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Hi Guys,


Sorry to bother you with a potentially stupid idea:


I was trying to come up with a solution to totally mask out the top/bottom spill for those of us who are using a 4:3 pj with a 16:9 screen and thought this might work:


Couldn't one make a template (probably constructed from sheet metal painted matte black) with a 16:9 cutout that you would position a few inches from the lense to block the 4:3 overspill from ever hitting the screen?


I guess the trick would be to get it positioned just right and get the tolerances of the cutout right to get the masking accurate as possible.


If one is already using a black screen border (ie Velux) and a black stage, would this improve things enough to be worth the effort?


Has anyone already tried this sort of thing?



Mike
 

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The problem is that light bends so that you can't use this method unless you have a border of about two feet above and below the screen of black material. You can block out a certain portion of the light completely at the very top or bottom, but the next 2 to 3 feet as you move towards the center of the screen will be an area where you have cut part of the light but not all of it resulting in sort of a shadow effect. In other words, you don't get a sharply masked area; it masks gradually. For example, in my set up, I only need to block the top spill (because I use a video processor to put the 16:9 panel in the bottom of the 4:3 panel), which spills up onto the ceiling. Above the screen for about 2 and 1/2 feet is the Velux screen border and then black background curtains. Therefore, when I use my mask on the lens (I used cardboard painted black), I can block out the light on the ceiling, and the shadow created on the next lower portion of the light spill is in the curtain area. On the screen, there is no shadow and I see the whole picture.
 

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I have been experimenting with this idea and ended up with the following. I placed a 16:9 mask about one foot in front of the pj lens. By the time the light reaches the screen, the mask shadow edge "blurs" to 4 to 5 inches. With a 3 1/2" border around the screen, this works out pretty well. Of course, you could move the mask, or change the size of the border so that the screen is fully lit, with no shadow, and the wall next to the border is fully shadowed. I have a very dark colored back wall, so this wasn't necessary. Either way, the effect is very satisfactory.
 

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I think that robbyc30 is correct. If you can move away from the lens a bit, you can decrease the amount of the blur or shadow. I have my mask right in front of the lens because I have a ceiling mounted projector and it is easy to just attach it to the front of the lens. But there was another post on this subject a while back and someone there also reported getting decent results with the mask about a foot or so in front of the lens.
 

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Light does not really "bend" much in my experience, although it can of course be reflected and re-directed. Still, the light expanding away from the projector creates the same effect. The problem is that the light is radiating out in an expanding pattern. Proof? Disregarding focus, if the screen is close, picture is small, if the screen is far, the picture is large. Thats why that tiny bit of light that is slipping by the edge of the black-out mask becomes 4 to 5 inches once it is at the screen.
 

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cpc - it is called diffraction and is a well known phenomenon in wave mechanics (be it light or sound or even waves in water). Light will bend around corners due to this effect.
 

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Why not use both methods. Masking out most of the light in front of the PJ. Then mask right in front of your screen both the upper part and lower part. The total light you will need to get rid at the screen will be less.


This was just a thought. I am not using masking myself in any form. I think it would be enough to mask at the screen.
 
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