From the Opticians Handbook website...... Is The Human Eye Well Designed? By Ari Siletz Tuesday, August 27, 2013 7:43 PM During courses in lens design one sometimes hears that the human eye is a poor performer when it comes to image quality. Employing a wide angle lens design the eye can resolve one minute of arc near its optical axis, whereas a similar commercially produced wide angle lens corrected for high order aberrations can deliver almost four times this resolution. Is the eye poorly designed, or is there more to visual acuity (VA) than just lens design? The latter seems more likely because retinal image quality metrics for high order lens aberrations—such as coma—arent good predictors of visual acuity. For example researchers at Murcia University in Spain measured wavefront aberrations in the eyes of 60 subjects with normal to excellent visual acuity (20/20 to 20/10) to see if the eyes of subjects with excellent visual acuity showed less high order aberration than those with normal vision. The measurements were made after defocus and astigmatism were carefully corrected. Conclusion: No significant correlations were found between VA measurements and the optical quality of the eye in young subjects with normal or excellent spatial vision. Some subjects with normal degrees of aberrations attained excellent VA. It isnt clear yet why visual acuity in the healthy eye is not well correlated with high order lens aberrations. Perhaps stray light management in the design of the eye needs to be factored in, or the density of light receptors in the individual plays a role, or neural image processing mitigates the effects of high order aberrations. The point is that assessing visual acuity strictly from a lens design perspective can be misleading. To be sure, a portion of the performance of any vision system—biological or artificial—depends on the optics; but the systems camera sensor, the electronics, and the software are of primary importance in assessing the overall performance of the system. Figure 1, from nasa.gov Lets explore the overall design efficiency of the eye by including its retina in our engineering assessment. The retina is where the image is formed on millions of light sensing cells and some preliminary image processing takes place. In artificial vision systems, say industrial inspection devices, the resolution of the optics and resolution of the cameras pixels are selected to match. It is wasteful to use a high resolution camera with a poor quality lens that can‘t deliver a high resolution image. In the human eye, the retinas sensor density matches the quality of the lens system only at the fovea, a tiny region close to the center of the visual field comprising 2 degrees of the eyes almost 180 field of view (Figure 1). Visual acuity degrades rapidly with distance from the fovea. Interestingly, this is not due to poorer off-axis image quality of the optics—optical resolution stays pretty much constant around the fovea over an angular range of 40 degrees (figure 2). Acuity degrades because the eyes high resolution color sensing cells, called cones, are rapidly replaced in the retina by low resolution gray level sensing cells called rods. Even though the image quality of the lens can handle a higher density of cones in areas outside the fovea, the eyes design doesnt bother to populate the region with cones (figure 3). This seems like a big design flaw. Why doesnt the eye take advantage of the relatively good image quality outside the fovea by putting more cones there? In defense of the eyes design, a good reason is that the rods, being much more sensitive to light than cones, allow us to see where it is too dark for cones, say under moonlight. A less obvious reason may have to do with image processing speed: low resolution images take fewer computations to analyze, whereas a sensor flooded with high resolution data across its entire surface could become an image processing bottleneck. To illustrate, hitting a tennis ball needs somewhat high visual acuity when the ball is far away and appears small. As the ball gets closer, however, quick updates on the location of the ball relative to the body (kinesthetic awareness) becomes a priority over detecting fine features. The eye easily accommodates this rapid change in sensor-resolution versus processing-speed requirement. With flexibility in mind, the retinas sensor layout makes good sense from a vision system engineering perspective: instead of maintaining a constant resolution, the retina has a resolution gradient across its surface to handle a range of situations each needing different trade-offs between visual acuity and image analysis speed. So, is the eye well designed? If it were just about image quality it is a mediocre performer. But if we ask whether the eye is well engineered as part of a very flexible vision system, the jury is out pending more research because it may turn out that our eyes are superbly optimized for how we use them. - See more at: opticianshandbook/eyes/anatomy/article/is-the-human-eye-well-designed/#sthash.uRkQufm9.dpuf
Posted on: Sat, 30 Nov 2013 19:09:00 +0000
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