Nowadays, camera manufacturers want you to believe their cameras are very sophisticated precision digital instrument that can take great photos effortlessly. There is some truth in the marketing literatures, but they don’t typically tell you at the same time that their cameras can also produce garbages if the users fail to use them properly. I admit, this is really not necessary for them to do so in the marketing materials and most camera manuals do come with such warnings. The matter of the fact is that many people have some unrealistic expectations from their cameras that cost them hundreds if not thousands of dollars.
When it comes to the very basic aspect of photography, exposure, the common complaints we heard is that the camera doesn’t product correct exposure. People can typically notice the obvious problems such as blown out skies, dark faces, grey snow, pale green leaves, etc.
Sometimes exposure problem is not very obvious because untrained eyes would think the exposure is OK if they can make out all the details. There are also elements of personal preferences: some like bright photos, others prefer darker ones. The definition of correct exposure various as a result.
We are not talking about your personal preferences or creative decisions that you can make about exposures, for example, high key and low key photos. The subject is: why does the camera sometimes fail to produce correct exposure in automatic exposure modes?
First, the scene may contain larger luminance range (dynamic range) than the camera can handle. The camera is forced to make a decision based on pre-programmed preference or information such as color, focus distance, and focus spot. The results is undesired exposure at some parts of the photo.
Secondly, the built-in exposure meter in the camera can be fooled. It doesn’t matter how intelligent or sophisticated they are. “3D Color Matrix”, “iFCL”, “1005 pixels RGB”, “63 zone” are just some of the fancy words used by big name camera manufacturers. All in-camera exposure meters have a fatal flaw: they are reflected light meters. They make decision based on the light intensity reflected from the scene. When the scene doesn’t reflect a statistically averaged 18% (or 12-13% as later studies found out) of the incident light, or the scene itself contains light sources (lamps, the Sun, sky, mirror reflections of flash, etc), the build-in reflected light meters will be fooled into incorrect exposure.
To obtain consistent exposure using reflected-light meter like the one in your camera, you can use a grey card, or compensate the exposure. The amount of compensation can be determined based on your experience or trial and error. You can also take multiple shots with various exposure compensation (exposure bracketing).
Another way of obtaining consistent exposure is to use an incident-light meter which measure the amount of light falling on the subject. The reading is not affected by the reflectance of the subject.
The following is an example of the exposure problem.
The photos were taken using a Nikon D200. I used aperture priority and matrix metering mode for the photo on the left. Exposure compensation was set at 0EV. The camera decided an exposure of 1/80s F/4 at ISO400. The photo on the right was taken using exposure of 1/200s F/4 at ISO400 in manual mode based on incident light reading from a light meter. The second photo accurately captured the late afternoon scene in my backyard. The first photo showed over-exposed pale leaves.
Keywords: Exposure, Grey Card, How-To, Light Meter, tips