Benefits of Stereo Vision You Didn’t Know About
Almost everyone is seeing in 3d stereo vision; two cameras displaced horizontally which give slightly differing perspectives of the scene. Our brain (specifically the occipital lobe) overlays the two live stream images, and uses the minor differences in perspective to determine depth and motion of the objects around us. Our vision is tremendously amazing when you think about it; the volume of visual information that our brain handles, live capture, overlay, and 3d rendering of images. The versatility and quality of human vision in many ways trump any modern human made cameras.
And there are even more advantages to having having stereoscopic vision that first meets the eye. (Don’t worry, i too will probably get tired of that pun quickly)
3D; The Depth Of Field (DOF) replacement.
About a year or two back | read or watched a seminar about what stock agencies were looking for in the current market. During one lady’s part of the presentation she said something to the effect of; “Don’t give me all these pictures with an endless depth of field, my eye doesn’t see focussed from 1 meter to infinity” Although she isn’t wrong, she wasn’t quite correct.
Close one of your eyes, hold your hand out a full length and focus on your finger, then something 10 meters away, then the horizon. (yes, get some exercise and move away from your computer) Pay attention to how sharp the areas are which you aren’t focussed on.
Play with that for a while, although its not sharp from front to back you can still see a lot. Its defiantly not a short depth of field like a aperture 1.4 lens focussed at 2 meters with a silky background.
Now open both eyes and do the same thing, paying attention to what you see in the areas you aren’t focussed on.
When you see with both eyes in stereo vision the background is much less discernible. Each eye’s depth of field hasn’t changed, instead the brain is receiving conflicting information from each eye about what lays in front of or behind the subject you are looking at. The image from your left eye has one background for your subject, while your right eye has a slightly different background for the subject. When your brain overlays these conflicting background (or foreground) images the result is a single image which includes both backgrounds at the same time. Since the images are unaligned it makes a messy unsharp background, a DOF from stereo vision. Your brain can still discern what is in front or behind, however its not sharp, and it isn’t distracting.
When that lady told the audience that she didn’t see detail front to back she was right, however it is because of stereoscopic DOF, not so much one eye’s aperture influenced DOF.
When you view the world with two eyes the DOF created by stereo vision makes the subject much more dominant, because everything around it isn’t sharp. However cameras don’t shoot in stereo, so subjects are much more susceptible to blending into the background, or generally having less impact. We do have ways to compensate; a fast lens has the capability for a short DOF -wide aperture. We can also learn lighting skills and angle choice which make the subject brighter than the background, which helps the subject stand out in the image.
Selective vision
When the information from one eye is not as useful as the information from the other your brain will use the information from the better eye to view the subject. Take your hand and cover one eye -don’t close it- then keep covering and uncovering that eye, you should still be able to read this text. If your brain took the information at 50/50 from each eye it would become much more difficult to read this text with a hand in front of one eye. Instead your brain is choosing the information from your uncovered eye and ignoring the other useless information.
One of the best applications for this property of your vision, is seeing reflective surfaces. If you are looking at something reflective your brain have two differing perspectives of the subject for your eye to chose from. In this way the eye is able to reduce the effect of reflections by choosing the best information from each eye to build the best image it can. When photographing scenes with water -especially near dusk- your camera will pick up much more bright reflected light compared to your vision which is showing more of the water’s colour. Or, if you are looking through glass you can have the same issue, walking outside looking into shop windows there will be a lot of reflections, however you stereo vision should be able to compensate.
When photographic reflective surfaces take note that your camera will be seeing a much more accurate -more reflective- version of what you are seeing. Adjusting the way you shoot to exploit this discrepancy can result in amazing images, or to compensate you can use a polarizing filter which willhelp to remove a lot of unwanted reflection. If you go the route of a polarizer make sure you get a circular -spinable- polarizer, and learn how the spinning motion effects the light.
Not all of your vision is 3d:
Close your right eye and keep looking forward, you lost about 1/3rd of the FOV, now do the same for the opposite eye. Only the area which is seen by both eyes is 3d; your peripheral vision isn’t covered by two perspectives, and therefore can’t be 3d. Although this doesn’t have any important photographic application its still pretty interesting. Cool eh?
-Click for the Introduction to, and Directory for; Foveas and Photographs series
Alright, here’s something weird.
I close one eye, hold a finger approximately 30 cm in front of my face. I then focus on the other end of the room…
What happens?
Well, according to what you might think, and what Reuben is suggesting I see the finger out of focus when focused at a longer distance. What I noticed though. That finger appeared (at least to me) to be more out of focus in the horizontal plane than in the vertical plane. Now each eye is circularly symmetrical in the vertical and horizontal planes, or at least to a very good first approximation. That means there shouldn’t be a difference in horizontal and vertical focusing of one eye, but because the brain normally processes images with the help of it’s stereo partner, perhaps when it looses that help there’s a difference in it’s image-processing power. The visual cortex is perhaps better prepared (probably, walking around with one eye closed a lot would un-train this) to deal with stereo vision than non-stereo vision.
Here’s what I propose… We’ve got a visual cortex that is trained to process images in stereo and it’s so good at it. The neural pathways and shape and image reconstruction algorithms running in our brains are trained to do the correction in the vertical with a lesser degree of stereo-provided help. When we loose the input to the stereo-helper, those neural pathways aren’t trained as strongly to operate the same way. Who knows the exact source of this confusion, perhaps the brain has some measure of the expected depth of that finger, and the focal point, it expects to see it doubled side to side but then doesn’t. Perhaps it’s shape forming capabilities are based on a reliable subset of rods and cones that isn’t uniformly distributed across the two retinas. The gist of it is this: What that visual cortex is well-trained to process, it doesn’t get. It’s interesting to note then, the effects of how the brain deals with this incomplete information, I’m sure it could be trained otherwise, but unless I’m going to try to make a living from what I can see this isn’t really a worthwhile investment of my time.
It brings to mind this clip from an excellent podcast. Start at about 16 minutes, and listen until at least the end of the section on phantom limbs. http://rkp.me/phantomlimb Of course I’d recommend listening to the whole show.