Mastering Your dSLR’s Controls
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Although every camera uses different buttons and menus to control key features, each includes some variation on the basic array of controls. This chapter provides an overview of the exposure and focus controls a digital photographer must master, and includes descriptions of how these controls differ between digital cameras and film cameras.
You’ll learn about automated exposure modes, using histograms, working with f-stops and shutter speeds, and selecting the right “scene” options. I’ll also cover some of the quirks of working with automatic focus systems, too. I’m not going to waste pages on some of the easier controls, like the shutter release, or on setup options such as white balance settings you make using your menu system. The emphasis here will be on the most important controls you use for everyday shooting.
Exposure Controls
After composition, the two most important aspects of getting a great photograph are zeroing in on the correct exposure and having the image focused properly. Your dSLR can take care of both of these for you automatically—most of the time. When exceptions occur, it’s time for you to step in and use your camera’s controls to fine-tune your images. This section deals with exposure; I’ll explain focus later.
Correct exposure is a necessity because, as you’ll recall from a previous blog post, no digital camera sensor can capture detail at every possible light level. In very dark portions of a scene, there will be too few photons to register in the individual photosites. If part of the scene is very light, the pixel “wells” will overflow and stop collecting additional photons. Some of the excess may spill over onto adjacent pixels, causing a blooming effect.
The goal of setting exposure is to either increase the number of photons available to register details in dark areas (by boosting the exposure) or to decrease the number of photons flooding the photosites in the dark areas (by reducing the exposure). Like film, sensors are unable to handle high-contrast situations in which there is a large variation in brightness between the dark and light areas. The degree to which a sensor can handle such variations is called its dynamic range. However, even a sensor with a broad dynamic range won’t handle the most extreme lighting conditions, so the “correct” exposure is likely to be a compromise that preserves detail at one end of the brightness scale at the expense of detail at the other end.
Generally, that means avoiding the clipping off of highlight detail caused by overexposure. Once a pixel bucket is “full,” that pixel is rendered as pure white with no detail at all. There is no point in collecting additional photons and, as I mentioned, doing so can spoil the image in surrounding pixels. On the other hand, information can often be retrieved from darker areas of an image, even if those areas are underexposed, usually by multiplying the data that is there.
Image-processing algorithms can often do a good job of this, which is why increasing your camera’s ISO sensitivity can improve the amount of detail captured in shadows.
Enhancing underexposed areas is likely to produce noise, though. You can see how this works at a primitive level in your image editor using the Brightness control. Moving the Brightness slider to the right lightens shadow areas enough that you may be able to see details that were previously cloaked in darkness. However, moving the same slider to the left to darken overexposed highlight areas won’t produce additional detail—it will only turn the white blocks into a featureless gray.
Digital camera autoexposure systems are optimized to attempt to preserve highlight detail (which is otherwise lost forever) at the expense of shadow detail (which can sometimes be retrieved). Any changes in how exposure is made that you make will simply be aimed at improving the relationship between the actual brightness levels in a scene, and what is captured by your camera.
