What Size Screen?...How Far Away?
"How big is the screen?"
"Diagonal or wide?"
When I ask the last question at a trade show demonstration of a projector and screen setup, crickets often sing. If an answer is given before the chorus, it's nearly always, "diagonal." Diagonal is, after all, a bigger, more impressive number.
For years there was no confusion on this issue. When screens were 4:3 (remember those?) nobody talked in terms of the screen width. It was always diagonal. But when 4:3 started to mingle with 16:9, all bets were off.
But no sooner had 4:3 screens faded into TV Land than anamorphic projection popped up. It's all over the place at shows, now there's a new source of confusion. Suppose you just want to know how far you should sit from your screen for the best immersion and the fewest visible artifacts? Does this depend on both the size and shape of the screen?
It seems like everyone out there has an opinion on viewing distance, but the most useful answer may come from THX. As you might expect from a company that got its start by certifying multiplexes and movie palaces, and wants to see your home theater replicate commercial theaters as closely as possible, its answer comes from the theatrical cinema world. It recommends a subtended angle of 40-degrees from your seat to the left and right edge of the picture—and that's for a 1.85:1 presentation. For 2.35:1, it recommends even more: just over 50-degrees.
But THX also acknowledges that many videophiles will sacrifice a little envelopment for a brighter, sharper picture—which means a smaller screen. I'm in that group. A 40-degreee angle of view from a distance of, say, 12 feet will require a screen 105-inches wide or (for a 16:9 screen) about 10-feet diagonal. This, in turn, will demand something more than a budget or even midrange projector for a great picture, and will not be kind to standard definition sources. I use a setup with a field of view of approximately 30-degrees for both 2.35:1 and 16:9 material.
The THX recommendations can also be a bit impractical with a one-piece HDTV and its inevitably smaller screen. For example, a 40-degree field of view with a 60-inch (diagonal) screen would position the viewer about 6 feet away. Apart from the fact that you'll see every little flaw in the display or source (though maybe that's your thing), this is too close for many speakers (particularly multi-way floor- standers) to sound their best, not to mention gel together to produce a unified, coherent front soundstage. 30-degrees is better, at about 8 feet, though I suspect most viewers will be more comfortable at 10 feet. That's about the same distance Home Theater magazine used in its last face-off (with slightly smaller sets), and no one on the panel chose to use an optional, closer seating distance.
Whichever option you choose (and sometime in the next year I'm seriously considering switching a wider, 2.35:1 screen as a reference), a few formulas might be useful. The first two involve some approximation, but they will get you inside a very small ballpark.
Viewing distance, 40-degreee viewing angle = screen width x 1.36 Viewing distance, 30-degreee viewing angle = screen width x 1.86
Screen diagonal x 0.87 = screen width (16:9 screen) Screen diagonal x 0.49 = screen height (16:9 screen)
Screen diagonal x 0.92 = screen width (2.35:1 screen) Screen diagonal x 0.39 = screen height (2.35:1 screen)
You can confirm these formulas for yourself if you didn't sleep through trig class. There will be a pop quiz on Monday.