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Medical Forum / General / Vision / January 2006QUESTION: 'Normal' minimum focal length
Hey, all. I am nearsighted, to the tune of 3.5, corrected by contacts. I was just having a close look at a polished stone I bought, and observed that I lose the ability to focus at about the 15cm mark; anything closer and I can't focus on it. I just wondered if that was a 'normal' distance for such a threshold? Is this distance something that one would expect to increase with age? Just semi-idle curiosity... ;) BD > I am nearsighted, to the tune of 3.5, corrected by contacts. I was just > having a close look at a polished stone I bought, and observed that I > lose the ability to focus at about the 15cm mark; anything closer and I > can't focus on it. I just wondered if that was a 'normal' distance for > such a threshold? Is this distance something that one would expect to > increase with age? Absolutely. But 15 cm is pretty good, for an adult. -MT If you are young then its good. If you are over 25 then its great! I am 23 and theres NO way I can focus to 15cm with full power contacts. Maybe 25cm if that. My sister is only 18 so her accomodation is good, she can focus down to 15cm. Without correction, I can focus as close as 10cm with my -4 pescription. Thats with both eyes open and crossing them and straining. Using both eyes increases the depth of focus and you can cross them for fusion of accomodation. However accomodative amplitude is measured with one eye open at a time. Uncorrected I see fine down to 15cm one eye at a time Thats 6.5 diopters of accomodation minus the 4 diopters of myopia I have. With full power contacts id be looking at 40cm minimum! I can not see fine print with contacts and newspaper print is somewhat blurry unless I hold it arms length. A slight undercorrection makes a huge difference. >If you are over 25 then its great! I'm 37. Guess that's alright, then. I never made a point of seeing how close I could get when I was younger, but I'd be surprised if I was ever able to get much closer than that. If only I could get rid of the damned floaters. ;) If you can still accomodate down to about 8 inches clearly, thats better than average accomodation for a 37 year old. Maybe your contacts slightly undercorrect? I find that as little as half diopter undercorrection makes a significent difference in how well I see from near. How well do you see uncorrected? corrected? How well do you see uncorrected? corrected? Corrected is _pretty_ good for me. Better with glasses. I have slight astigmatism in my left eye, and it was decided that it was mild enough not to sweat it in terms of the contacts. So there's a very subtle difference between right and left with my contacts. Almost unnoticeable. Certainly a 'hit' I'm willing to take to get off the glasses. I figure that if the contacts undercorrected I would experience eye strain, which I don't. Uncorrected is just silly. I'm useless. And I thought _my_ prescription was strong. I have a good friend who's 9.5. Yuck. 9.5 is insane, much more so if contacts due to vertex distance. Your glasses pescription should be -4.5 give or take a half diopter. Different people appear to behaive differently in how much contact power they need vs. glasses. I know someone that sees 20/20 with -3.5 contacts but needs -5 glasses to see perfect. I know another that needs -4 contacts but only -4.5 in glasses. The average is 1.25 meaning you multiply your contacts to arrive at glasses. My -3.25 contacts multiplied by 1.25 come at about -4 in glasses(I rounded) I see MUCH better with -3.25 contacts than -3.25 glasses and in fact theres next to no difference between -3.25 contacts and -4.25 glasses. I know some optometrists disagre but like I said, it depends on the person and their eyes. You can compare glasses vs. glasses but you cant compare contacts vs. anything as its variable on the person and their eyes. Undercorrection isnt silly, you will understand in a few years once you get presbyopia. Imagine not seeing clearly as much as 2 meters in advanced presbyopia. Youll need reading glasses for everything 2 meters and closer. Talk to someone across the table? His face will be blurry! Try to use the computer? You wont be able to read a thing on the screen! Newspaper? Forget it your arms arent long enough! I dont like contacts for several reasons, one is they arent comfortable, they can be felt. Two is I cant see clearly less than about a foot with contacts and my eyes feel strained and ache if I do any near work with them. I do not wish to incude more myopia that way. Three is I can just take my glasses off and see great from near. I am happier with glasses than contacts. Ill only wear contacts for seeing in the distance such as to play sports.
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