Archived posting to the Leica Users Group, 2006/03/31
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]<<? >If it was so importand, the I am sure our eyes would also be super sharp >right up to the edges. ? This isn?t a valid analogy because you?re comparing a mechano-optical lens and a visual system that includes a lens, capture medium and post-sensory cognitive input.? I doubt very much that the elements in any of your camera lenses can change shape through their focal range, or that any of your photos have displayed Sheerer?s phenomenon.? The lenses in human eyes with normal vision *are* sharp right to the edges of the field.? The decreasing acuity and other phenomena we observe when using our own visual system are due to characteristics of parts of the system other than the lenses, including the receptor and nerve cell density in the retina (capture medium) and other neurological effects (post-processing).? ?>> -------- Pretty close. Considered purely as a lens, the optical system of the human eye is not too good. The lens is essentially a separated doublet with a focal length of about 20 mm and a maximum aperature of between f 3.5 and f.4.0. Most of the converging power is due to the spherical shape of the cornea, the lens itself is used primarily to adjust focus. Compensation for defects in corneal shape is the main reason why we wear glasses. As we age and the lens can no longer adjust the focus, we must wear reading glasses to help out nature. The eye's optical system has just about every abberation possible. Spherical abberation, chromatic abberation, astigmatism, etc. are all present. Barrel distortion and curvature of field are significant. Typically the overall resolution of the eye is about 6 to 8 l/mm at a distance of 200 mm. This corresponds to an optical resolution of 120 to 160 l/mm on the retina. The theoretical resolution of a lens corresponding to the specifications of the eye is about 400 l/mm. Why, then is human visual performance so good? First, the eye is NOT a camera. It is just the sensor of an enormously complicate d image processing system. The field of view covered by the eye's optical system in normal daylight vistion is just a couple of degrees. The macula, the high resolution color sensitive area of the retina, is very small, subtending an angle about the size of your thumbnail at arm's length. The retina is curved to compensate for curvature of field. It's just like the curved film plane of the Minox camera - whose optical system was designed to emulate that of the eye. Second, a complex image processing process rectifies most optical errors and creates our percepts out of the relatively crude optical image delivered to the retina. It's like a Photoshop expert correcting the image produced by a box camera. Most of the eye's optical abberations are corrected in software. The sharp "edge to edge" visual image we perceive is built up of a number of the narrow angle views sensed by the retina as the eye scans the scene. We remember the images, store them, then reconstruct the scene i n our minds. A number of the "corrections" have nothing to do with the image picked up by the eye. We know that doors, walls and windows are rectangles so the we compensate for the barrel distortion of the eye and perceive straight lines. We adjust the apparent size of objects to compensate for distance. We fill in the "blind" spots with a pattern like the surrounding area. We compensate for perspective distortion as we move closer to objects. This perspective correction accounts for the attention getting effect of extreme wide angle and telephoto lenses. We don't see the perspective exaggeration naturally but are forced to recognize it when presented in a photograph at a normal visual distance. Finally, we configure the final perception as we want it to be. Our needs, desires, experience, and expectations all enter into our perceptions. What I perceive in a picture posted on the LUG may not be quite what you see. We all have different life experiences which enter into the percept. Interpretation of images from other cultures is particularly difficult since we lack the "local knowledge" necessary for proper understanding. In fact it is possible to have a strong perception without any sensation at all. Just ask anyone who has had a religious vision. When I was young there was a popular song whose lyrics went: "As I sat upon the stair, I saw a little man whi wasn't there. He wasn't there again today. Oh how I wish he'd go away." So don't compare the eye to a camera. It's not. It is one of the the most complex systems in nature, a visual sensor connected to a flesh and blood computer of exceptional capability. Larry Z