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Couldn't you get an equivalent focal length by wearing reading glasses?


Only to a limited degree. The problem is that the eye becomes inflexible with age, lowering the ability to modify it's "lens power".

Lens power is defined as the inverse of the focal length. Lets say you have glasses that, when combined with you eyes, create perfect focus at a range of 5m. If you look at something 10m away, you need to add (or subtract) the difference by modifying your built-in lense.

The lens stength adjustment for your eye needed for that, is

dP = 1/10m - 1/5m = -0.1m^-1. This is not much, and even old eyes can do this easily. Also, even if the eye is not able to do so, the image is going to be pretty sharp at 10m or any greater distance away, or even at distances of about 3-5m.

Now consider having your lenses adjusted to be perfect at 0.5m away. If you move the object you look at to 1m away, you get:

dP=1/1m - 1/0.5m = -1m^-1

This is still easy for a young person, but for someone older, this can be straining, or even impossible. Also, even slightly away from the perfect distance, perceived blurriness will be significant if not able to adjust.

Furthermore, to look at something much further away, eyes must be ajusted by dP=-2m^-1. This is doable for a young person, but an older will move around in the fog.

In other words, a screen that is 3m+ away from you requires a lot less lens compensation from the eye (and thus far less strain) than a screen 50cm away, even with perfect glasses, as long as you are not sitting perfectly still at that distance. Even variation in distance between the center of the screen and the edge of the screen may be enough to cause problems.


Thanks for taking the time to write this. I learned a lot.




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