The ISO of the human eye

According to Clarkvision the ISO of the human eye adapted to night is 800 whilst that of the human eye in daylight it is 600 times less sensitive which would equate to an ISO of 1.

Here's what he says:

"At low light levels, the human eye integrates up to about 15 seconds (Blackwell, J. Opt. Society America, v 36, p624-643, 1946). The ISO changes with light level by increasing rhodopsin in the retina. This process takes a half hour our so to complete, and that assumes you haven't been exposed to bright sunlight during the day. Assuming you wear sunglasses and dark adapt well, You can see pretty faint stars away from a city. Based on that a reasonable estimate of the dark adapted eye can be done.

In a test exposure I did with a Canon 10D and 5-inch aperture lens, the DSLR can record magnitude 14 stars in 12 seconds at ISO 400. You can see magnitude 14 stars in a few seconds with the same aperture lens. (Clark, R.N., Visual Astronomy of the Deep Sky, Cambridge U. Press and Sky Publishing, 355 pages, Cambridge, 1990.)

So I would estimate the dark adapted eye to be about ISO 800.

Note that at ISO 800 on a 10D, the gain is 2.7 electrons/pixel (reference: http://clarkvision.com/imagedetail/digital.signal.to.noise) which would be similar to the eye being able to see a couple of photons for a detection.

During the day, the eye is much less sensitive, over 600 times less (Middleton, Vision Through the Atmosphere, U. Toronto Press, Toronto, 1958), which would put the ISO equivalent at about 1. "



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