I'm preparing example photos for a lecture I'm giving on digital photography at the SparkArts: Digital Arts Festival, October 12, 2007.
These photos were created to illustrate the importance of creating proper exposures when you're shooting with digital cameras, even if you're shooting photographs that are intentionally low-key.
In the film world, low-key exposures are often created on film for a variety of reasons, primarily to create low-key images, but also to create a nice grainy effect with push-processing (where the photo is exposed dark, and processed to look like it was exposed normally). You can get away with that on film, because film grain is made up of tiny tonal gradations that can actually look pleasing.
Digital grain is harder, brightly and unpleasantly colored. It's often accompanied by rough tonal transitions, rather than smooth gradients, and an unpleasant striping effect caused by discrete differences between rows of sensor pixels. If you want to add a grainy effect, there are Photoshop plugins that simulate film grain beautifully!
In other words, in the digital world, it's imperative that you expose your photos bright enough to capture the detail that's important to you. It may seem counter-intuitive, but in a low key image, shadow details are vitally important, so it's important that you expose bright enough to capture those shadows with as much detail as possible.
To illustrate, here are two images shot for a low-key portrait, along with their corresponding histograms:
Low-Key Exposure as Shot
This is unedited low-key exposure as shot. The contrast looks nice, but what you don't realize in this version is that it's more grainy, the transitions in the gradients in the deep tones are not smooth, and this does not look as good enlarged.
Also note that it has an unnatural looking red cast to it. That's because the sensor is more red-sensitive in the dark.
As you can see, nearly half of the tonal range available on the sensor is completely unused! This can lead to noticeable banding. If you try to boost dark shadow details (i.e., to make the model's shaded eye visible), you will introduce significant digital grain and ugly striping to the image.
Normal Exposure as Shot
This is the normal exposure, raw and unedited. By shooting at normal exposure levels, we take advantage of the full range of tones that the sensor is capable of recording, giving us much more tonal depth to work with. Also, because the sensor is optimized to be color balanced at normal exposures levels, skin tones look very natural.
In the normal exposure a much broader range of tones is recorded by the sensor. The more data you can capture to describe your photo, the better! This will give you a lot more head-room and flexibility as you develop the image to suit your liking on the computer, and no matter how big you enlarge it, you won't notice any grain or banding in the image.
Normal Exposure Edited for Low-Key
Notice there is a lot more detail visible, and her skin color looks much more natural. I like the high contrast in the Low-Key as shot, but I could still get that look with this image, just by boosting the contrast a bit.
In this illustration, I just matched the histogram as closely as I could to the Low-Key exposure so you can make an apples-to-apples comparison of how much more detail is actually recorded by the sensor.
Low-Key Exposure Boosted
The low-key exposure is boosted here to show the lack of recorded detail in the shadows. Her left eye is unrecoverable, and there is a severe lack of smooth tonal gradation between the darker shades.
I know a lot of old-skool film guys who shun editing your photos on the computer. The old adage, "get it right on film" clearly does not apply here.
Even if you're aiming for a low-key shot, it's important to always shoot for exposures in normal ranges if you want to maximize the quality of the end result!