How can I use a histogram to expose correctly when the scene is very dark?

Question

If I am photographing a scene with a lot of dark background, how can I use the histogram to tell if the exposure is correct? For instance, at an outdoor concert in which the performers are moderately illuminated but their surroundings are dark. Or a view of a cityscape viewed across a dark body of water and with a dark sky above it? In other words a scene in which much of it is expected to be almost or totally black. When shooting RAW in such a situation, what should the histogram look like to make the final processed image dark without being noisy? And without the black areas becoming posterized?

_{Image information: Canon EOS 7D + 70-200mm f/2.8 L IS II, ISO 6400, f/2.8, 1/60 second. Cropped from 5184x3456 to 3872x2581 before downsizing to 1536x1024 for web viewing.}

_{Developed at 3900K, exposure adjusted -0.17 stops, Canon 'Standard' Picture Style, Contrast -4, Highlights -2, SHadows -1, Color Saturation +1, Unsharp Mask: Strength 6, Fineness 7, Threshold 3, NR: Lum. 9, Chrom. 10 using Canon's DPP 3.}

Answer 1

A histogram is meaningless in this situation. Only a spot meter could give you a correct exposure. And even then it would be difficult because your subject is moving.

Answer 2

The general rules for histograms still apply, it's just that most of the "weight" of your histogram will be leaning to the left:

Your aim will be the same: keep as much of the data in the histogram from clipping at the right hand edge, without leaving too much way down the left hand side. You should be able to see some data reaching all the way across the histogram to the right, even if it's low compared to the higher peaks over the left.

If it goes flat on the right like this, it's underexposed:

If you have very bright highlights, such as the lights in that picture, it will be unrealistic to avoid all clipping without underexposing quite a lot, hence the small uptick on the right of the first histogram above.

I've shown mono histograms here just for illustration (and because it's what I found with a quick Google search) but the same goes for RGB histograms.

Answer 3

The histogram for your posted image is:

However, I assumed that there'd be nothing but black sky and expanded the canvas out to your original resolution:

As can be seen, in the cropped image, the histo-rule-of-thumb posted by thomasrutter applies. But, the general rule completely falls apart at the native resolution.

In these situations - if it's possible to fill the frame for an exposure test shot, one would go that route. If that is not possible (and I imagine it'd be pretty hard if you were focal-length maxed out shooting airplanes at night), then I think the histogram becomes decreasingly useful.

Answer 4

Everyone who said the histogram was pretty much irrelevant for this type of image is pretty much spot on. The exposure of the original frame was about as close to what I was looking for as can be expected. Reducing exposure as a way of eliminating minor banding in the very dark areas also has the consequence of reducing overall detail of the non-black parts of the image.

Shooting an airplane doing aerobatics while going about 300 mph from a position a half mile or more away - in the dark - is a challenge. I was using a Canon EOS 7D with the EF 70-200mm f/2.8 L IS II lens. Exposure settings were ISO 6400, f/2.8, 1/60 second. I was panning with the plane using IS mode 2 to get anything resembling sharp. Some of the pans I shot matched the speed of the smoke more than the speed of the plane (the smoke slows ever so slightly as it is buffeted in the plane's turbulent wake before eventually slowing until it 'hangs' in the air several hundred feet behind the plane).

Since the question has garnered some recent attention, I decided to revisit this image, originally shot back in October, 2012. This time I'm using Canon's latest version of Digital Photo Professional 4 as compared to version 3 (or possibly 2) that I used back then. Hopefully I've learned a little along the way as well.

The differences are subtle, but they are there. About a year after I asked the question above I learned how to raise black levels to "crush" the blacks and get rid of much of the 'blotchiness' and noise in areas that should be totally dark with no detail. Unfortunately, to fully crush the blacks in this image also would have meant losing any detail in the black painted leading edges of the plane's wings. I might eventually try a multi layered mask to give most of the dark areas a uniformly black appearance.

Answer 5

What constitutes proper exposure depends on the photographer's visualization of the final image.

As usual when thinking about exposure in a DSLR, a good place to start is to expose to the right to produce the range of unclipped (and perhaps clipped) highlights consistent with your vision. Noise in the very dark portions -- i.e. Zone 0 of the Archer-Adams Zone system -- of the scene can be eliminated by raising the black point when developing the RAW file in the computer.

Example

The image was shot at ISO 12,800. Raising the black point eliminated most of the noise in the background so that only moderate denoising was required and the image could remain sharp.

The black point was similarly raised in this image to reduce stochastic noise in the shadows and move more of the pixels into Zone 0.

Alternatives

• Standard HDR techniques are an option.
• Many cameras will highlight underexposed and overexposed areas of an image after capture. This can be used to indicate clipped highlights in cases where that is a concern.