PC World stated in a review that unless you are sitting very close to a TV most people would not be able to tell the difference between a 720p and a 1080 television image.
Given the same screen height, measured from the bottom edge of the lowest row of pixels to the top edge of the highest row of pixels:
Compared to NTSC, 720p offers 2.25 times the pixel density.
Compared to 720p, 1080p offers 2.25 times the pixel density.
A viewer must sit 1.5 times as far from an NTSC screen (displaying 4:3 aspect ratio video) as they need to sit from a 720p screen in order to perceive similar resolution from the two screens.
A viewer must sit 1.5 times as far away from a 720p screen as they need to sit from a 1080p screen to perceive the same resolution from the two screens.
If we, instead, hold the viewing distance constant, and vary the screen size:
A 720p HDTV (16:9 aspect ratio) with a screen size of about 1.8 times that of an NTSC (4:3 aspect ratio) will appear to have the same resolution as the NTSC television.
A 1080p HDTV with a screen size of 1.5 times that of a 720p HDTV will appear to have the same resolution as the 720p HDTV.
In other words, a 32 inch NTSC television, a 59 inch 720p HDTV and an 88 inch 1080p HDTV will all appear to have approximately the same resolution when viewed from the same distance. Obviously, when viewed from the same distance, the 720p screen will appear much bigger than the NTSC screen, and the 1080p screen will appear much bigger than the 720p.
(Note that all sizes are based on viewable size, measured from the top left corner of the top left-most pixel to the bottom right corner of the bottom right-most pixel. Advertised screen sizes may differ slightly.)
Most people
will be able to notice the higher resolution of 1080p compared to 720p. The difference between the two is similar to, but greater than the difference between a 2 Megapixel camera and a 1 Megapixel camera.