Hey. I did abit of research about the eye for my science project, so this might help.
Optics
According to the
Standardised Human Eye website which is extremely cheem and has details on just about everything mechanical about the eye, the iris opening of the eye is 7.0mm+ in bright daylight and 2.0mm- in dark conditions. Given that the focal length is about 17mm for most people, eye aperture is about 7/17 =
~f/2.5 in dark conditions, and
f/8.5 in bright conditions.
In comparison,
the cat's eye has a max aperture of f/0.6-0.8 at night. Of course, it's optimised for speed over resolution, as the cat's eye doesn't have a fovea (the central area of extremely sharp vision).
Sensor system
As for ISO Sensitivity, that's a function of the retina and not the lens, but here goes. There are 4 pigments present in the eye of the Rhodopsin family, R-5,7,9 and 11 and these are sensitive to red, green, blue and UV(!) light respectively. We don't see UV because our lens blocks it, but for those who have had cataract operation, you should be able to see UV as a purple-grey.
The gain of the eye is about 3500:1 electron
hoton when fully dark adapted. To translate to ISO i'm not sure, but I've had to use ISO 3200 30 seconds f/2.8 to make a dark image of my room just as my eyes saw it, even though I could see fairly clearly.
The spatial resolution of the fovea is 45 linepairs/mm, and the temporal (time based) resolution is about 45 hertz.
Transmission and First-level image processing
The eye doesn't act like a conventional photodetector system, which integrates the input over a certain time and outputs a value. Rather,
it operates on a difference system, where if two points compared have a different value, the output is proportional to the difference. Also, the output value decreases to zero with time even with the same amount of difference. This was shown in an experiment where a picture was artifically stablised on a subject's retina, and it disappeared from view in a few seconds.
This difference system can possibly explain why the eye has a
super dynamic range.
The difference in brightness between small adjacent areas is likely to be small, so detail will still be retained. I'm not sure how to articulate this fully here. blehh.
In addition, the equivalent of an unsharp-mask processing occurs in the retina, where edges are enhanced.
Your brain doesn't get the RAW, it gets the processed JPEG :bsmilie:
Movement and Image stabilisation systems
The eye is moved by a set of 3 pairs of muscles which act antagonistically (they work against each other like biceps-triceps). One set moves up/down, one left/right and one oblique (diagonal). There are 4 forms of movement, 2 are conscious, one semiconscious and one unconscious (the IS system).
The first two are Saccades and Smooth pursuit, which are used for target tracking.
Saccades are discrete movements (step-wise) and they occur frequently (3 per second) and are quick (eye movement at 400-700 deg per second). Their purpose is to lock onto a target. For example, if you see something in the corner of your vision, and you want to look at it better, a saccade occurs which turns your eyes towards it.
Smooth pursuit occurs after lock-on, where your eyes follow the object continuously. It's slower at 50-70 deg per second but allows you to view the target better.
The third movement is
Vergence, which is the crossing of your eyes as the object becomes closer. This allows both eyes to focus on the same object at close distances, as well, as
providing distance information. Something like the
(D) system in Minolta/Nikon lens.
Lastly, there is the
Vestibolo-Ocular Reflex (VOR), which is the
Image Stabilisation system in the eye. It works by measuring the acceleration of your head using sensors in your neck and ears, and conunteracting it by turning the eye in the opposite direction. It's essential as someone who loses this function will be unable to read without taping himself to the bed as his heartbeats would blur his vision.
You can find out more about Eye movements
here.
Okae. That's about it.
Hope this helps! =)
QX
