Re: What is the shutter speed of the human eye/brain?

Kevin – thanks for a great question! In fact it’s more than one question, 
and raises many thought-provoking issues, from simple optical 
considerations to much deeper psychological ones. 
You first ask “What is the shutter speed of the human eye/brain?”. 
Forgetting the brain for a moment, let’s consider the comparison between 
the eye and a camera, where shutter speed is easy to understand. In a 
still camera a single “snapshot” of the scene relies on an exposure 
determined by aperture and shutter speed. If the scene contains moving 
items, or if the camera is hand-held, shutter speed must be high or open-
time must be short otherwise motion blurring of the item occurs.
While there are many analogies between the eye and a camera, we of course 
do not use our eyes as still cameras. We see movement and usually are not 
aware of motion blur, because we “refresh” the image in our mind very 
frequently. This is what motion picture cameras do, in that they take a 
very rapid sequence of still pictures, which we subsequently view at the 
same frequency, and perceive the result as continuous motion. Of course, 
we are actually seeing a rapid-fire series of still images and in 
projection systems the still images are separated by blank (dark) periods 
which add up to significant proportions of the total time – maybe as much 
as 50%. However we are unaware of this, because they are very short.
So, I think your first question is not really so much to do with the 
analogy of cameras and shutter speeds as with motion picture cameras and 
scene refresh rates.
So why do we perceive continuous motion when a rapid sequence of stills is 
projected? Well I think it is indeed to do with the natural frequency with 
which our brains and eyes combine to refresh our perception of the data 
coming from the eye. Cinematography soon understood that a frequency of 
50Hz was about right to completely eliminate flicker – this means that our 
eye/brain combination refreshes no faster than this – every 1/50th of a 
second.
A lot of people would naturally think of persistence of vision, as an 
important phenomenon in understanding these things – but this is actually 
irrelevant. There are two kinds of persistence of vision. The first - 
negative (e.g. staring for a while at a red spot on a white background 
causes you too see a cyan spot on a blank sheet for a while afterwards) - 
is due to fatigue in the retina and is a simple physiological effect. The 
second – positive – is the eye/brain hanging on to the image after the 
image has disappeared. This latter effect causes us to be unaware of 
blinking most of the time, but has nothing to do with seeing continuous 
motion at the cinema. If we stored the previous image in our minds it 
would overlap with the next one and cause the effect of motion blur.

You also ask “Is there a known speed at which an object cannot be seen? At 
what speed 
does an object appear blurred to the human brain? What processes determine 
this?”
Well, firstly consider a technique for stills photographers to get sharp 
images of fast moving objects - called panning. By sweeping across the 
scene and following the subject photographers skilled at this produce 
sharp images of the subject with a blurred and streaked background. The 
very blurred and streaked background gives the still image a dynamic 
appearance and conveys motion to the eye. It must therefore relate to what 
happens when we pan with our head and eyes, while watching a fast moving 
object go by. So things moving fast across our field of view will get 
blurred at a point where they move faster than the combination of our 
panning and scene refresh-rate can cope with. There isn’t a simple number 
to quote, because when awareness of blur sets in will depend on lighting 
conditions, the size of the object in question or at least its size in our 
field of view which also depends on how far away it is. As for objects 
being invisible or undetectable because of the speed of motion - if you 
are unable to observe a bullet whizzing by – maybe you would be more aware 
if it was the size of a car, but travelling at the same speed. It would 
obscure a given area of the background for longer simply because it 
subtends a larger angle of our view or because it hides a point in the 
background for a longer time . So again, there isn’t a fixed number.

The one number which is more or less fixed, though, is our refresh rate, 
and it prompts me to suggest some simple experiments for you or anyone 
reading this to try out. One involves rotating disks. Imagine half the 
disk being black, the other half white. At what rotational speed do you 
fail any longer to be able to see that it has two separate areas? If you 
divide it into alternate quarters of black and white – what comparative 
result would you expect? What if you subdivide further?
The other experiment involves taking a longish tube – say a cardboard 
mailing tube - about a couple or more feet long. Cover one end with a 
black card in which you have cut a narrow slit – say a couple of mm wide, 
and almost the full diameter of the tube. Looking through from the other 
end, hold the tube so the slit is vertical and begin to swing the tube 
horizontally. See how fast you have to do it before the slit disappears 
and you see more or less the whole scene you are scanning.

As I said at the outset – a great question, and thanks for asking it.