" The dynamic range of the human eye is huge, but within a scene, it is smaller. " I need an explanation for this statement

Question

Why the dynamic range within a scene is less for human eye? Also, what's the HDR contrast ratio for human eye to perceive?

Answer 1

I presume the reason for this is that a human eye can adjust exposure from scene to scene, but within a scene, the exposure is fixed.

The same is true for cameras, by the way.

For example, a f/4-22 aperture, 1/8000 - 30 seconds exposure time, ISO 100-102400 camera+lens combination, has about 33 stops more dynamic range between scenes than it has within a single scene. So if the dynamic range of the camera within a scene is 13 stops, between scenes it's 33+13 = 46 stops.

Answer 2

The DR of the human eye is estimated to be approx 12 stops with a fixed iris opening (i.e. chemically dilated).

It is estimated to approximately double with the variable iris opening, which occurs near instantaneously as you scan a scene. I think this is about the maximum you would need to account for with an HDR image. Easy enough using a reasonably current DSLR w/ ~12 stops DR (base ISO), bracketed +/- 2 images, w/ a 3EV exposure step.

That is for daylight viewing. If you add in the ability of the eye to dark adapt, and the increased contrast of light points in a dark scene, the DR capability of the eye is even higher; exceeding 24 stops (low light/star viewing).