Why do my digital photos often look better at about 1 stop under the camera's meter reading?

Question

When shooting high-contrast daytime scenes, especially landscapes, I’ve found that bracketed shots exposed about 1 stop darker than the camera’s suggested exposure often look better than the “normal” exposure. They usually keep more realistic color, preserve more detail, and have better overall balance. This seems to happen whether there’s a lot of sky in the frame or very little.

I’m trying to understand why. Is this normal for digital cameras and sensors? Is it related to dynamic range, metering assumptions, or how the camera defines a standard exposure? Or could it point to calibration differences in the camera or lens system?

I use Nikon bodies and mostly Nikon lenses, and these examples were shot in natural daylight.

user13555 · Accepted Answer

I'm not sure if this is a "how is exposure defined" question or an "is my camera busted" question, so I'll try to address both. :)

Definition of proper exposure

ISO standard 1271 contains a definition for photographic exposure.

Bypassing the math, "correct" exposure averages a scene's luminance and renders that luminance at a particular (but arbitrary) level, measured in lux-seconds, at the image plane.

That level has been chosen based on an assumption that the the average scene's peak luminosity is <=7.8x its average luminosity (again, an arbitrary figure).

The standard provides manufacturers with a small amount of wiggle room (it specifies a constant K, the value of which the manufacturer can select, within a narrow, defined range) to compensate for transmission light losses in the optical pathway, as well as for a rendering a particular manufacturer feels is most pleasing.

In simpler language, "correct" exposure maps a particular shade of "average" grey in a scene to a specific RGB value in the image.

Anything in the scene brighter or darker than this "average" simply falls where where it falls in your image. Or put another way, depending on the average luminosity of the scene, dynamic range of the scene, dynamic range of your imager, etc., etc., it is entirely possible to experience clipping (in shadows and/or in highlights) in a "properly exposed" image.

Is my camera exposing correctly?

In practice, manufacturers have developed sophisticated metering systems to properly weight or discount areas of an image to achieve a higher rate of pleasing images. In effect, this adjusts the the shade of grey the camera considers to be "average" in the scene.

The fact that you prefer the image your camera delivers when its meter says the scene is one stop underexposed may indicate that your camera meter is out of calibration, or may simply be revealing your personal preferences.

Your histogram may offer some clues (be sure to evaluate a RGB histogram, not just a single-channel histogram), but it would be best to shoot a calibrated grey reflective target to see where your meter places the grey in your image file--should be at least 2.96 stops below saturation for 12.8% grey target or 3.46 stops below saturation for 18% grey target.

Hope that helps,