Recently, I got an e-mail from Jim Stewart in Davis, Calif.: Bob, I wear glasses for nearsightedness and astigmatism and I'm 51 years old. For the last 15 years or so I've had problems with driving at night. My father and my brother-in-law also have had the same problem, so I don't think it's just me. Trips to the optometrist only got me the reply that I was getting old. Being both an engineer and serious amateur photographer, I know something about optics and I was convinced that there was a solution. (You are correct to be skeptical. You must have seen my previous columns on "Doctoring" \[Electronic Design, Sept. 30, 2002, p. 110, and Oct. 14, 2002, p. 86\]. Don't JUST trust the darned doctors! /rap)
I noticed that the lenses in my glasses were selected as a compromise between near and far vision, with the compromise assuming lots of light. "Lots of light" implies that the iris is "stopped down," giving a lot of depth of field. The problem with this is that at night, my iris is wide open with very little depth of field, and I couldn't focus well on anything of any distance away.
I went to a new optometrist and told him that I'd like him to make me a set of glasses for night driving and that I'd tell him exactly how to make them. He agreed. After he examined my eyes and gave me a prescription for new normal glasses, I had him prescribe a pair that were set for good distance vision plus another 1/2 diopter of sphere correction towards infinity.
The glasses worked better than I ever expected. Just the joy of going outside at night and seeing the stars as pinpoints of light instead of blobs would make it worth the cost. Night driving is no longer a dreaded chore for me. I still have a little problem with headlight glare, but it's minor now that I can focus clearly.
Jim (via e-mail)
As soon as I read this, I recognized instantly that this is a perfect continuation of my theme from Doctoring Stuff, Parts 1 and 2, as referenced above: Do NOT just let your doctor tell you what is good for you. Listen to what he (or she) says, and ask questions. THINK. In this case, Jim's first optometrist was not interested in communicating. He was assuming that a compromise lens prescription would make the patient happy. He didn't want to hear any dissent. So, Jim had to go out and find another doctor who could help him avoid the unacceptable performance of a compromise. Wise. Great engineering! Now Jim can drive cheerfully and much more safely at night.
I'm lucky that at 63 my eyesight at night is still almost perfect. But I know several people who have complaints such as this, and they may have the exact same problem, subject to the exact same solution. And we will check it out. Is 0.5 diopters the best correct amount of compensation? Quite likely it is really close, but we are still checking into this. Most optometrists just have a mirror so a 20-ft viewing distance can be folded into a 12-ft office. Are they prepared to test your eyes for proper focus at 200 ft? In the dark? White letters on a dark field? Ahem....
So if you have trouble focussing on what is going on at night, and your lenses for nearsightedness are suspect, consider this outside-the-box solution: Get a pair of "night-driving glasses" that are optimized for best focus at 200 ft rather than at 20 ft, and you may be much happier and safer. (Contact lenses likewise? I'll guess so!) And, keep on thinking!
Comments invited! [email protected] —or:
Mail Stop D2597A, National Semiconductor
P.O. Box 58090, Santa Clara, CA 95052-8090
