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Tuesday, March 27, 2007

The human eye is equivalent to a 576 megapixel camera... (kottke.org)

The human eye is equivalent to a 576 megapixel camera... (kottke.org)

The human eye is equivalent to a 576 megapixel camera and has a nighttime ISO of 800 (~1 during the daytime).

Notes on the Resolution of the Human Eye

What is the resolution of the human eye, or eye plus brain combination in people? There seems to be a lot of different numbers quoted.

Visual acuity is defined as 1/a where a is the response in x/arc-minute. The problem is that various researchers have defined x to be different things. However, when the different definitions are normalized to the same thing, the results agree. Here is the problem:

Usually a grating test pattern is used, so x is defined as cycles in the pattern. Different researchers have used a line, a line pair, and a full cycle as the definition of x. Thus, they report seemingly different values for the visual acuity and resolution. It is easy to recompute the acuity to a common standard, when the study defines what was used.

So when we define x to be a line pair, as is normally done in modern optics, the 1/a value is 1.7 under good lighting conditions. This was first determined by Konig (1897 [yes that's 1897] in 'Die Abhangigkeit der Sehscharfe von der Beleuchtungsintensitat,' S. B. Akad. Wiss. Berlin, 559-575. Also in: Hecht (1931, 'The Retinal Processes Concerned with Visual Acuity and Color Vision,' Bulletin No. 4 of the Howe Laboratory of Ophthalmology, Harvard Medical School, Cambridge, Mass.) A summary plot of numerous subjects of visual acuity as a function of brightness appears Pirenne (1967, "Vision and the Eye," Chapman and Hall, London, page 132).

The acuity = 1.7 when the light level is greater than about 0.1 Lambert. A Lambert is a unit of luminance equal to 1/pi candela per square centimeter. A candela is one sixtieth the intensity of one square centimeter of a blackbody at the solidification temperature of platinum. A point source of one candela intensity radiates one lumen into a solid angle of one steradian according to the photonics dictionary http://www.photonics.com/dictionary.

The acuity of 1.7 corresponds to 0.59 arc minute PER LINE PAIR. I can find no other research that contradicts this in any way.

Thus, one needs two pixels per line pair, and that means pixel spacing of 0.3 arc-minute!



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