|MadSci Network: Neuroscience|
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.
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