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Scheimpflug and Optimisation

Discussion in 'Projectors, Screens & Video Processors' started by Lux In Umbra, Jul 7, 2004.

  1. Lux In Umbra

    Lux In Umbra
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    Could the experts here help me with the following "problem" (I'm a novice, particularly when it comes to "Scheimpflug!):

    I'm going to be setting up/installing a Barco 708 soon, which has been used up to now for displaying a 2.4m wide image.

    Since I have a constraint on the minimum height above ground of the screen (~115cm), and am projecting upwards (as was in previous setup), a 2.4m width image would put the projector lens height at aprox 160cm above ground, which is a little stratospheric.

    I also think a 90" image (2.0m width) is better suited to my set-up (~4.0m screen to eye).

    (Both situations unfortunately enforce a viewing position right at the edge of the screen, to avoid the image being blocked by the projector!).

    Now (finally!) to my questions:

    This projector has three Scheimpflug ranges, with "optimal screen widths" of 1.4m ; 2.4m and 4m.
    Can someone explain these "optimal" widths?
    Does it mean that setting up for a 2.4m width means simply setting the Scheimpflug ring to the preset position will give the perfect results, whereas with (say) 2.0m, extra adjustment is required? Or does it mean that one always sets to the closest range setting, and puts up with the imperfections??

    Given this, is it any better to put up with a cloud-mounted projector, and huge images, or can it be set up just as well for 2.0m wide images?

    Thanks for your patience (if you made it this far!!)



    PS: Does the screen ratio (16:9, versus "standard" 4:3) make any difference from a Scheimpflug perspective?

    PPS: Also, if I were to use the existing 2.4m width, since it think it was well-callibrated for that size, would I only require small adjustments to get a good image? (I think that 2.4m is the projector default, as well).
     
  2. Lux In Umbra

    Lux In Umbra
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    Which is more believable:

    1) The installation manual for my model?
    2) The Barco lens program set up for this model/lens?

    They give inconsistent results wrt projector distance, and more importantly, position wrt the screen (vertical dimension).

    I think this is probably due to the assumption of 4:3 screen in the installation manual's equations.

    The Barco techie I spoke to previously had said the projector should be located 1/3 up the screen height, but the lens program places it below the screen for 16:9 use.

    Can I believe the lens program? Are there trade-offs to placing the projector below the screen height? (And what alternatives - short of using 4:3 - are available)

    Your help is appreciated!
     
  3. Roland @ B4

    Roland @ B4
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    Lux
    Sorry for the slow response.
    2nd post first. use the lens program it is more accurate the manuals did have some errors and rarley made reffernce to 16x9 images.

    As regards the schimflug settings. I have installed loads and loads of 708 projectors and have yet had to move the setting off the mid point. My advice would only be to move the adjuster if you can see the corners are focusing differntly
     
  4. Lux In Umbra

    Lux In Umbra
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    Thanks, Roland.

    I think I've worked out the difference between the figures wrt 4:3 and 16:9 ... the 16:9 image only uses (aprox) {the top} two-thirds of the phosphor area, so the projector lenses can be placed (aprox) level with the bottom of the screen, rather than the 1/3 up required for 4:3.

    On the Scheimpflug-related issue, does this mean that:

    1) If I use the same screen width as the previous user, I shouldn't need to make modifications to the tubes-alignment, only the lenses? (And the projector should be fairly easily set up - not requiring electrical focus adjustments, etc)

    2) Only minor adjustments to the bow/skew, linearity, etc are required.

    BTW: Finally, if the projector has been being used @ 2.4m width with seemingly good results, in your (substantial) experience, would 2.4m width be an acceptable width for viewing from 4.2m (fits the ~30 degree field quite well, and seemed bright enough to me).

    Thanks!
     
  5. Roland @ B4

    Roland @ B4
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    The shiemflug will only adjust the diagonal corner focus.

    The single most important thing to do with your projector is make sure that each image uses the centre part of the tube. By using the settings of the previous user is potentially dangerous because they may not has got it right either.

    I use the following method.
    Use an interanl pattern of a low frequency the 15kz PAL or NTSC will be fine. Adjust the H and V size to way over scan the tube edges. Turn the contrast down to make sure that your projector doesn't get too hot.
    Select the green image by the colour select menu (or cover the other colours) and make sure that the image overlaps by a few inches the top of your screen (for a ceiling mounted projector). the amount of over lap should be the same on either side.
    Next display the red image and make sure the position of the overscaned image is exactly the same as the green. I.E. if the green image was 2" wider on either side of you screen mark the wall with some sticky tape and make sure the red is the same. there are x4 7mm bolts that you will have to loosen to move the tube 2 are buried right inside the projectorat the base of the tube. Do the same for the blue too.

    I would always be inclined to ignore previous settings on the projector and asume the ower got them wrong.
     
  6. Lux In Umbra

    Lux In Umbra
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    Thanks again.

    I thought that overscanning like that was the biggest no-no in the business! I assume that after setting it up that way, one very quickly returns the H & V settings back to normal?

    You think that the tubes will need readjustment even at the same image size? (I probably wont be replicating the exact positioning of the projector, I suppose - but same distance from screen).

    With regards the last user, they were supposed to be somewhat barco-knowledgeable, and I think the previous setup was by a Barco service expert (but may have had a few minor kludges since...).

    Thanks for your advice!
     
  7. Roland @ B4

    Roland @ B4
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    Lux
    you are quite right once you have set the lenses. reduce the width so that the active picture area does not come close to the edges of the tube face.
    I should have made that clear
     

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