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Perfect alignment?

Discussion in 'Projectors, Screens & Video Processors' started by PC_112, Jun 10, 2005.

  1. PC_112

    PC_112
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    Hello,

    Does the projected image align perfectly on your screen?

    I have a screen hanging from the ceiling with black borders and I am trying to align the image perfectly on it, but I am always missing about 5 millimeters on one side or another. (e.g while on the right bottom side the projected image touches the black border, on the right bottom side it is about 5mm inside the black border - the screen is 203 cm wide)

    So far I am just moving the projector and using lens shift in order to align it. Should I also try to use keystone, or I shouldn't bother? I can only notice that is not perfectly aligned when I go close and because of the black bars on the screen that I am trying to align it to.
     
  2. gingerone

    gingerone
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    I would stay away from keystone at all costs, it will ruin your picture. I'd overscan the picture just slightly in to the black.
     
  3. Nick_UK

    Nick_UK
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    That is a very broad statement. Surely it depends on the projector ? I use the keystone correction on my Benq 6200, and it does not "ruin the picture".
     
  4. Gary Lightfoot

    Gary Lightfoot
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    If one side slopes down in a corner, then the projector is too far to one side.

    If the image is lower in the bottom r/h corner, then the projector is a little off center to the left, so moving it to the right (try an inch to start) should level it up.

    What you have there is horizontal keystone - with the projector a little too far to one side, the left of the lens is nearer to the screen then the right, because you've rotated the projector to the right to line the image on the screen.

    HTH

    Gary.
     
  5. PJTX100

    PJTX100
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    At the screen sizes we are dealing with I'd consider 5mm to be within tolerance. As you say you'd barely notice it...PJ :)
     
  6. PC_112

    PC_112
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    My prjector has horizontal and vertical lens shift. Is doing some horizontal lens shift the same as moving the projector to the right?
     
  7. PJTX100

    PJTX100
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    No. Horiz lens shift keeps the image dimensions static.

    "Moving to the right" is better described as "swiveling to the right" which will have the effect of making the right vertical edge longer and the left vertical edge shorter. Tolerances here could be fractions of mm's!..PJ :)
     
  8. Gary Lightfoot

    Gary Lightfoot
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    I actualy meant physically moving it to the right and not swivelling, because the position is probably wrong and swivelling is what is probably causing the problem. :)

    Keystone might correct it, but I would prefer to get it correct by positioning if possible.

    If it really is that small and un-noticable from normal seating distance, then it might be easier to do as others have suggested and use the black border to absorb some overspill if you make the image a little larger to hide the gap.

    Gary.
     
  9. PC_112

    PC_112
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    ok, but if I swivel to the right wouldn't that skew the other sides also?

    I noticed. I didn't find a good way to do it yet with the ceiling mount. If the screws are too loose it moves a lot. If they are tight you have to force it to move, and in that case it usually moves a lot again. I guess there should be some middle of the way where the screws are tight but not too tight?
     
  10. PC_112

    PC_112
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    Ok, and just to be sure I will be trying the right thing, when you say "right" is this the same direction in both projector and screen? Ot it is the opposide because when I look at the screen I am turned looking to the east and when I look at the projector I am turned looking to the west?
     
  11. PJTX100

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    Ah, yes, see what you mean. I'm probably confusing the issue.

    In my defence, it's probably because the way I did it I put the PJ up 1st, got it all square, then put the screen up in the right place.

    I'll shut up now! :)

    ...PJ
     
  12. Gary Lightfoot

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    If you're looking at the back of the projector so you can see the screen in front of it, you will move the projector to the right. It doesn't have to be a lot either. I had exactly the same problem with my very first projector so it's easily done. :)

    I think you can do it accurately if you make sure the distance from the top left and right corners of the screen are the same distance from the center of the lens. You can use a piece of string to do this rather than a tape measure.

    Hope that helps.

    Gary.
     
