October 5, 2012

Getting the Exposure You Want for White or Black Subjects

One of the challenges of photography is creating an exposure that accurately depicts the subject. Most of the time, our digital cameras' automatic metering and exposure systems do an excellent job. But there are two types of subjects that our cameras need help with to create the best exposure: anything white and anything black.

Properly exposed white building at +1 2/3 stops

The problem arises because all light meters are designed to select a shutter speed, aperture and ISO to produce a medium brightness exposure. This is sometimes called a "middle gray" or "18% gray" exposure. All it means is that the brightness of the picture overall is halfway between pure white (without detail) and pure black (with no detail); hence, the short-hand of "middle gray".

But white subjects that take up 90% of the picture space or black subjects that fill 90% of the image area are NOT medium brightness. White subjects are much brighter than medium and black subjects are much darker than medium. So our job as the photographer is to add light to brighten the white subjects or subtract light to darken the black subjects. You can do this using the Exposure Compensation feature or manually adjust the shutter, aperture and/or ISO to achieve the same effect. (For more about Exposure Compensation, see my previous blog post here.)

But how much more or less light do you need for white or black subjects? This depends partly on personal taste, but the visual guideline is as bright or dark as you can and still see detail in the subject. You can do your own test for white and black subjects ahead of time and then know exactly how bright or dark to make each one. Here's how:

Testing for the Best White or Black Exposure
  1. Find an all white (or all black) subject outside. It should be large enough that you can completely fill the frame with white (or black). It should also have details in it to help you judge when your exposure adjustment has gone too far. The side of a building, a coat or shirt, or similar subjects with fine texture are perfect. (I don't recommend cars or trucks because the smooth metal does not provide enough visual texture for you to judge the results.)
  2. Be sure the subject is evenly lit. There should be no dappled light or large shadows falling on your subject. And the light should be constant while you take the entire series of photos. So avoid partly cloudy days in favor of completely clear or completely overcast weather.
  3. Set up your camera on a tripod and adjust the camera position and/or zoom setting to fill the frame with white (or black). There should be no other colors in the photos. If you are using a smaller subject, you may need to supply a white (or black) background.
  4. You can capture either JPEG or raw files. Choose the format you most commonly use.
  5. Set the camera to Program exposure mode. You can use other exposure modes as long as they give you access to the Exposure Compensation feature. These are normally the "letter" settings (P, M, A or Av, and S or Tv) and not the "picture" settings (portrait, landscape, action). Or you can carefully adjust your manual settings, preferably changing the shutter speed.
  6. Choose the camera's lowest ISO setting (usually 100 or 200). Do not use Auto ISO or the test won't be accurate.
  7. Change the camera's white balance to automatic (AWB).
  8. Make a series of 7 to 10 pictures, varying the exposure compensation by 1/3 steps.
    1. For a white subject, start with the Exposure Compensation set to zero (0) and take a photo at every plus 1/3 stop up to +2 (or +3 if your camera allows).
    2. For a black subject, start with the Exposure compensation set to zero (0) and take a photo at every minus 1/3 stop down to -2 (or -3 if your camera allows).
    3. You should see your photos get brighter or darker as you change the Exposure Compensation amount.
Series of 10 exposures of a white subject
starting at 0 compensation and
getting brighter by 1/3 stops to +3

Series of 10 exposures of a black subject
starting at 0 compensation and
getting darker by 1/3 stops to -3
  1. Download the photos to your computer and view them large on the monitor. I recommend you enlarge the photo to 100% view and look at all parts of the picture.
  2. Determine which exposure for the white subject is very bright but still retains details and texture. This is your ideal exposure adjustment for a white subject.
  3. Determine which exposure for the black subject is very dark but still retains details and texture. This is your ideal exposure adjustment for a black subject.
Best white exposure at +1 2/3 on the right
compared to loss of detail at +3 on the left

Best black exposure at -1 2/3 on the right
compared to loss of detail at -3 on the left

Now when you face a white or black subject, you will not have to guess about how much to adjust the exposure in order to get a photo with appropriate brightness. You may still need to make minor changes to the settings, depending on the shot and your intentions. For example, you can usually expose a black subject darker and a white subject brighter when they are in the light of an overcast day or the shade than when they are in direct sunlight.

Completing this little test should take a lot of the guesswork out of exposure! Try it for yourself!