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Thursday September 18, 2008
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One good thing about the Internet is its ability to open up research for all to see. No need to reinvent the wheel when the answers you want have already been unearthed. It's that spirit that brought me to Dr. Roger N. Clark's website. With a PhD in Planetary Science from MIT (like who doesn't have one of those), Dr. Clark still considers himself an "amateur" astronomer. I don't think he'd shy away from being characterized as a dedicated observer, which partially explains his research into "the human eye and how the eye and brain functions in low light conditions."
This is where our paths cross. Dr. Clark's research led him to publish answers for a few questions I've been mulling for years, especially as they relate to optics versus human eye. How do the cameras and lenses we use stack up against what evolution produced? I'll give you a taste of his answers, but if this entices you at all you've really got to dig into his site for the full treatment.
To compare an eye to a camera you've got to first establish the eye's acuity. You're getting an idea of what the eye can see, but in an extremely limited space. That's just a starting point because your eye is a wide angle device and it's not limited in the same ways a camera is.
"The eye is not a single frame snapshot camera. It is more like a video stream. The eye moves rapidly in small angular amounts and continually updates the image in one's brain to "paint" the detail. We also have two eyes, and our brains combine the signals to increase the resolution further. We also typically move our eyes around the scene to gather more information. Because of these factors, the eye plus brain assembles a higher resolution image than possible with the number of photoreceptors in the retina."
Clark estimates a 120° field of view (though in reality it's probably wider) and runs the numbers to estimate your eyesight is the equivalent of a 576 megapixel image!
I was surprised at Clark's estimate of the human eye's greatest sensitivity only being the equivalent of ISO 800--less than most modern cameras. During the day your eye's sensitivity actually ramps itself down to the equivalent of ISO 1. The eye is a whole lot more versatile than anything you can buy.
Though the eye can adapt over time and resolve a bright to dark ratio of 10,000,000:1, it can't do that quickly. The best it can do in real time and under the right circumstances is resolve objects with a contrast ratio of 10,000:1. That's still a pretty impressive number.
Bottom line, if our eyesight was being advertised on eBay, we'd be listed as 25mm, f/3.5 and ISO 800. Humans are high end gear!
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