    Now I'm assuming it's just the way the Lens's are positioned inside the machine, but to be honest, I'm not sure HOW the projector gets the image up so high above it's centerline without having the top of the image splay outwards (like you would if you tilted it back)

    So, can anyone actually explain HOW a projector manages this trick, of keeping everything square, and yet above centerline so much ?

    Cheers
     
  2. hornydragon

    hornydragon
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    interesting...... but possibly the LCD panel and light path is not straight behind the lens....ie already physically keystoned to a set value.....
     
  3. explicitlyrics

    explicitlyrics
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    This can be done with lens shift. Keystone damages the picture but lens shift moves the image.
    From the physics I know I am assuming that it does this by keeping the image the same but by moving a lens it can change the shape the picture comes out of the projector. Thus when it has travelled a bit further at the top of the image it has spread out a bit and the picture is rectangle. It wouldnt lose any detail as nothing is actually happening to the pixels - they are just squashed so they fill the right size when the light reaches the screen.
    I hope this makes sense,
    Chris
     
  4. explicitlyrics

    explicitlyrics
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    If my theory is correct, if you move a screen so that it is parallel to the lens but closer than the real screen (50cm away ish) then the picture will be a funny shape.
    If so then it is working on the basis that light spreads out over distances.
    Anybody confirm this?
     
  5. Tempest

    Tempest
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    That sounds good (though don't really understand it!!!)

    I'm just sticking with, it's the way the lenses are positioned inside the projector :D
     
  6. explicitlyrics

    explicitlyrics
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    if the lenses are closer together at one end (or corner) then the light will bend in this direction. If you bend it towards the screen then the picture should fill the screen correctly.
    It would be about poisitioning two lenses at the right distance and angle to get the picture in an off centre position.
     
  7. JonMace

    JonMace
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    I'm probably not understanding you, correctly but if you more the screen (or the projector) closer but still parallel all you will get is a smaller immage with the same shape.
     
  8. explicitlyrics

    explicitlyrics
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    if you shine a square image at a wall from an angle, then the screen will be taller at the end furthest from the projector. The light is spreading as it travels from the small lens on the projector.
    If you make that same end smaller than the side near the projector, then by the time it reaches the wall/screen it will have expanded so that it is as tall the rest of the image.
    Hope this makes more sense
     
  9. theritz

    theritz
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    Tempest,

    the very clear diagram you've posted is correct - in a projector without lens shift (such as the AE1/200 and others) the image is projected as you've described - the light is "bent" through the lenses. the degree to which the image is off-centre of the centre of the lens is usually called "offset" - for instance the HT1000 has a pretty strong amount of offset, it throws its image way off the centre line of the lens. JonMace is also correct, if you move the projector nearer to the screen in the same plane (ie you don't tilt it in either the horizontal or vertical axis) then the image just gets smaller.

    Lens shift does exactly what it says, it shifts the lens so that the image is projected more or less in the horizontal or vertical axis. With the AE700 at the extreme of it's vertical lens shift, the image is being projected more or less as your diagram describes (the normal arrangement for an AE1/200 etc).

    explicitlyrics, I'm sorry but I can't really understand your posts (not messing.....) - it does what it does as described by Tempest and JonMace.

    Sean G.
     
  10. Tempest

    Tempest
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    Thanks for your advice guys.

    Will try and summerise this in my email back to him......
     
  11. zoolap

    zoolap
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    That diagram is so nice it's almost worth adding to the FAQ ?
     

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