  13. Gary Lightfoot

    Gary Lightfoot
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    PJ,

    I had to do it the other way round due to the roofline restrictions in my loft, so the screen didn't have much lee-way for moving other than an inch or so each way (it can slide a bit). Once that was up I had to position the projector.

    Gary.
     
  14. PJTX100

    PJTX100
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    Yep, can see that's what the majority do.

    Sorry for not shutting up as I promised :) , but if PC has horiz lens shift (which is effectively the same as moving the PJ albeit you may bring in extra issues like uneven focus if you do it too much... catch breath) then the solution may well be to swivel the PJ slightly to make sure both vert edges are equal, then to use horiz lens shift to line it up on the screen...PJ :)
     
  15. Gary Lightfoot

    Gary Lightfoot
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    That might work. :)

    Gary.
     
  16. beecee

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    also, if the projector is not level, this can have an effect on the picture, as i ceiling mounted mine and the picture was off down one side but perfect on the other.

    so after ages tweaking i decided to adjust the screws that hold the pj to the mount at this helped no end, so just make sure the pj is level front to back and side to side.
     
  17. martian1

    martian1
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    Gary is right try string here is a shot :thumbsup:
     

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  18. PJTX100

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    Absolutely right that string is an excellent way to ensure the PJ is central to the screen, and it is vital for PJ's without optical lens shift.

    The whole point of optical lens shift though it to allow you to place the PJ slightly off centre. As long as the PJ is square to the screen, it's possible to move the image a fair bit horizontally either way without compromising the image.

    Either way, on a "green field" installation such as your pic, I agree get the PJ central whether it has lens shift or not....PJ :thumbsup:
     
  19. PJTX100

    PJTX100
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    I don't think it does depend on the projector. And neither will it ruin the picture. What it does do is make the image inferior to what it would have been had no digital keystone been employed.

    I'm not sure how digital keystone correction works, but what I'd imagine is going on is some sort of extra scaling. But instead of it being consistent across the whole pic, it's an increasing (or decreasing) scale for each horizontal line of pixels.

    ...PJ :)
     
  20. PC_112

    PC_112
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    Thanks!

    My projector is indeed placed about 5cm off center, but fortunately it has vertical lens shift.

    With you directions I managed to get it 99.9% perfect, which is good enough for me :)

    The problem is that now I temporarily have a Da-lite model B 4:3 screen that I might replace with a da-lite model B 16:9 screen. I hope such a change will not mess up everything again since the new screen will be placed on the same hooks.
     
  21. Ekko Star

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    You need to get your PJ square on in all planes to your screen if you can.

    Using keystone correction is a compromise as the PJ is digitally compensating for whatever the offset is. This deteriorates the picture and ideally you would want to minimise this.

    Lens shift is a great idea as it actually moves the lens assembly physically so keeping distortion to a fair minimum. However even at the outer movements of any assembly you may have undesirable affects.The lens is moving away from the prism block and there is only so far it can possibly go.
     
  22. Nick_UK

    Nick_UK
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    Digital keystone works by distorting the picture in a mirror image of the distortion seen on the screen. This means that the picture is being digitally re-mapped, and inevitably some loss of quality will occur. However, this loss of quality is very small, and does not "ruin the picture" as was suggested by another poster.

    A lot will depend on how many pixels are on the display device. My projector has a resolution of 1280 x 1024 - I can lose some resolution there due to digital keystone, and still end up with a better quality picture than projectors with a lower resolution.
     
  23. Mark_a

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    According to the Sanyo Z3 brochure, I've got, its lens shift allows for a full screen height either up or down, and half a screen to the left or right. On a 2.6m screen that seems a massive amount of movement to me and having your projector 5cm off centre seems quite unimportant. Mine will have to be about two foot to the left of where it should be as that's unfortunately where the door is.

    Though I could be talking cobblers as I don't actually have one to play with yet, though the electric screen arrives on Monday. Yay! :)
     

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