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Message: <blockquote data-quote="WJC" data-source="post: 3584370" data-attributes="member: 25191">19 “IT’S ‘TACK SHARP’ FROM THE CENTER TO THE EDGE.”More than one knowledgeable observer has praised his binocular for providing “tack sharp” images from the center to the edge of the field. However, those who make such claims don’t have the capacity to observe images on axis and at the edge of the field in the same instant. The observer may see a “crystal clear” image in the center of the field, then, on seeing an equally crisp image at the edge of that field, supposes all points in between must provide images of equal sharpness, all the time. But to the brain, it just isn’t so. A Reality CheckConcentrate on the tiny space between “image” and “in” in the preceding paragraph. If you’re unwavering in your concentration, you’ll notice the most you can make out are those two words; some people can’t even do that. Now, with your eyes frozen on that tiny space, try to see the “r” in “clear” or the “c” in “center.” Although the separation in this example is far less than one would find in a binocular’s field of view, it’s plain to see our sharply focused field is very restricted, not by the physical optics of the binocular but by physiological optics controlled by the brain. An observer with a high quality instrument can easily come away with the wrong conclusion because he’s not paying attention to the millisecond eye movements that place the edge of the binocular’s field of view in the center of his own or the slight dioptric accommodation that accompanies that movement. A Talk with an OptometristWanting to be certain not to share more than I know about this subject, I called on optometrist, Dr. Edward R. Ford, of Ford Family Eye Care in Twin Falls, Idaho who offered the following:“The corneal thickness centrally is thinner than it is peripherally, thus causing a change in refractive error from the eye’s central line of sight to its peripheral line of sight. Now, aside from the optics of the cornea, aqueous, pupil, lens and vitreous not being perfect, perhaps the largest issue with the eye’s peripheral vision lies at the level of the retina.*As you know, the retina consists of photoreceptors called rods and cones. The rods provide light sensitivity and motion detection, while the cones provide the detail and color vision. The center of the retina, known as the macula, consists of the highest concentration of cones while the peripheral retina has the higher concentration of rods. With that in mind, it makes sense that the central retina or macula gives us the sharpest acuity, while the peripheral vision does not.”This explains the value of the millisecond eye movement and dioptric accommodation that most observers never consider.*:cat:Just a thought,Bill</blockquote>

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