Ask Dave, November 26

Ask Dave, November 26th
I get tons of questions each week, I respond as quickly as possible, and I might just answer them here too. If you have questions you can email me at <a href="mailto:Learn@DaveShumway.com">Learn@DaveShumway.com</a>.

Question:
Tony asked...
Why do you use an exposure bias?

Answer:
Often when working in rapidly changing light, with dynamic subjects, I (and many professional photographers) switch from manual to aperture priority mode (some will go to shutter priority mode, depending on the situation). When working with most modern cameras the photographer needs to be aware of what the camera is thinking when metering and suggesting an exposure for a scene, so they can step in and override the camera as needed.

On a basic level your camera wants everything that it meters to be 18% grey (see note). For most "normal" scenes that works well, but nature is full of black (100%) and white (0%). Think about a black bear or a tundra swan. A head shot of a black bear will tend to be overexposed as the camera thinks the black bear should be 18% grey. A similarly tight shot of a swan will be under exposed as the camera attempts to make the white swan 18% grey.

Some overcome this issue by using a middle/neutral grey exposure card (or something that is about 18% grey) with the camera's spot meter to dial/lock in a correct exposure for the scene. Others use high end light meters to know what the "true" setting should be, but neither of these methods work well in rapidly changing conditions with wild subjects. However I have and still use both of these methods in the studio.

Regardless of the shooting mode you should understand what your camera is trying to do, so you can override as needed, to get the correct exposure for the light at the moment your camera goes "click." This normally means taking countless photographs with your gear in many conditions, so that when the moment of truth comes you can set your camera correctly, with little to no thought.

Who cares, I shoot RAW?
That is a legitimate argument, but I strive to get my image as close to "perfect" at the moment of capture. Also adjusting exposure is a good way to increase the visibility of nose, and there are the situations where you miss by more than a stop (about what can be easily adjusted with a RAW file). There are also the times when over exposing a black bear could completely blows out the grass or snow in the background, or when shooting a landscape that missing the desired exposure dilutes the power of the color and most software has a difficult time recovering those lost elements.

NOTE:
If I recall, it was Ansel Adams that suggested 18% as middle/neutral grey and many followed suit. Ansel was speaking of reflected light not luminance, which happens to be what ANSI's standards are based on, they use a luminance of 12%. Oddly Sekonic uses a reflectance of 14% (they are the top handheld light meter manufacture). The difference between 12% and 18% grey is about 1/2 stop, but a luminance of 12% is typically going to measure a reflectance of 18%. I know it all seems a bit crazy, convoluted and pointless. If, like most, you use your camera in an evaluative metering mode that means the camera wants most of your scene to show a reflective light of around 18% grey (the last sentence is all that matters). 


A few general exposure compensation tips to remember:

• Brightly lit snow is one and one-third to two stops brighter than medium/neutral gray;
• a bear or bison filling the frame is normally around one stop darker than medium/neutral gray;
• a white swan or pelican is normally around one stop brighter then medium/neutral gray;
• a raptor's dark underside against bright sky is normally around two to three stops too dark (your camera gets confused by the bright sky);
• dead grass of early spring and fall is very near white (0%) expose accordingly;
• When in doubt, check the histogram. Still in doubt, bracket. (heck always check the histogram)

Happy shooting