I was just doing a search for some THX related info and can't believe the amount of misinterpretation there is of THXs basic seating recommendations.
Nearly everybody who 'quotes' THX suggest that THX recommend you sit between 26 degrees and 36 degrees, which isn't what they're saying at all. Those viewing angles
are for the back row and beyond. Pretty much where they don't want you to sit in fact
What res is your projector?
1080 source material is similar to what we get in commercial theatres, whether it be film (usually from a 2k res master - Digital Intermediate), or 2040 x 1080 DCI Digital source, so in the home we can apply the same rules that are applied in those theatres.
I often refer to THX recommendations because their information is readily available (see their website
THX.com) and is the standard to which they will build commercial theatres, screening rooms (where the director sees it before sign off) and domestic ones (if you can afford it). SMPTE and THX along with Fox seating distances are shown here (for 2.39 screens). The bold figures are screen height ratios (seating to screen height relationship) and can be applied to all screen ratios since the width is irrelevant:
THX recommend a 40 degree viewing angle for full HD 16:9 displays:
HDTV Set Up « THX.com
http://www.thx.com/files/2009/12/seat-distance-display-setup-400x300.jp
40 degrees is the equivalent to 2.4 x the image height, so whatever your screen height is, if your eyes are 2.4 x that height away from the screen (i.e the screen is 4 feet tall, so you sit 9.6 feet away), you will be at a 40 degree viewing angle. If you had a 2.35:1 screen, the same applies, as around 50 degrees is the angle recommended by THX for 2.35 material. If you stay in the same seat with 40 degrees for 16:9, with 2.35 movies at the same height and wider (as in a correctly implemented commercial set up), you will be at 52 degrees.
THXs 36 degrees is where they suggest the
back row should be. SMPTE 33 degrees is their
furthest recommended. The geometric centre of the seating area in many commercial theatres is 50 degrees.
if you want to replicate where you may sit in a commercial theatre with your home set up, then somewhere between 2 to 4 times your screen height will get you there. For a more immersive experience, then somewhere around 2.4 x IH would be a good start. 2 x IH is approaching the limit where pixel visibility may start to become an issue, depending on the display technology being used and the visual acuity of the viewer.
If a 2.37 screen and Constant Image Height is your aim, then it's recommended you use good anamorphic lens if you want to achieve the above recommendations. Zooming works well, but as you're making pixels 33% larger in both axis, it's like moving your seating 33% closer, so you will have to take that into account.
THX Certified Cinema Screen Placement « THX.com
http://www.thx.com/files/2009/12/thx-certified-cinema-screen-placement-616x462.jpg
CEA and CEDIA appear to have adopted SMPTEs 'reference' seating distance of 3 x IH, so again, anywhere between 2 and 4 times the image height will put you within commercial seating distances, though closer ones may present problems with pixel visibility. You'll need to check before you make a decision.
If your set up is along similar lines to how the director saw the movie before he signed it off, then you know you'll be seeing things close to how he did. You may also want to consider an ISF calibration to get the display to D65 if accuracy is also part of your intent.
However, depending on where you may prefer to sit in a commercial theatre may have an impact on where you'd like to sit at home, but experiment and see what works best for you. If you project onto a wall first, you can see what sizes work best from where you sit and adjust accordingly before buying a screen. Not everyone likes to sit in the same place.
Here's another couple of links to another site with similar info, but more concise:
Home Theater Viewing Angle | The Home Theater Book
http://thehometheaterbook.com/home-theater-viewing-distance/
HTH
Gary