A list of terms both technical and common to the OO community that you may be uncertain of. Suggestions and contributions to this list welcome.
Accomodation.
This word refers to the amount that the lens of the eye can focus closer. It varies with age, and it is the decrease in accomodation in middle-age that is associated with presbyopia, the need for reading glasses. The less accomodation, the harder it is for the eye to focus close up. With lower accomodation (and/or higher hyperopia) the lens cannot even bring the horizon into focus, it's too damn close!
The chart below shows approximate accomodation by age, in diopters, plus the nearpoint at which sharp focus typcally is lost:
Age 5 16.00d 2.5 in
Age 10 14.00 d 2.8 in
Age 15 12.00 d 3.3 in
Age 20 10.00 d 3.9 in
Age 25 8.50 d 4.6 in
Age 30 7.00 d 5.6 in
Age 35 5.50 d 7.2 in
Age 40 4.50 d 8.7 in
Age 45 3.50 d 11.2 in
Age 50 2.50 d 15.7 in
Age 55 1.75 d 22.5 in
Age 60 1.00 d 39.4 in
Age 65 0.50 d 78.8 in
Age 70 0.25 d 157.5 in
Age 75 0.12 d infinity
In practice there is some variation. We have all met aged men who read without glasses. People who are slightly myopic are often able to read without glasses in later life. Conversely, occasionally younger people have less accomodation than normal, so this chart is only a guide. Most people find 12-14" to be a comfortable reading distance, so as you can see from the chart, it's no surprise that in the fourties as this distance becomes too close (or the arms get too short!) reading becomes uncomfortable. Any slight, perhaps previously uncorrected, hyperopia will hasten the need for correction for close work. Hyperopia also gradually increases with age.
Relative strength.
An often heard term is "His glasses are very strong". How strong is strong? Doctors, Joe Public, and fetishists have different ideas on this, and really, it's all relative. If you, your friends and family members wear glasses in the -2.00 to -4.00 range, and that's what you're used to seeing, you may meet someone who wears -8.00 and consider it "strong", whereas he thinks his glasses are "weak" compared to his Uncle Jim who wears -12.00. Take a look at the off-the-shelf reading glasses in the pharmacy. Some aren't maked in diopters, but as "weak", "medium", and "strong". Chances are these "strong" reading glasses are about +3.50, which is laughably "weak" to many a hyperope who has been wearing +8.00 since childhood. You can't judge by appearances anyway. With today's aspheric and high-index lenses the thickness is also relative, and the size of the lens in today's smaller frames creates less edge thickness, so a modern -6.00 lens can easily be thinner than a 1970's -2.00.
Generally speaking low or mild myopia is under 5 diopters (-5.00), moderate myopia is 5 to 10 diopters, severe or high myopia is over 10 diopters. It's a bit different with hyperopia, in which mild or low is considered to be under 2 diopters, moderate 2 to 4 diopters, and high or severe 4 to 6 diopters. Above 6 diopters hyperopia is considered to be extreme, and over 10 diopters may not be correctable to 20/20 vision.
To try to gauge the strength of a lens just by looking at it, a harmless hobby many OO's enjoy, try our Rx Spotter Page
