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Lens shift + keystone correction question

Discussion in 'Projectors, Screens & Video Processors' started by tjobbins, Jan 2, 2005.

  1. tjobbins

    tjobbins
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    Hi guys,

    Was wondering if anyone could help me understand lens shift better.

    I'm planning a 84" diagonal screen with a Panasonic AE700. Using the calculator on Projector Central, I've worked out that my PJ will need to be a minimum of 8.4' away (2x zoom) to achieve this. However ideally I'd like to run at slightly less than maximum zoom - say 1.75x, which gives a distance of 9.5'

    In my living room, this would place the project about 1 foot in front of my sofa, and directly in line with a ceiling light, prevent ceiling mounting. So I thought instead I could put it on a table by the side of the sofa.

    My first question is - if I were to shift the image horizontally by the full 25% allowed by the AE700, would I require any keystone correction? The projector and screen would still be exactly parallel, however the middle of the PJ lens would now be hitting the screen at 75% of its width instead of 50% as normal. Is keystone correction required here?

    A second, and much less important question is - how does lens shift actually work? :) I can't quite understand how a lens can project an image which is outside of its viewpoint.

    Thanks in advance!

    Tom
     
  2. KraGorn

    KraGorn
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    Lens Shift is purely an optical effect, Keystone Correction is done by re-processing the image .. thus LS is good and KC is bad, KC is bad because it makes use of the projectors scaler which often isn't as good as that in the source and if you've obtained 1:1 mapping with a PC or DVD player then the benefit is lost.

    How LS works exactly I don't know, but presumably by adjusting the lens or prism it squeezes the image to retain a rectangular projected area as the light moves off-centre.

    I don't know the 700, but IMX lens shift can move the image +/- at least 1/2 height and/or width, depending on whether it has one axis or two movement, ie. the lens can be in line with, say, the top edge and the image can be centred .. this assumes that the centre of lens is in line with the centre of the image.

    The only caveat is with that some projectors, like the Z2 I had, using the shift at the extreme limits can introduced small focusing problems due to imperfections in the optics, but that wasn't a big problem when viewing video and only noticeable using a Windows desktop on the PC.
     

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