# How can you determine picture size of a projector?

Discussion in 'Displays' started by Rob Varto, Mar 7, 2004.

1. ### Rob Varto Supporting Actor

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I was thinking about installing a projector in my family room and since I have a cathederal ceiling, there is no chance for me to mount to a ceiling. Therefore, I would have to mount to the wall across the room (about 20' away) and I was wondering how big of an image that would project onto my wall? Any idea how to calculate this? Is 20' totally unrealistic? Are there options for mounting when you have a catherderal ceiling?

2. ### Kirk Patrick Stunt Coordinator

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3. ### Guy Kuo Supporting Actor

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20 Feet is fairly unrealistic unless you set your image quality expectations very low.

Estimate your lumens requirements as below. (If you are too lazy to understand what you are doing, skip forward three paragraphs. Unfortunately, that laziness will mean you won't be able to apply the informatin here to other situations very well)

Square footage of screen x 16 / screen gain ---> raw Lumens needed for darkened room viewing.

Then take that raw lumens figure and multiply x 1.5 to take into account bulb dimming over the life of the bulb.

Then take that result and multiply by 2.0 to account for manufacturers publishing light output for uncalibrated projectors with everything opened wide without regard for image fidelity. Expect to lose 40 to 50% from the published figure for a projector once you calibrate it to produce a picture with correct color temp and good fidelity.

Assuming an average 1.3 gain screen, this all means you should look for a machine with a lumens output rating of...

Square footage of screen / 1.3 x 16 x 1.5 x 2 lumens or
about 37 manufacturer published lumens for every square foot of screen area.

For instance....

A 16x9 80 inch wide screen is about 25 square feet.

25 x 37 is about 920 lumens minimum published output required.

Your 20 foot screen (17.4 feet wide x 9.8 feet tall would need a projector which outputs just over 6,000 lumens!

4. ### Gabriel_Lam Screenwriter

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It really depends on your projector.

For instance, I have a JVC DLA-SX21u, which has a relatively long throw lens.

At 20' away, you can zoom the lens down to about a 117" diagonal screen, which is very reasonable for that projector (many people use it for 120"+). I have mine 17' away and am using a 100" screen.

If you figure you'll need a few inches behind the projector for cords and stuff, and a projector is about a foot long or so, you'll really be 19' away, which would be down to about a 110" screen on full telephoto (and about 145" on full wide).

So much of it depends on your projector and lens choice.

5. ### Guy Kuo Supporting Actor

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This thread illustrates how four different limits all need to be considered when sizing a screen and projector.

1. The amount of light output which a projector can output. Too large a screen and the picture becomes too dim.

2. The physical throw ratio of the projector. The projection distance and offset angle limit the range of screen sizes you can accomodate in a room.

3. The panel resolution and screen door effect. Too large a viewing angle makes the projector's individual pixels visible and this degrades the image.

4. The acoustic effects of placing your screen prime viewing position where the screen size and viewing ratio dictate.

Ideally, you take into account all four factors in designing your room. If you are very careful, you'll get all four factors aligned just right with your choice of projector, screen size, viewing distance, and listening position.

I realize this is one of the less technically advanced forums for front projection, but there really are things to think about once you get past the initial questions of economy.