Monday, March 1, 2010

A quick word on 'snowfall' photographs

(Alternate Title: "A foot of snow later...")

Thanks to it being an El Nino winter, there's been quite a bit of wintery mix precipitation that's been hitting the Northeastern USA this past month.

As such, there's the temptation to go take a digital photo that you're going to email to family & friends (probably to get some sympathy for snow shoveling).

So you go out and take a photo during the storm ... and all that nice white snow turns out a yucky dark grey in the photo - - that's probably not what you wanted, so why did this happen?

The short answer is "technology", specifically, your camera's automatic exposure system.

Yes, we've become quite accustomed to auto-everything cameras, but a downside of this is that the camera never knows what you're photographing, so it guesses.

Simplistically, this "guess" is known as 18% grey, and while it works great 90% of the time for us, where it doesn't work so well is when we have a low contrast scene - - and during a heavy snowfall is a low contrast scene.

With a low contrast scene, the camara's educated guesses often goes wrong: it is looking for "dark AND grey AND bright" contrasts, but it can't find what's not there.

So while a white cow in a snowstorm (light on light) or a black cat in a coal mine (dark on dark) are both intuitively obvious to us, for the camera, it sees both as low contrast and hard to figure out. Subsequently, it can get the overall exposure settings wrong.

Typically, the camera's bad guess is that the cow/snow is "too bright" of an overall scene (it can't find true black), so it sets for a short exposure, which turns white into grey (underexposed). Similarly, the cat/coal is interpreted as "too dark" (can't find bright white), so it calls for a long exposure...and this turns black into grey (overexposed).

Here's an example of an image that's was a low contrast light scene, so it was auto-exposed to be a bad "grey" shot (underexposed), which was corrected later in post-processing:

The trick to avoid this problem is pretty simple: be aware of how your 'automatic' camera settings work, and anticipate what it will do based upon what you know is in the picture. The simple rule of thumb to remember is that the camera will want to turn everything grey if the scene lacks a good cue (bright sunny sky for contrast) to help it out.

With digital, its pretty easy to take a test shot and then adjust your settings. And if you forget (or not bother), you can fix the incorrect exposures later in post-processing. Its up to you to decide how much its worth a little bit more effort upfront when taking the picture, both to get better overall results, as well as to save time later from less post-processing.

And of course, if you do choose to override the default exposure while taking the photo to get what you want, do also make sure to remember to set your override settings back to normal afterwords.


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