Reading Glasses Work When Accommodation Does Not
The Accommodative reflex is a scary sounding phrase that essentially means our eyes are doing everything they can to focus up close. And when they can no longer do this, that's when we slip on a pair of reading glasses to give our eyes that much anticipated relaxer.
There are actually three functions that all occur simultaneously to allow us to focus up close. The major factor is accommodation itself, that is to say when the crystalline lens inside the eye is acted upon by the surrounding body of muscles and the lens thickens. Much is still studied about the actual forces and muscles here, but all we need to know is that the lens contracts and adjusts in power.
By adjusting in power, we are able to focus up close. The power is considered plus power, which magnifies. (have you ever wondered why the numbers on your reading glasses always start with a plus power (+)? Whether they be a +1.00, +2.00 or +3.00 they all replace a portion of plus power that the lens can no longer provide.
The other factors that make up the accommodative reflex are convergence of the eyes. When you focus up close, your eyes actually both move in towards your nose a bit, this allows both eyes to see the same image at such a short distance.
And finally, pupils will get a little smaller. This is referred to as miosis. When your pupils constrict, less light is allowed in and sharper focus can be had. Now it makes a little more sense why that menu at the candle light dinner was so hard to read.
The process works wonderfully well, until the crystalline lens slows down around the age of forty and presbyopia sets in. Then, well thank goodness for reading glasses.