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Medical Forum / General / Vision / July 2005How Does Pupil Size Affect Refraction?
I have read that lage pupil size aids near vision (accomodation) and that narrow pupils aid distance vision. iam myopic and I have at baseline large pupils. This is independent of amount of light. In fact I'm sensitive to sunlight i think because my pupil tends to stay large even in brigtness.What is the optics of this. Is there a refraction effect independent of the lens. Is this just the pinhole effect. Is there a reference somewhere, I'm not an optometrist but i think I can follow a fairly technical explanation. Why do contacts (RGP) make me even more ligt sensitive? Mike Samsel >I have read that lage pupil size aids near vision (accomodation) and that >narrow pupils aid distance vision. Then you've been reading the wrong stuff. Normal pupils all constrict when you try to accommodate. The effect of pupil size is exactly the same as what a photographer calls "depth-of-field." Large pupils make measuring refraction more accurate because the depth-of-field is shorter. Small pupils make near _and_ far vision better because depth-of-field is greater. > iam myopic and I have at baseline large pupils. Myopes have larger pupils. Young people have larger pupils. > This is independent of amount of light. In fact I'm sensitive to sunlight > i think because my pupil tends to stay large even in brigtness.What is the > optics of this. More light = more glare (a vision problem, not physical pain), more pupillary constriction and for some, a tight feeling in the forehead (physical pain). There's also facial squinting and discomfort that are partly emotional. > Is there a refraction effect independent of the lens. Dunno whut ya mean. > Is this just the pinhole effect. Yes, a pinhole has the optimum depth-of-field. Every image improves as the aperture is decreased to 3/4 mm. Smaller than 3/4 mm, the image starts degrading again. > Is there a reference somewhere, I'm not an optometrist but i think I can > follow a fairly technical explanation. Google for ray-traces of pinhole optics and the "camera obscura." It becomes pretty obvious just by geometry. Light from one point on a tree (the "object") passes through a hole in the wall and casts a "blur circle" on the far wall. As the pinhole gets smaller, the "blur circles" get smaller until you can resolve a pretty good image on the wall. > Why do contacts (RGP) make me even more ligt sensitive? Usually that's different. RGPs rub your cornea constantly and that often sets up a low-grade but irritating sensitivity in the _iris._ Bright light makes the iris contract and it hurts. Often this is a warning sign of a poor fit, but many GP wearers tolerate a small amount of light sensitivity. -MT, OD If smaller pupils equal better vision (well better visual acuity) near and far, and myopes have large pupils, it seems desireable to reduce pupil size on average if one has larger pupils. What are the mechanisms/techniques that can do this?? Sunning the eyes? Decreasing anxiety?? Something else.?? What is the mechanism by which myopes pupils become/stay larger? Is it a response to blur or the cause (partial) of blur? Mike >>I have read that lage pupil size aids near vision (accomodation) and that >>narrow pupils aid distance vision. [quoted text clipped - 52 lines] > > -MT, OD > If smaller pupils equal better vision (well better visual acuity) near and > far, and myopes have large pupils, it seems desireable to reduce pupil > size on average if one has larger pupils. If the eye is "normal" (no refractive error), large pupils aren't much of a problem. In an evolutionary sense, they seem to be advantageous in attracting mates. That's why women in Rome put deadly nightshade sap in their eyes. If there is refractive error, it can be "compensated" by shrinking the pupil. That would work, in bright light, but it's still blurry in dim light. So we usually take the approach of correcting the refractive error instead. > What are the mechanisms/techniques that can do this?? Sunning the eyes? > Decreasing anxiety?? Something else.?? I don't know any behavioral techniques for shrinking pupils. Pilocarpine drops work great but there's still a problem seeing at night, and often a headache too. > What is the mechanism by which myopes pupils become/stay larger? Is it a > response to blur or the cause (partial) of blur? They don't "become" larger; they start out that way. Over time the constrictor muscles in the iris become weaker, especially in the elderly. -MT > If smaller pupils equal better vision (well better visual acuity) near and > far, and myopes have large pupils, it seems desireable to reduce pupil [quoted text clipped - 3 lines] > What is the mechanism by which myopes pupils become/stay larger? Is it a > response to blur or the cause (partial) of blur? Pupils become smaller with age and are larger with emotions (good and bad -- ie happiness, excitement, anxiety and fear all have pupil enlarging effects). I would suggest that myopes don't have "larger" pupils but that hyperopes have "smaller" pupils due to the accommodation that they exert 100% of the time. The pupil constricts with accommodation. Since 75% of people are slightly hyperopic, they would skew the average towards smaller. There isn't much you can do to affect your pupil size, outside of various drugs. Dr Judy > Mike > [quoted text clipped - 54 lines] >> >> -MT, OD |
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