I’m guilty of over-underexposing my photos. I like how shadows provide texture and ambiguity to an image. More importantly, when there’s not enough light, sometimes you just have to underexpose in order to get a clean, usable photo.
As we learned in the lesson on exposure, underexposing a photo means telling your camera that you want less light than the camera thinks the photo should have. The result is an overall darker scene.
How do you know better than your camera?
There are artistic reasons. But sometimes, you have no choice. In a darkly lit scene, there may not be enough light period to create a well-exposed photo. This is the case when shooting the stars.
If you leave the camera to do what it must to get the light needed for a well-exposed scene, it will do things to the photo that you may not like. For example, imagine taking a picture of stars in the sky. On a pitch black night, there’s so little light that the camera will keep its shutter open for a very long time.
And this is what the stars will look like (I’m using a Wikipedia photo for now because I’ve never actually seen stars while living in New York, nevermind photographed them):
What happened here? The camera kept its shutter open (we’ll get to that in the shutter speed lesson) for a long time. Long enough that things – in this case, the stars (actually, the earth) – had time to move, causing blur.
If you don’t want blur, then you have to tell your camera, “Hey, I’m OK if there’s only dots of light. It’s what a star-filled sky is supposed to look like.”
You express this artistic sentiment by dialing down the exposure:
What do you lose by underexposing a photo?
The whole range of illumination in the photo will shift down. Any details that were in the shadow will be black. What would’ve been midtones will now be shadows. And (some) details in the highlights may be more visible.
What happens when you underexpose in the dark?
Since everything that is dim will be even darker than before, the only things that will be visible will be the actual light sources, such as stars. At the terrestrial level, street lamps and neon signs will will serve as the bits of light:
If you don’t underexpose too dramatically, then objects that are close to the light sources will have a dramatic reveal:
What happens when you underexpose in a well-lighted scene?
Typically, this situation will come up when the background is overwhelmingly bright, such as an a clear day at noon. Anything that isn’t in the direct path of light will be dim by comparison.
Sometimes, these dim details will be distracting. So, underexpose to make those dim areas black. Now you have silhouettes:
So when wouldn’t you want to underexpose?
This is all at your discretion and it depends on the goal of the photo. But it’s pretty simple, don’t underexpose when your goal is to get a clear, lit photo of the subject. For a corporate portrait, for example, the dark and mysterious photo with creepy shadows may not be desirable.
I underexposed when I shouldn’t have; what can I do?
As with any undesirable effect, you have the option to fix it in post-processing in a program such as Photoshop.
But keep in mind that you can’t create detail where the camera captured none. If your camera captured black, it’s not as if it’s hiding some detail there that is waiting to be uncovered by PhotoShop or CSI’s plucky tech expert.
Isn’t it better to play it safe then, and not underexpose?
Again, it depends on the situation. Photoshop may not be able to recover all of the details within the shadows. But this is a less fatal flaw than if your shutter speed was so slow that the entire photo is a blur.
You do have some leeway if you shoot with RAW files, an option available in most expensive cameras (and some cheaper ones too). These pack more detail into each pixel, allowing you to recover as much as a full stop or more of light. The tradeoff is that the files are massive.
I’ll cover RAW processing in a future chapter.
