Histograms: Helpful Exposure Tool, or Should You Avoid Them Like the Plague?
A few months back, we shared some tips on how filmmakers can use histograms to properly expose their images. Despite the fact that histograms can be a helpful exposure tool in certain situations, there are quite a few downsides to using them in a filmmaking setting. Cinematographer Ryan E. Walters explains:
The most common criticism of histograms is that they're wildly inconsistent. For instance, it's almost impossible to look at just a histogram and tell whether the image it's representing is properly exposed or not, as can be seen in Walters' example in the video above. With that said, however, given that the shooter knows the context and style (whether it will be dark, bright, balanced, etc.) of the image they're creating, the histogram can be an excellent visual reference tool. If you know that you want a dark, contrasty image, or a bright, washed-out image, a quick glance at a histogram will tell you if you're on the right track. Then again, for judging over and underexposure, a waveform monitor is a much better tool.
Then there's the issue of Exposing to the Right (ETTR), a practice widely used by stills photographers to maximize the range of luminance values captured by the camera when shooting RAW. In theory, ETTR is a solid practice because it results in cleaner images with less noise in the shadows. As such, ETTR is the law of the land when exposing with histograms for many photographers. However, for filmmakers, many of whom are shooting on 8 bit 4:2:0 cameras with limited dynamic range, ETTR can be a risky practice because it can very easily lead to accidental overexposure and unrecoverable highlights, especially if the histogram is the only exposure tool being used. Then there's the issue of maintaining exposure continuity throughout a range of shots. As can be seen in Walters' example, ETTR can lead to some unsightly shifts in exposure between shots if you're not extremely careful.
Ultimately, the histogram can be a helpful exposure tool when you need a visual representation of luminance values within a certain shot. It can help you determine whether or not the camera is exposing the way you want it to be, whether dark, bright, or neutral. However, histograms should almost always be used in conjunction with other exposure tools like waveform monitors, false color displays, and zebras. If you rely on the histogram alone, you might be in for some unfortunate exposure surprises when you review your footage.