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Zoom Lenses

Discussion in 'Projectors, Screens & Video Processors' started by charles, Jan 1, 2005.

  1. charles

    charles
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    An increasing number of projectors have large zoom lenses.To give an example the SONY HS50 has a 1.6 zoom which means that with a 100 inch diagonal screen the picture could be viewed at a minimum of 118 inches and a maximum of 179 inches.
    What effect does this amount of zoom have on picture quality and brightness?By having a projector at its closest range would the light emmission still be equal accross the screen?On the other hand by using the maximum zoom would the brightness of the image be reduced since the beam would be travelling at a greater distance,added to which would there likely to be a deterioation in picture quality by using the maximum zoom?
    Charles
     
  2. Nick_UK

    Nick_UK
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    The optics on most budget projectors is pretty good, so you shouldn't see too many problems with the uniformity of brightness across the screen. However, the bigger the area that you illuminate with a light source, the dimmer the light will be. A picture with a 36" diagonal will be twice as bright as one with a 72" diagonal. If image size and brightness are important factors, you should think about getting a pj with a high ANSI brightness rating.

    With regards to picture size, all digital pictures are made up of pixels, and the bigger you make an image the bigger the individual pixels will be. Even if you choose a pj with a good native resolution (1024 x 768), each picture element on the screen could be a couple of millimetres or more in size when projected on to a big screen. It's a well-known fact that the bigger you make a picture, the more imperfections will show up. However, if you choose the correct size of screen for the environment, and have people sitting the correct distance from that screen, it should not be a major problem.
     
  3. David_of_Surrey

    David_of_Surrey
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    Shouldn't a 36 inch be FOUR times a bright as a 72 inch screen?

    Dave
     
  4. Gary Lightfoot

    Gary Lightfoot
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    I was thinking the same. It works on the square rule IIRC.

    Gary.
     
  5. Nick_UK

    Nick_UK
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    Yes, of course it will be four times, not two :)

    My brain cells were not working too well yesterday - possibly due to an alcohol overload the night before :rolleyes:
     
  6. PJTX100

    PJTX100
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    This sounds logical. But I don't get this at all.

    Smallest screen I can get is approx 50" and largest approx 90", if the 50" was 4 times brighter than the 90" I'd certainly notice, and it doesn't look that much brighter.
    If they rate a machine at say 1200 lumens, then what size screen are they quoting that for? The smallest, in which case the largest would only be 300, or the largest in which case say hello to a whopping 4800 when projecting at the smallest size.

    Perhaps I've drunk too much tonight too! :D

    ..PJ
     
  7. Gary Lightfoot

    Gary Lightfoot
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    The eye is very good at making adjustments for light due to the iris for one thing, so reducing the amount of light by half looks a lot brighter than you think. I good way of trying this is to use an ND2 filter in front of the lens - it will reduce the lumens by half, but it doesn't seem to make as much difference as the measurements would reveal.

    Reflected light from the screen is measured in foot lamberts, and to calculate it, you divide the pj lumens by square foot area of the screen (and then multiply by any screen gain if it is not unity), and you should aim for 12 to 16 if you want something like you would get at the cinema. Many prefer a brighter image though, so don't worry if it's too high. Too low might be a problem though.

    So, a 50inch wide 16:9 screen is 9.76 sqr ft, and divide that into 1200 equals 123ft lamberts. Very bright. However, most projector lumens are over rated by the manufacturers, and also will dull down after a couple of hundred hours and this can be as much as a 40% reduction from the manufacturers stated figures, so lets reduce the 1200 by 25% which gives us 900lumens for this example (720 for 40%).

    50" = 900 / 9.76 = 92ft lamberts. (73 for 720lumens)

    100" = 900 / 39 = 23ft lamberts. (18.5 for 720)

    Twice the size, but a quarter of the light is reflected back. Though twice the width is actualy 4 times the area.

    HTH

    Gary.
     
  8. PJTX100

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    Thanks Gary, makes a bit more sense now...PJ :smashin:
     

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