My eye doctor, the last time I got checked, told me that lighting was a key
factor in getting good focus as we age .... much like a camera, our depth of
field depends on iris opening ...

I noticed that things did not look as sharp when I was wearing my
sunglasses, although I could find no flaw in the lenses. It finally
dawned on me that since my sunglasses allowed only 15 percent of the
light to reach my eye, I had to open my "aperture" about three
"f-stops" -- e.g., from f/16 to f /5.6 -- so my lenses were not as
sharp as when stopped down.

vince