Display Resolution and the Human Eye Have you ever wondered how video resolution numbers correspond to what we really see? There is a simple, mathematical answer. The human eye has its own “resolution” whereby picture details that are below the lower boundaries of visual acuity are lost.
Optometrists use a Snellen chart to assess the “resolution” of eyesight. Everyone knows about 20/20 vision; it really means that a subject can read the line that defines average vision from twenty feet away. The elements on the Snellen chart are analogous to measuring the angular limits of human vision.
Everyone learns in high school geometry class that a circle is 360 degrees. For angles smaller than 1°, we use minutes-of-angle (MOA) as a measurement. "Normal" visual acuity is considered to be the ability to recognize an optotype (letter on the Snellen chart) when it subtends 5 MOA. Our actual “visual resolution” threshold is slightly less than half the limit of acuity. In other words, the average person cannot distinguish two pixels as discrete elements if their angular size is 1 MOA or less. Display resolution below this threshold is imperceptible.
The resolution of a display device is limited, in part, by the device’s pixel size. To determine the size of the picture element of a display, divide the horizontal image size by the horizontal resolution. A typical 16:9 50-inch panel will have a horizontal image about 43.5 inches wide. For a display rated at 720p, divide 43.5 by 1280 (720p = 1280 x 720) and you will find a picture element size of .034 inch. For a display rated at 1080, divide 44 by 1920 to find an element size of .022 inch.
To determine the viewing distance where increased resolution in the display does not add to the viewing experience, S all we do is solve a simple trigonometry problem. Here is a formula that will give you a good starting point: D = /TanV where S is the picture element in inches, TanV is the tangent of .016667 degree (1 MOA) and D is the viewing distance in inches. Using our 1080 example above (D = .022/Tan.01667), we calculate a critical distance of 75.6 inches, or about 6 feet 4 inches. In other words, a typical viewer more than 6 ½ feet from a 50” 1080 display will not see any difference between 1080 and 720 in terms of detail. Other performance parameters will impact the viewing experience, however.
Try this simple calculation on your next project and you’ll know that you’re getting all the resolution the audience can see for your installation dollar.
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