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Projector to screen angle

Discussion in 'Audio Visual' started by jc69817, 6 Apr 2019.

  1. jc69817

    jc69817

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    Hi guys. I think I can guess the answer before I ask you the question but here goes anyway. If (as I expect) it does, is there a simple cheap alternative ?

    Does a projector have to be at 90deg to the screen and if not will the shape of the picture be distorted ? I’m thinking of getting a cheap projector and screen for the kids in the back garden but don’t want the projector in the middle of the garden. I was thinking of putting it on the corner of a nearby shed.

    Thanks in advance guys.
     
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  3. pete50

    pete50

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    Yes it does although most projectors do have a "trapezium adjust/compensate" button but generally it is pretty poor. Also if you are thinking along the lines of buying a very cheap projector such as one of these DON'T. They are complete rubbish. LED technology just isn't good enough at the moment. Get one with a proper bulb that you wont have to have the kids sitting in total darkness watching an out of focus screen. I know I have one although I have now bought a proper projector. s-l225.jpg
     
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  4. Lucid

    Lucid

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    The picture distortion is called keystone. Unsurprisingly then, the fix for it is called keystone correction. There are two types.

    Electronic keystone is simple and widely used. The picture is electronically shrunk and distorted within the boundaries of the imaging chip to compensate for the optical distortion caused by the projection angle. However, there are two problems: First, by remapping the image it generates a lot of image artefacts so you get staircasing on what should be straight edges (jaggies), and a general loss of resolution. It affects everything, but you'll notice it most with onscreen menus and fine detail.

    The second issue is one that folk rarely consider. Reducing the image size on the chip has the effect of messing up the projection distance. The projector no longer follows the manufacturer's throw ratio guide. The projection distance required to fill the screen becomes longer. When you buy your screen based on the distance from the shed, then find that the corrected image won't fill it like you planned, but all the maths looks correct, then this is the reason why.

    There are a couple of alternatives to electronic keystone correction. The first is Optical Lens Shift. The second is short-throw and ultra-short-throw projection.

    Short-throw is easiest to understand. The projector has a lens that allows for a relatively large image at a relatively short throw distance. This is quite common on gaming and entertainment projectors. The image width equals the projection distance, and the projector sits below the bottom line of the projected image. Some can get closer than that. The trade-off is the loss of a zoom function. This means to make a bigger or smaller image requires moving the projector further or closer to the screen.

    The other solution is Optical Lens Shift.This uses a projection lens that can move left-right and up-down which has the effect of shifting the image in the corresponding direction. The advantage with lens shift is you retain zoom and don't introduce any image artefacts. The catch is cost and a limit on how far the image can be shifted off-axis. The norm is 1/3 to 1/2 of the image size in either axis, but not maximums at the same time. For example, a 6ft-wide image could be shifted up to 2 or 3 feet left or right.

    The previous poster is correct about those cheap LED projectors. They sound great on paper, but the adverts are written in a way that makes them out be to be far higher specified than they are in reality.

    In this situation, a used projector would be a far safer bet.

    There are plenty of really good 720p and 1080p used projectors going for peanuts. Mostly it's down to lamp replacement costs.
     
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  5. Mattatooi

    Mattatooi

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    Ideally you want the projector perpendicular at 90' and centre horizontally as close as possible before adjusting anything digitally. Make sure you check what the ratio of the lens is so that it will be suitable for your screen size. IE if you have a 8' wide screen and a projector with say a 1.5:1 ratio, then you will need the lens to be 12' from the screen. Of course if you have to apply heavy keystone then you will need to move it closer. This is because you will have to "pull" in the image to make it have square corners. This is digitally done. Your light from the projector will still shine in the trapezium shape, but it will be be black in the areas you have keystoned in. Whey you keystone heavily you reduce the lumens of the projector.

    So what I am trying to say is, no dont stick it at a weird angle.
     
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  6. pete50

    pete50

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    Yesn it is called keystone my mistake. It's still rubbish on most projectors if not all. You will still need to get the projector pretty damn near to spot on for a decent undistorted picture. I reiterate don't buy an LED one.
     
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  7. Lucid

    Lucid

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    There are a couple of used projectors for sale on AVForums right now that might meet your needs.


    Panasonic PTAX200 with up/down & left/right lens shift
    • 720p 3-chip LCD projector: HDTV/gaming/Blu-ray/DVD/PC compatible - YES
    • Max brightness 2000 ANSI lumens
    • Replacement lamp cost £60-£100 / 2000hr
    • Location: Gosport, Hampshire
    Offered at £200 with a spare lamp, but a cheeky £170/£180 cash might snag it


    Epson Powerlite 8100 with up/down & left/right lens shift
    • 1080p 3-chip LCD projector: HDTV/gaming/Blu-ray/DVD/PC compatible - YES
    • Max brightness 1800 ANSI lumens
    • Replacement lamp cost £70-£120 / 2000hr
    • Location: Milton Keynes, Bucks
    Offered at £150


    Although older and used, either of the above would be a massively better choice than the cheap LED projectors being offered on ebay/amazon.


    If this or any other reply was helpful to you, then do the decent thing and click the T-H-A-N-K-S button. It appears when you hover the mouse pointer near the Quote Multi-quote buttons. This is the proper way to show your thanks for the time and help someone gave you.
     
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  8. DIYnot Local

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