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Tip #733: How Much Resolution is Too Much?
The eye sees angles, not pixels.
This article, written by Phil Platt in 2010 discussing how the human eye perceives image resolution, first appeared in Discovery.com. The entire article is worth reading. Here are the highlights.
As it happens, I know a thing or two about resolution, having spent a few years calibrating a camera on board Hubble, the space telescope.
The ability to see two sources very close together is called resolution. It’s measured as an angle, like in degrees. For example, the Hubble Space Telescope has a resolution of about 0.00003 degrees. That’s a tiny angle!
Since we measure resolution as an angle, we can translate that into a separation in, say, inches at certain distance. A 1-foot ruler at a distance of about 57 feet (19 yards) would appear to be 1 degree across (about twice the size of the full Moon). If your eyes had a resolution of 1 degree, then the ruler would just appear to you as a dot.
What is the resolution of a human eye, then? Well, it varies from person to person, of course. If you had perfect vision, your resolution would be about 0.6 arcminutes, where there are 60 arcmin to a degree (for comparison, the full Moon on the sky is about 1/2 a degree or 30 arcmin across).
To reuse the ruler example above, and using 0.6 arcmin for the eye’s resolution, the 1-foot ruler would have to be 5730 feet (1.1 miles) away to appear as a dot to your eye. Anything closer and you’d see it as elongated (what astronomers call “an extended object”), and farther away it’s a dot. In other words, more than that distance and it’s unresolved, closer than that and it’s resolved.
This is true for any object: if it’s more than 5730 times its own length away from you, it’s a dot. A quarter is about an inch across. If it were more than 5730 inches way, it would look like a dot to your eye.
But most of us don’t have perfect vision or perfect eyesight. A better number for a typical person is more like 1 arcmin resolution, not 0.6. In fact, Wikipedia lists 20/20 vision as being 1 arcmin, so there you go.
[Phil then summarizes:] The iPhone4 has a resolution of 326 ppi (pixels per inch). …The density of pixels in the iPhone 4 [when viewed at a distance of 12 inches] is safely higher than can be resolved by the normal eye, but lower than what can be resolved by someone with perfect vision.
LARRY’S EDITORIAL COMMENT
There’s a lot of discussion today about the value of 8K images. Current research shows that we need to sit within 7 feet (220 cm) of a 55″ HD image to see individual pixels. That converts to 1.8 feet to see individual the pixels in a UHD image. And 5 inches to see individual pixels in an 8K image on a 55 monitor.
Any distance farther and individual pixels can’t be distinguished.