Google this topic and you will discover a war of sorts, a raging disagreement between those who say you should overexpose digital photography as much as possible, referred to as “exposing to the right” since it piles up the histogram toward the right edge, and those who recommend the opposite: underexpose and create a histogram that is left-leaning.
You must expose to the left because clipping is bad, some say. Overexposure is a non-concept in digital photography. Even when shooting RAW, the rightmost edge of your histogram is a sharp cliff, and it must be avoided at all costs. While you can sometimes recover highlight information from a RAW file, you’ll never get much, and if you miscalculate and clip some highlights, the results will be un-film-like and harsh. Expose to the left and always be safe.
Others purport that you must overexpose your digital photography, i.e. expose to the right, because of how digital sensors work. Unlike film, which has a logarithmic response to light, digital sensors have a linear response. So while film grain is evenly distributed across perceptual values, sensor noise lives predominantly in the shadows. It’s easy to see why when you imagine a linear sensor trying to hold four stops of exposure—if 100% sensor charge is four stops up, then one stop down is half that, or 50%, and one more stop down from there is 25% and one more down is 12.5%. As Jason Rodriguez commented on my dynamic range post, fully half your chip’s sensitivity is devoted to the brightest stop you can hold. Each stop you drop from there doubles your noise. So to maximize your signal to noise ratio and get the cleanest image, you must overexpose as much as you can, to distribute your image across the chip’s least-noisy sensitivity range.
Like the raging war between the half-black, half white aliens in Star Trek episode 70 (oh yeah, I went there), this is a non-argument. Both philosophies are 100% correct, and should be in play in the digital photographer’s mind when deciding on an exposure.
It’s so simple to state the combination of these two philosophies that renders both extremes silly: You should expose as bright an image as you can without clipping.
Man, that’s so much easier. I don’t know why people put so much effort into the debate.
Let’s look at some images. Here are some trucks at f/11, 1/500:
(click images for larger size)
Here’s that same view at f/8, 1/250 (for a total of two stops brighter):
The first image is a about a stop underexposed, although it does have small highlights that are just barely being held. The second image is clearly blown-out, and appears clipped in the highlights, but holds nice shadow detail.
But these images are raw, so we have some flexibility. Here they are again, color corrected in Lightroom into a similar look:
They almost match, but if you look closely at the overexposed image you can see that, while Lightroom was able to recover a surprising mount of detail in the highlights, the backs of the white trailers are still a flat, featureless white, with abrupt, cyan-to-white transitions in shading. Almost worse are the highlights in the clouds, which reveal Lightroom’s desperation.
Meanwhile, in the shadows, the underexposed image is a bit noisy, whereas the overexposed image is cleaner.
But the difference is not very noticeable. In this case, it seems the advantage goes to the underexposed image. Had I opened up a stop I could have reduced noise in the grays by half, but I’d be missing some highlight detail on the foreground truck. My fear of blowing out caused me to expose to the left, with happy results.
And now the counterexample: Another pair of images two stops apart:
Another attempt to make them match:
And while in the second image one could say that the clouds are a bit clipped, and the sky a bit low saturation, this is hardly as noticeable as the noise in the shadows of the underexposed image:
So our second image would seem to indicate a victory for exposing to the right.
You’re probably way ahead of me on the conclusion: You cannot apply a blanket philosophy of underexposure nor of overexposure to digital photography. Instead, you must learn your camera’s nuances and seek the correct exposure for the scene—which will almost always be as richly exposed as possible without clipping. Sometimes the resultant histogram will be left-leaning, and sometimes it will be piled up to the right. Make the shot, not the histogram.