Peter T.

I think you've got it.

Some change in primary focus (focal length) that affects us as we age can be corrected by adding a single lens.

The troublesome thing is that the ability to focus over a range of distances, by "stretching" the lens of the eye, decreases. The distance between the "farthest" and "nearest" things we can "focus" is referred to as the "accommodation range."

When focused at any particular distance, there is a small range of things nearer and farther that are sufficiently "near focuse" that they can be seen with fairly good clarity. The difference in distance between the nearest and farthest things that are "visually clear," when your eye is fixed at a particular distance is your "depth of field."

A smaller aperture gives greater depth of field.

When near vision is such that the nearest distance at which you can focus is at about arms length, people will generally admit to becoming somewhat "far sighted."

In an ordinary room light, you might be able to focus at a nearest distance of - for a crude example - 2 feet. Objects may be fairly clear at 20 inches and out to, say 30 inches or so. (Depth of field usually extends further "beyond" the focal distance than "inside" focal distance.)

A very bright light might contract your pupil to half the diameter it has in ordinary room light. This will not change the fact that your "nearest focal distance" is still at 24 inches, but the pinhole focusing produced by the smaller "aperture" might now allow you to see things as near as 16 inches and out to perhaps 40 - due to the increased "depth of field."

A bright light does let you read things closer than you can read in a dim light.

It should be noted that even though you can see the fine print well enough to read it in a bright light, you may be reading "inside" the distance at which your eye is focusing. Done for extended periods, this can cause eye strain.

The ideal is to have reading glasses adjusted to the distance at which you would normally read, AND use a fairly bright light. Here, the increase in depth of field from the bright light lets you scan across a page (over a fair range of distances) without "working" your eyeball to keep sharp focus.

(The ability to scan without refocusing is helpful, even for persons who don't need vision aids, hence the recommendation that reading be done in good light by everyone.)

A bright reading light will let you read things closer - and over a wider range of distances - than you can comfortably see in a less bright situation.

John