The same TV, two seats
Each angle turns into a multiple of the screen diagonal: 1.2 diagonals for 40 degrees and 1.6 for 30. For three common TV sizes:
- 55 inch: 5.5 feet (1.7 meters) for THX, 7.5 feet (2.3 meters) for SMPTE
- 65 inch: 6.5 feet (2.0 meters) for THX, 8.8 feet (2.7 meters) for SMPTE
- 77 inch: 7.7 feet (2.3 meters) for THX, 10.4 feet (3.2 meters) for SMPTE
The SMPTE seat is always farther back, so the same TV fills a smaller part of your view from there. A seat anywhere between the two numbers gives a view between 30 and 40 degrees.
Where the two angles come from
THX presented its 40 degrees at the 2006 CES show. It was described as the theoretical maximum horizontal view angle, based on average human vision, and THX treats it as the best seat for an immersive, cinematic picture.
The SMPTE number is older and murkier. EG 18-1994, Design of Effective Cine Theaters, used 30 degrees for movie theaters, and SMPTE withdrew the document in 2003. There seems to be no direct SMPTE recommendation for home TVs, so the 30 degree figure is widely quoted for living rooms without being a current standard. Treat both angles as references, not rules.
Which angle fits your resolution
Pixels decide how close you can sit before the picture breaks up. A 65-inch 1080p TV has a Retina distance of 101 inches (258 cm), and a 65-inch 4K TV has one of 51 inches (129 cm).
The SMPTE seat of 8.8 feet (2.7 meters) is past the 1080p limit, so 1080p still looks smooth from there. The THX seat of 6.5 feet (2.0 meters) is inside it, so 1080p pixels can start to show and 4K earns its keep. One home theater guide puts it the same way: 2K is great at 30 degrees, and 4K is worth it at 40.
Working backward from your couch
If the couch is fixed, start from the seat. At 8 feet (2.4 meters), the diagonal that fills a 40 degree view is 80 inches (204 cm), and the one that fills 30 degrees is 59 inches (150 cm). A TV between those two sizes fills a view between 30 and 40 degrees from that seat.
If the TV is fixed, start from the TV and use the table above. The Viewing Distance Visualizer works it out for any size, in feet or meters.