My rule of thumb is that for regular TV, use a 3:1 ratio and for HT, use a 2:1 ratio.
(i.e. If sitting 12 feet from the screen, use a 48 inch (4 feet) for regular TV, and if for HT use a 72 inch (6 feet).
A 3:1 ratio for HT is just not quite like being in the theater, IMHO. Unfortunately, I have a 3:1 ratio at home, (46" TV @ 12 feet) and while it is okay, especially for TV, it is not quite good enough on moives to convey that feeling of being in the theater. But then again, I prefer to sit about 1/4 of the way back from the front seats in the theater, wo we appear to have a different perspective on watching movies.
That being said though, if you're getting a headache from being too close, you should take that into account. My guess is that you should keep to a 3:1 ratio, (at most), which means that your 56" is just a little too big. (Perhaps we should trade your brand new 56" DLP HDTV, for my 4 year old 46" RPTV? No, huh?!)
My two cents worth anyway.
Good Luck in your quest!
I'm by no means an expert in this, but use a THX rule of thumb for 1080P. The width viewing angle should be no more than 36 degrees. That means the sitting distance should be at least 1.6 (approximately) times the width of the screen or you can theorhetically resolve pixels (not with my eyesight however). However, I don't think this has anything to do with the headaches you are experiencing. Some people get headaches from the ghosting on LCD, some from the halos around DLP, others from a bright screen in a dark room.
11 feet is too far away from a 42 inch plasma ( I have one). What might have happened hear is that your eyes needed to converge more than usual for a fixed period of time. The over used muscles needed to fix in the adjusted position relative to your usual screen size would likely cause a headache.
Or the alcohol consumed at the same time casued a hangover.
All theory is just theory and is just a guideline but the headache is yours to suffer. You should not force yourself to accept what other people are saying.
Don't feel ashame if you need a couple more feet to feel comfortable and DON'T ask the front row seaters for their advise.
You should make your own judgement by trying at your friends place. The only cost may be just pop corn, pop or wine depanding on the time of visit.
Or you will get used to it. I had 12' distance from a 42" Panny Plasma. At first, I went "Wow" with my head tilted back to increase further distance. Over time, I get used to it. Now I think that it is too small and wish that I had a 50" instead.
I have a 50" Plasma at just over 10 ft. and at first it seemed huge but now we are all used to it and it seems just right. I think 56 would be great at 11'6".
The headaches may not be due to the screen being big. Generally, the headaches are a result of getting used to another stimulus, whether it be the "sudden" change in screen size (going to a much bigger screen or a smaller screen than what you are used to), a brightly lit screen in a dark room (which you might not be used to), or some type of display artifact (plasma, LCD, and DLP each have their own types of artifacts). Sometimes you get acclimated to the change after time, and sometimes you don't (could you ever get used to listening to a "bright" system?).
The "accepted" values ave already been posted. 3:1 for TV viewing, and 2:1 for movie viewing. You could even go a step further and use 1.5:1 for high definition movie viewing (720p, 1080i, 1080p). But 2:1 or 1.85:1 is a nice distance to shoot for with movies.
But I can help but comment on something here. I do see a number of audiophiles seem to take pride in bucking the trend with videophiles and shooting for longer viewing distances and smaller screens, and being proud of that fact. That strikes me as odd. It's almost like a reverse snob thing. But then again one also sees similar parallels with non-audiophiles. One complaint from that side is that audiophiles listen to music "too loud." That's not meant to be overtly conclusive, but rather just a passing observation.
My point of reference.
-In the theater I sit 1.5 times the screen width back from the screen (about row 5 or 6).
-At home, I sit 1.85 times the screen width from a 800x600 front projector
-watching TV at home, I sit about 4:1 viewing distance (bedroom). it was about 3:1 when we had a bigger TV, which was sold for financial reasons.
-I listen to classical music between 60-72dB. And rock 68-76dB.
It really seems odd that a 42 inch at 11 feet, which is over 3.5:1 viewing distance, caused you that much problems. Your 34 inch at 11 feet is about 4.5:1. If anything, I'd think it was due to some other stimulus (alcohol, food, unfamilar setting, poor source material, display or display technology artifacts, too bright, too dark, etc). Or just the fact it was a sudden screen change in side (if that's it, you will get used to it after a while). Or maybe just laugh all the way to bank with a smaller screen. I dunno.
About 1.5 screen widths is required to achieve an acceptable sense of immersion. This is no bigger than the subtended field of vision in the farthest seat from the screen within a decent THX certified commercial theater.
I run a 9" CRT projector (no visible projection artifacts) on an 87x49" screen (100" 16:9 diagonal) with seating at 11'. This is small for nice DVD scope transfers, a little large for lower aspect ratios, and the best compromise for a standard screen shape. I'm leaning towards a change to 101x43" constant height.
Also note that it is impossible to visually resolve 1080 line HD sitting beyond 3.3 screen heights (about 1.8 widths).
Re KJL: I doubt screen size had anything to do with the headache, but an obvious answer is to watch another movie at the friend's without ingesting anything.
We sit 92" from a Sony 36 XBR as a compromise set by an equalateral triangle determined by the separation of the front LR speakers. That's a ratio of 2.5, a bit too far away to be immursed in the picture. Although that's a near field listening configuration for stereo, I prefer listening at my desk chair, a bit over 11' from the center line of the speakers. An obvious solution for movies and HDTV is front projection, but I need to figure out a way to accomodate TV without running through expensive projector bulbs or sitting in a dark room.
As others have responded, light, flickering or sudden changes in light to dark can stimulate headaches. Also keeping your eyes focused on a single point for extended periods that are different than your usual focal length can do the same.
With respect to screen size, use your own subjective experience and taste. Going by the numbers is a good place to start, but you may find a smaller screen to be more comfortable if you regularly experience headaches. Room decor can also be a big difference. If a 42" screen is mounted on a dark background, you may get the same immersion as a larger screen on a light-color wall. I recently made this change, and the difference was spectacular.
Plasmas can be very bright. That can be the cause of eye strain, particularly if you are not used to the brightness.
It is possible to cause more eye strain from sitting farther, not closer, because the bright spot (the picture) in a small part of your field of view can cause eye strain. Your brain averages the light level in determining how wide to open the iris. A small screen makes that average low, causing the iris to open up and this makes the bright spot (the screen) seem even more intense. That is why it is recommended, particularly for smaller sets, that a light source be located BEHIND the set to reduce the apparent intensity of the set's brightness.
Check out this thread in reference to lighting: http://www.dvdtalk.com/forum/printthread.php?t=408684 Hope this helps!
One additional word of caution. If you are sensitive and subject to eye strain from watching tv, be particularly cautious about DLP. A lot of people report becoming fatigued by viewing DLP sets using a single chip (all rear projection sets and, generally, front projection sets under $30,000). If you are considering such sets you need to spend some time viewing a friends set to be sure you can live with them.