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For years, friends have asked my advice on camera exposure, typically on-site with a train bearing down on us. Politely, I’ll offer suggestions—based on conditions, but such advice can be deceiving since conditions change quickly. For my photography, I often refine exposure as the scene unfolds. A train entering a scene may alter my anticipated exposure, which requires subtle adjustments at the last moment.
Using the camera’s histogram to judge exposure is part of my latest technique for refining exposure and making optimum use of the digital camera sensor. A histogram reflects exposure information collected by the sensor. This is displayed as a graph that offers exposure quantification: it shows the range of data recorded by the sensor and alludes to data lost. The histogram allows me to gauge when the scene is over- or under-exposed. It solves much of the guesswork previously necessary when shooting film, while providing real information by which to adjust future exposures. What it doesn’t tell me, is as important to what is displayed on the graph.
Using film, ‘over-exposure’ inferred that too much light reached the emulsion and resulted in an image that appears too bright, while ‘under exposure’ inferred that too little light, thus and a dark image. It was never as simple as that, but that’s good enough for the moment.
The advent of digital imaging combined with the ease of post-processing using digital technology has changed the definitions of exposure, so far as I’m concerned. I can now use information from camera sensor on-site to help capture the greatest amount of information.
This is not much different than my traditional approach to black & white photography. The new tools offered by modern digital cameras have altered my means for calculating exposure. More to the point; the need for obtaining desired visual balance between light and dark in-camera isn’t part of my exposure technique because the appearance of the exposed image in the thumbnail on the camera display doesn’t accurately reflect data collected, while the final image may be best refined after exposure.
Here’s a difference between film and digital: Film sensitivity is less definitive than with digital sensors; simply, the data accumulated during a digital exposure fits between definite parameters, while with film significantly more information may be retained than is readily visible to the naked eye. Beyond these limits with digital, data isn’t recorded (to the best of my understanding). Thus to obtain the greatest amount of visual information a digital exposure must be calculated to be carefully placed between the image’s deepest shadows and brightest highlights. The tool needed to gauge this decision is the camera’s histogram.
A histogram displays a series of lines progressing from dark to light. These lines reflect the number of pixels exposed in the various gradations. How this data is collected isn’t important for this exercise. Crucial, is the assessment of the histogram in order to make future exposures that don’t lose critical information in extreme highlight or shadow areas.
When I make snow photos, I expose in a manner to place the bulk of information toward the center of the graph. I pay close attention to highlight falloff. Losing detail in the brightest parts of distant clouds, or at the center of locomotive headlights isn’t a problem, but losing detail in snowy foreground is undesirable. Ideally, the graph will taper gently into the extremes, indicating the smallest degree of loss in the deepest shadows and brightest highlights.
The histogram is extremely useful when exposing bright snow scenes, because most camera automatic settings are not tuned to expose for large fields of white and tend to grossly misjudge a brightly lit and largely white scene. This typically results in under exposure which renders snow gray rather than white and, risks opaque shadows (a substantial loss of information). It renders many elements too dark (such as the train passing through the scene). However, a few modern digital cameras have ‘snow settings’ that should overcome these difficulties.
Before making my desired image sequence, I’ll make a series of test exposures to check the effect of camera settings. Based on information displayed by these graphs I’ll make exposure adjustments to place highlights and shadows appropriately. As my subject approaches, I’ll further refine my exposure by making adjustments in 1/3-stop increments. I’ll continue to compensate for exposure changes caused by the train entering the scene (including variations caused by locomotive headlights and ditch lights).
Displayed here are both hypothetical graphs to show how I read histograms, and images of the real graphs from my Canon 7D exposed in snowy scenes last Sunday, February 10, 2013. Both types of images are intended to illustrate how I’ve selected exposures.
I use the histogram feature all the time, but find it most useful in extreme situations. It has proved its value by eliminating uncertainties previously caused by the extremes of snow photography.
Some advice for the graph-adverse photographer working in snow: use the camera meter to gauge base exposure then override the meter by opening up by 2/3 of a stop (for example open from f11 to f9).
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