|ISO 100, f/22, 1/60 second, 24 mm|
Wednesday, 2 January 2013
Exposing for Snow
As smart as we think they are, cameras are simple dumb devices. The light meter in the camera is set up to expose for an "average" amount of light falling onto the sensor; what camera manufactures long ago have determined is the average scene that we photograph. This was empirically determined to be medium or 18% gray. This means that if you place an 18% gray card into the light of a scene, and allow the camera to meter off of that, the resultant exposure setting the camera shows will render a well exposed scene.
If you were to replace the 18% gray card with a white card, what would happen? The camera would meter out an exposure which would turn that snow white card into a nice dull medium (18%) gray...
And what happens if you again replace the 18% gray card with one that is black? The camera meters out an exposure which renders the black card to medium gray...
So, you're standing out in a snowy field and you've left your 18% gray card in your warm house. What next? Well, leave the gray card and turn to the gray matter between your ears.
Lets think through it together.
If you meter a white card, and the image which results from the camera chosen exposure is underexposed (turning the white card into medium gray). We would then need to manually increase the exposure to render the card the proper colour. But, by how much to we increase the exposure?
That's where experience comes into play. In my experience, a snowy scene on a winter day needs to have the exposure boosted between 1 and 2 stops over the camera's meter recommendation. Otherwise the scene will be dull and, well... gray...
Take the scene below. I picked the creative concept that I wanted from the exposure as I walked up to the scene. I wanted to get close to the crystal encrusted plants in front of the post, and I wanted the ones on top of the hill in focus as well. Plus, I wanted all of the post in the frame, so it called for a wide angle. I picked a very small aperture - f/22.
Metering suggestions from the camera (set on "Evaluative metering"), the camera told me I should expose at 1/200 s. From experience, I knew that this would render the scene darker than I wanted it. I wanted the white to pop! I set my shutter to 1/60 of a second - a full stop and a third slower. The result is that the snow and ice crystals are vibrant; and the dark side of the post is deep into the shadows. Exactly what I was looking for!
There is simply no substitue for knowing your equipment, and how light affects it. Shoot LOTS and learn from your mistakes. Change those mistakes into experience, and bring that experience forward into great images!