The human eye, what can it see?

[Big Mike]

So most sunset pictures are "HDR"? Multiple exposes blended together?

No, not at all.

But if you look at a lot of sunset shots with those deep, rich colors, anything in the foreground (like a palm tree) is likely a dark silhouette. We don't often remember seeing only the dark silhouette of the tree, because our eye adjusts to it, but that's what the photo records.

 

[Big Mike]

As far as ISO goes, unlike film or dig., our eyes can distinguish detail in some pretty dark shadows.

Film or digital can distinguish detail in dark shadows just fine...maybe better than our eye. It just takes a longer exposure time. For example, a photo taken for 30 seconds (and decent aperture size) will likely pick up anything with any little bit of light on it.

The difference is that our eye will look to the shadow, adjust to it, see what's there, then look away and adjust again.

 

[Dwig]

[...
The difference is that our eye will look to the shadow, adjust to it, see what's there, then look away and adjust again.

Quite true, and it does this for both "exposure" and focus while, at the same time, not usually fixating on any one point, scanning the field instead. You brain then computer processes the incoming data in real time forming a composite image. You are rarely aware of anything being "out of focus".

When it comes to color and exposure, the human eye is a four sensor system rather than the 3 common to color film and most digital cameras (all current commercially available color digitals). The fourth sensor set is a luminance only set that is primarily sensitive in the middle of the spectrum around green. This sensor system is more sensitive to light than the other three and in very low light is used almost exclusively, yielding nearly monochrome images. In bright light it doesn't work well, but the other three cover that end of the intensity range and are each sensitive to different sections of the spectrum yielding color images in much the same way as modern color film and digital sensors.

 

[Hamtastic]

Excellent, short vid on human color perception

Beau Lotto: Optical illusions show how we see | Video on TED.com

 

[djacobox372]

Eyesight has actually two sensor types: rods and cones.

Cones are more plentiful (higher resolution) and centered in the middle of the eye, they are sensitive to color and less sensitive to light. They are your "day vision."

Rods are less numerous and located away from the center of the eye, they are not sensitive to color (especially red). They are very sensitive to light and prone to be blown out by bright light. This is why your peripheral vision is actually better in the dark then it is in the daytime, it's also why it's easier to see things slightly to the side at night then it is dead center (where your rods are less numerous).

Interesting fact: rods can actually accumulate light for up to 15 minutes, meaning a very dim light source may not be visible unless you stare at it for 15 minutes... that's a slow shutter speed!

 

[Josh66]

Here's some interesting reading about the human eye, and how it relates to photography:

Clarkvision Photography - Resolution of the Human Eye