Peter T

Think pin-hole camera. When you look through a small enough hole, depth of field is essentially infinite.

Since the iris of the eye contracts in bright light, there is a definite increase in depth of field, since the pin-hole effect of the iris aids the focusing normally done mostly by the lens.

In a camera, you generally use a small aperture to get lots of depth, and a very large aperture (and corresponding very fast shutter) if you want to "blur out" a nasty background or foreground.

Focal length changes will also have an effect, but they don't matter much to the eyeball (unless you have one of those you can screw out and replace.)

A trick used by lots of old-timers I've known - who definitely didn't know much optical theory - is to make a fist, and then raise the fingers enough to open a small hole between the palm/first finger and the thumb. Peek through the hole, and distant objects will be somewhat sharper (but of course a little dimmer). Used to see which one of the neighbors kids was on the tractor next door (up to say a half-mile off?).

Technical references, compare Fraunhoff vs Fresnel focusing theories in most any "optics" text.

John