Stephen Johnson Tutorials: Digital Exposure
Use Histograms and Expose to the Right
Use the Histogram on the back of your camera to ascertain the actual exposure you just made rather than just trust the light meter's guess. Below is an excerpt from my last book explaining why...
Use the built-in light meter to establish your basic exposure. Check the results with your histogram display. Think of the digital photograph much as you would with slide film, expose for the highlights so as no to blow them away. The shadow detail will fall where the sensor is capable of holding it. Use the camera’s built-in histogram to check the data. Many cameras will warn of over-exposure by flagging areas of the image that are at or near over-exposure. A quick minus exposure compensation exposure can be made to take care of the potential problem. The exposure should be re-checked.
The problem gets a little tricky because of the way digital data is measured and bits counted. The highlights produce more voltage and are counted with more numbers, thus the upper values are dramatically better sampled than the shadows. If important dark detail is present, it is better to record it lighter (as long as you don’t blow out the highlights), then darken to your desired appearance in Photoshop later. This gives you more recorded shadow and mid-tone information even if they don’t look quite right at first.
Caption to graphics to the right: It is better to weight the exposure as high up the tonal values as possible, while being very careful not to blow out the highlight detail you need. Digital brightness values are counted in bits upward from 0 for the shadows. Every step in brightness upward is basically twice the previous number of electrons, building to the full tonal value available, or white. Thus counting from top down a typical camera 12 bit space, the brightest, values 2048-4096, occupy fully half of the tonal values available in that space, thus affording the smoothest gradation of tones. As the data goes into the shadows the numbers of bits available to characterize the brightness plummets dramatically. Again, give the digital file as much exposure as possible to keep the brightest tonal data high up on the histogram. This will maximize the data points underlying the tonal differentiations.
Above excerpt from Chapter 7: Digital Camera Techniques. Stephen Johnson on Digital Photography
What You See May Be Far Less than What the Camera Can Hold
Of course, using the camera histogram so carefully does call into question another issue. If we set our camera to Raw capture, and since the raw file is unprocessed data, how do we see the preview on the back of the camera and a histogram?
Well, the camera converts the Raw data to a JPG with whatever picture processing setting may be dialed into the camera. Naturally, these settings are pre-set to be fairly aggressively contrasty as people like punchy images.
High contrast settings in the JPG processing essentially pushes the resulting histograms out in both directions, much like an expanding bellows, adding contrast and perception of potential clipping of shadows and highlights to your experience of the raw exposure now pushed through JPG processing.
A simple change to the JPG processing controls make the resulting histogram much more representative of the Raw data capture capabilities of your camera. Lower the Contrast in Picture Processing controls to the lowest contrast possible, and presto, chango, your histogram will no longer be dramatically clipped from the raw data capture dynamic range.
Of course, your JPG previews will look grayer, less punchy and for many less emotionally satisfying on-site. The pay off is better understanding of where shadow and highlight detail is held and clipped in the Raw file, giving you more informed decision making in the field exposure.
A Grayscale ramp from black to white, as we see it.
A Zone Scale depicting how a 12 bit camera allocates its 4000 levels of gray in brightness values represented. 2000 levels of tonal differentiation represent the brightest values in the image, only 8 represent the darkest values.
A camera screen histogram with a light-meter based exposure, wasting almost 3000 levels of gray.
A camera screen histogram with an additional stop of exposure, utilizing almost all 4000 levels of gray in the 12 bit space
Example: Canon Picture Style Controls
Example: Canon Picture Style Controls Detail Set with Contrast dialed all the way down from middle "0" to the most "Minus" setting.
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