Taking Control of Your dSLR

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Unveiling the secrets of foolproof exposure Metering for fun and profit Finding the advantages of self-exposure Getting focused f n terms of buttons, dials, and controls, comparing a typical point-and-shoot camera with a digital SLR is like comparing a hang glider to a Boeing 767 cockpit. Certainly, the more sophisticated option has more controls, but most people find it easier to flip a switch and shift a control than to yank on a lever, lean to the right, and pray.

Because you can do so many different things with a dSLR, the learning curve is a bit steep. You might have four or five exposure modes to choose from, a half-dozen different ways to focus, and the ability to fine-tune details such as white balance, sharp- .^MiiSSS ness, contrast, and color. Because you're a more serious photographer, those options are probably the reason you bought a digital SLR in the first place.

Okay, champ. You've got this incredibly versatile gadget in your hands. How are you going to make it work? This chapter helps get you started with the toughest of the tough: exposure and focus.

Discovering the Secrets of Exposure

If you're lazy or not up to using your brain on any given day, you can set up your digital SLR to operate much like a glorified point-and-shoot camera. Turn on autofocus, set the exposure control to Auto, and fire away. As you depress the shutter release, your camera tries to guess which subject in the frame is most important, and it focuses on that. The camera attempts to ascertain what kind of picture you're shooting (landscape, portrait, close-up, for example), and it chooses an exposure that is probably fairly close.

82 Part ll:0h- Shoot!

Clever little algorithms choose a shutter speed that eliminates subject- or camera-related blur (most of the time) and an f-stop that provides a decent compromise between depth-of-field and proper exposure. It's almost a given that your $1,000+ dSLR then produces pictures just as good as a $200 snapshot camera.

But that isn't what you bought your digital SLR for. Auto is the mode you use when you hand your camera to your fumble-fingered brother-in-law and ask him to take a picture of you and your kids. You'd rather maintain control over your exposures so you can apply f-stops and shutter speeds creatively.

Understanding why exposure is tricky

In a perfect world, your dSLR's sensor would be able to capture all the photons that reach it from your subjects. Dark areas would be represented by photosites (the individual picture elements) that received few photons, and light areas would be registered by photosites that captured lots and lots of photons. All the intermediate tones would fall somewhere in between.

But it doesn't work quite that way. Some dark areas might produce too few photons to produce an image at all; some very light areas might be represented by so many photons that excess light overflows from one pixel into adjacent pixels, causing a light smear known as blooming. (I introduce this in Chapter 2, where I explain how sensors work in more detail.)

Proper exposure helps ensure that the sensor receives enough light to capture detail in the dark areas of an image, but not so much that light areas are washed out. Figure 5-1 shows some of the choices you might have to make. Exposing for the shadows provides lots of detail in the top version (the original version of this image), but the highlights are completely washed out. The bottom version uses improved exposure to trade off some of the brightness in

Figure 5-1: Expose an image so highlights don't wash out; they are impossible to retrieve.

the shadows for more highlight detail. (Actually, I played with the exposure by using an image editor on the RAW version of the file, but this represents reallife choices nonetheless. You can find more information on using RAW files in Chapter 8.)

It's generally a good idea to try to preserve highlight details; they are impossible to retrieve if washed out. Brightening dark shadows is a lot easier than retrieving lost highlights.

Achieving such an ideal exposure can be tricky, because sensors have a fixed dynamic range, which is the range over which detail in both light and dark areas can be captured. No sensor's dynamic range covers the full gamut of illumination levels that you're likely to encounter in everyday photography, so the "right" exposure is likely to be a series of compromises.

The first compromise comes when the continuous range of brightness and darkness in an image is converted from that analog form into digital bits and bytes. During the conversion, an infinite gradation of tones is sliced up into a limited number of different shades — 256 different shades per color in the case of the 24-bit "full" color you probably work with in your image editor. Your digital SLR might capture more different tones than that — as many as 4,096 variations if your camera captures 12 bits per color channel — but you still end up with a measly 256 tones for each color in Photoshop (except if you're working with an HDR, high dynamic range, image).

Your mission, if you choose to accept it, is to make sure that the available tones are the right tones to represent your image. You don't have many to spare. Perhaps you're shooting a night scene and have lots of detail in the shadows that you want to preserve, but there are also some details in highlights illuminated by a street lamp. How can you even guess how the camera will capture each of these details? Let the histogram be your guide.

Getting exposure right with the histogram

Digital SLRs include a kind of display called a histogram, which is a chart shown on your LCD that displays the number of tones being captured at any brightness level. The number of pixels at each brightness level is shown on the histogram as a vertical bar, and there are 256 of these bars. The far-left position represents the darkest tones in your image, and the far-right slot shows the tones in the very lightest parts of your image.

Typically, a histogram looks something like a mountain, as shown in Figure 5-2. Most of the tones are clustered in the middle of the image because the average image has most of its detail in those middle tones. The bars are shorter at the dark or light ends of the scale because most images have less detail in the shadows and highlights. However, images that have a great deal of detail in the dark or light portions can have histograms that look very different, reflecting that particular distribution of tones.

Figure 5-2: With a well-exposed image, the histogram looks like this.

It isn't really possible to manipulate the shape of the histogram in your camera. For that, you need to use an image editor. What dSLR owners can do with a histogram display is use it to judge whether the current exposure is correct for the image. That's fairly easy to do:

1. Take your picture.

You have to take the picture first because, unfortunately, dSLRs can't display a histogram "live" (in real time). The sensor doesn't see the image until the exposure is actually made. (Point-and-shoot digital cameras might show a live histogram.)

2. Examine the histogram with your picture review function.

• If an image is overexposed, the graph is shifted towards the right side of the histogram, with some of the pixels representing lighter tones clipped off entirely, as you can see in Figure 5-3.

• An underexposed image has the opposite look: The tones are crowded at the left side, and some of the shadow detail is clipped off, as in Figure 5-4.

3. If you see either condition, compensate by changing the f-stop, shutter speed, or EV (more on these later in this chapter) to correct the exposure error.

If you're shooting RAW (see Chapter 8 for more about that), you might be able to adjust exposure and contrast when importing the image into your image editor. However, most of the time you'll want to get the exposure correct in the camera.

Figure 5-3: An overexposed image clusters all the information at the right side of the graph.
Figure 5-4: An underexposed image moves all the picture information to the left side of the histogram.

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