Contrast Management using DiMAGE A2 in-camera Settings

The curve below was obtained by taking three series of exposures at ISO 64 of a standard target.

• Camera contrast minimum (-5)
• Midrange contrast (+0)
• Camera contrast maximum (+5)

This set of curves shows three things:

• The luminance used by the camera to calculate exposure (here, a middle gray value), is placed slightly below the midpoint (127) of the intensity scale (0-255). This is a favorable location, as upper values tend to be higher-contrast, and clipping and consequent loss of texture occurs above about 250, only 3 stops above middle gray.
• The camera's imaging algorithm clearly uses the middle intensity value, 127, as the fixed value in adjusting contrast. dcraw uses the saturation point as the fixed value. As exposure,by convention, is calculated on the middle value of the object, the camera's algorithm is easier to work with because metering systems assume middle values should be used to calculate exposure. (One could argue with this; for example, the the threshold could be used, but this would require that we re-frame all of our exposure conventions, but this would be more complicated and the phenomena would remain the same.)

  However, if the photographer's main interest is preservation of highlight detail, then the following technique can be used to yield proper exposure:

  • Set the "exposure compensation" to -2. (This tells the camera that you're going to meter the exposure from a bright area near the pixel saturation threshold.) In this camera, do so by pressing the exposure compensation button on the camera's back and rotating the front control wheel. Remember, it's - when metering bright areas and + when metering dark areas. I often reverse this, myself. But at our age, we sometimes have trouble telling right from left. Fortunately, the viewfinder display tries to replicate the effects of the exposure, which helps us avoid brain short-circuits.
  • Set the camera to use "spot metering." In this camera, do so by rotating the "function" dial at the top left side of the camera body to the (o) symbol, press the button in the center of this dial, and use the front control wheel to select spot metering. Or use the menus.
  • While framing the image, briefly place the metering circle on the bright area for which you want to preserve image detail, and press the AEL button to lock the exposure. Hold the AEL button until you've made the exposure (unless you've used the camera's recording menu to make this button function like a toggle).
  .
• Digital sensors have a long, low-contrast toe and high-contrast high values, prone to blocking. This camera increases contrast more by decreasing low values than by increasing high values. I can perceive neither tone nor texture below an image intensity of about 10. Thus, at contrast -5, middle tones are darkened and texture is held only about 2 stops into the shadow, which can make shadows pretty empty. For this reason, I've found the +5 contrast setting useful only in flat light.

  The natural tendency, for photographers who understand this, is to underexpose to protect highlight detail -- but doing so does not increase shadow contrast or detail, and increases noise in shadow areas. It appears to be a simple fact that digital cameras have little exposure latitude and precise exposure is a really good idea.

  My own experience is that having a real-time histogram and taking manual control of exposure is the best safeguard, but when highlight areas are small, their spike may be too small to see in the viewfinder, and then the photographer doesn't quite know how to adjust for them.

  In addition, I am finding out that the DiMAGE A2's exposure calculation is not as accurate as I had assumed. This is a reason to bracket exposures of important scenes.

Next topic: DiMAGE A2 Exposure Accuracy