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And of course including how far away from it should you sit.
So here's a handy calculator to crunch the numbers and further down the page some notes on how to use it and a discussion of the terms used.
No time to read the user guide because you just want some results then enter the measurements of your screen, either Diagonal or Width and click on the button.
| How to use this calculator |
Just in case you haven't worked it out the green section is where you enter your screen size and possibly the screen gain and the lumen output of your projector.
As the most common screen measurement is the diagonal in inches (even for the vast majority of the World's population that have been using the metric system for a very long time) that's the first input box in the calculator.
You can however enter this measurement in mm in the next box or if you prefer the screen width in inches or mm.
To do a fresh set of calculations click the button to first clear the calculator.
Projector Output
The output of a projector is given in ANSI Lumens but forget about using the maximum rated output in the spec sheet if that's the only number quoted.
I suspect that in order to arrive at this number the manufacturer adjusts every control the projector has in order to maximise its light output with little regard to what the image looks like because they're not looking at an image but rather a 100 IRE white screen and they're trying to make that as bright as possible.
By time you calibrate a projector for optimum image quality and select a picture mode suitable for movies, possibly put the lamp into economy or low output mode as well, you may end up with less than 50% of its rated output and that's with a brand new lamp.
And a lamp can lose as much as 50% of it's brightness over it's lifetime with most of that happening in the first couple of hundred hours.
Proper technical reviews will give you some idea of the actual Lumen output of your projector.
Image Brightness
Image brightness is expressed in Foot Lamberts (ftL), ftL = (Projector Output/Screen Area) X Screen Gain where Projector Output is in ANSI Lumens and Screen Area is in Square Feet.
For a cinema the Society of Motion Picture and Television Engineers (SMPTE) suggest 12ftL for a 100% white frame and 16ftL for an empty gate, but you can get away with less in a home theatre with good ambient light control.
Unless you're trying, for whatever reason to replicate the look of a Plasma screen as little as 8ftL can look pretty good in a dark room.
Front projection CRT enthusiasts regularly see these sorts of levels and even less and see no problem at all with the projected image.
As a guide, to achieve 12ftL on a 100" diagonal screen with a gain of 1.0, the projector would need to be outputting 355 ANSI Lumens and for 8ftL it needs 240 ANSI Lumens.
THX 26º SMPTE 30ºTHX 36º
These are just guides provided by the people that have a fair bit to do with setting up cinemas and all specify the horizontal viewing angle from screen edge to edge and back to where you're sitting.
26º (furthest from the screen) is the minimum THX recommendation and 36º is considered the optimum.
SMPTE suggest roughly midway between these with 30º as a minimum.
In terms of viewing distance to screen width ratio, these work out at 2.2, 1.6 and 1.9 screen widths.
It should also be noted that these angles and ratios are really for a CinemaScope 2.37:1 screen and not a 1.78:1 (16x9) one.
Recommended THX viewing distance (1.78:1 screen)
For a 1.78:1 screen THX suggest simply taking the width of the screen and dividing by 0.728 to arrive at a recommended seating distance.
Or you could multiply the screen width by 1.37 and this is equivalent to a horizontal viewing angle of 41.5º and way closer than I'd like to ever sit, but that's just me.
Maximum distance to fully resolve 720p and 1080p
Based on Visual Acuity data which is simply our eyes ability to fully resolve detail for a 1080x1920 and 720x1280 pixel display.
A person with perfect 20/20 (or 6/6) eyesight can resolve 1/60 of a degree of Arc and this equates to a viewing angle of 32º for a 1080p display and 21.3º for a 720p display from which a viewing distance can be calculated.
So it's expressed here in terms of the maximum distance from the screen you can be and still be able to see all the detail that is available to you.
If you move further back then you're not seeing everything that the display is capable of and therefore you're missing things you should be seeing.
As such these numbers should be considered as the maximum viewing distance.
It should be noted that for a 1080p display both the SMPTE 30º and THX 26º minimum viewing angles (and therefore maximum viewing distances) exceed this maximum distance.
15º Vertical Viewing Angle
There are no real guidelines to seating distance and vertical viewing angles other than a suggestion that the maximum vertical angle measured at the seated eye height to the top of the image shouldn't exceed 35º (to avoid neck strain).
There is also a suggestion that if the screen size occupies less than 15 degrees of the viewer's vertical field of view, then the image appears small.
The first set of distances given are for a normal 1.78:1 (16x9) screen and the second for a nominal 2.37:1 image shown on that screen (with black bars above and below obviously).
3 Times Image Height
Another popular guideline for viewing distance included because if I didn't people would probably ask me to do so.
Both it and the 15º guidelines above are interesting though in reinforcing the concept that you need to sit closer to a 1.78:1 (16x9) screen when displaying 2.35:1 - 2.40:1 material.
And no doubt something the CIH (Constant Image Height) fans would point out as a reason why CIH is better.
Obviously how big a screen you use or how close you sit to it is entirely up to you but there are a few things to be said for not sitting too close.
Whilst the Visual Acuity 1080p and 720p data sets the maximum viewing distance, there is also for every display resolution and technology a minimum viewing distance.
This absolute minimum viewing distance is set by the presense of visible pixel structure or SDE (Screen Door Effect) for fixed panel displays or scan lines with CRT projectors.
As you move further back these distractions disappear and the image takes on a more solid appearance.
However even at a distance where this occurs you can still get the occasional "peek a boo" scan line or artifact appearing with some program material or during vertical pans.
As such, the optimum minimum viewing distance for any display is further back still from the point where the image "gels".
There is also a lot to be said for not sitting so close to a screen that you continually have to move your eyes, or even your head from side to side to follow the action.
Sort of like sitting in the front rows on the net line at the tennis.
Great to see all the action but by the end of the day, very tiring.
Having said that some people like the screen to completely fill their peripheral vision so that they can be immersed in the action.
These would be the people that like to sit in the front rows at the cinema.
Others like to sit in the middle and some like to sit right up the back (but probably for reasons other than enjoying the cinematic experience).
Explanation of terms Too big or too close?