Getting familiar with the concept of depth of field is one of the biggest steps you can take to becoming a more artful photographer. I introduce you to depth of field in Chapters 2 and 5, but here's a quick recap just to hammer home the lesson:
II Depth of field refers to the distance over which objects in a photograph appear sharply focused.
✓ With a shallow, or small, depth of field, only your subject and objects very close to it appear sharp. Objects at a distance from the subject appear blurry.
✓ With a large depth of field, the zone of sharp focus extends to include distant objects.
Which arrangement works best depends entirely on your creative vision and your subject. In portraits, for example, a classic technique is to use a shallow depth of field, as in Figure 6-9. This approach increases emphasis on the subject while diminishing the impact of the background. But for the photo shown in Figure 6-10, I wanted the historical marker, the lighthouse, and the cottage behind to have equal weight in the scene, so I used settings that produced a large depth of field, keeping them all in sharp focus.
Shallow depth of field
Shallow depth of field
Note, though, that with a shallow depth of field, you can just as easily throw foreground objects into soft focus as background objects. In the lighthouse scene, for example, if I had used settings that produced a short depth of field and I set focus on the lighthouse, the historical marker and the cottage both might be outside the zone of sharp focus.
So how do you adjust depth of field? You have three points of control, as follows:
✓ Aperture setting (f-stop): The aperture is one of three exposure settings, all explained fully in Chapter 5. Depth of field increases as you stop down the aperture (by choosing a higher f-stop number). For shallow depth of field, open the aperture (by choosing a lower f-stop number).
Large depth of field
Large depth of field
Figure 6-11 offers an example. Notice that the trees in the background are much more softly focused in the f/5.6 example than in the f/11 version. Of course, changing the aperture requires adjusting the shutter speed or ISO to maintain the equivalent exposure; for these images, I adjusted shutter speed.
f/5.6, 1/1000 second f/11, 1/200 second f/5.6, 1/1000 second f/11, 1/200 second
✓ Lens focal length: In lay terms, focal length determines what the lens "sees." As you increase focal length, measured in millimeters, the angle of view narrows, objects appear larger in the frame, and — the important point for this discussion — depth of field decreases. Additionally, the spatial relationship of objects changes as you adjust focal length.
As an example, Figure 6-12 compares the same scene shot at focal lengths of 138mm and 255mm. I used the same aperture, f/22, for both examples.
Whether you have any focal-length flexibility depends on your lens: If you have a zoom lens, you can adjust the focal length — just zoom in or out. (The Rebel XS/1000D kit lens, for example, offers a focal range of 18-55mm.) If you don't have a zoom lens and can't switch to a lens offering a different focal length than your current lens, scratch this means of manipulating depth of field.
For more technical details about focal length and your camera, see the sidebar "Fun facts about focal length."
✓ Camera-to-subject distance: As you move the lens closer to your subject, depth of field decreases. This assumes that you don't zoom in or out to reframe the picture, thereby changing the focal length. If you do, depth of field is affected by both the camera position and focal length.
Together, these three factors determine the maximum and minimum depth of field that you can achieve, as illustrated by my clever artwork in Figure 6-13 and summed up in the following list:
✓ To produce the shallowest depth of field: Open the aperture as wide as possible (select the lowest f-stop number), zoom in to the maximum focal length of your lens, and get as close as possible to your subject.
✓ To produce maximum depth of field: Stop down the aperture to the highest possible f-stop setting, zoom out to the shortest focal length your lens offers, and move farther from your subject.
Greater depth of field:_
Select higher f-stop Decrease focal length (zoom out) Move farther from subject
Shorter depth of field:
Select lower f-stop Increase focal length (zoom in) Move closer to subject
Figure 6-13: Aperture, focal length, and your shooting distance determine depth of field.
Here are a few additional tips and tricks related to depth of field:
✓ When depth of field is a primary concern, try using aperture-priority autoexposure (Av). In this mode, detailed fully in Chapter 5, you set the f-stop, and the camera selects the appropriate shutter speed to produce a good exposure. The range of aperture settings you can access depends on your lens.
✓ Some of the fully automatic scene modes are also designed with depth of field in mind. Portrait and Close-Up modes produce shortened depth of field; Landscape mode produces a greater depth of field. You can't adjust aperture in these modes, however, so you're limited to the setting the camera chooses. And in certain lighting conditions, the camera may not be able to choose an aperture that produces the depth of field you expect from the selected mode.
✓ The Rebel XS/1000D also offers a special autoexposure mode called A-DEP, which stands for automatic depth of field. In this mode, the camera selects the aperture setting that it thinks will keep all objects in the frame within the zone of sharp focus. You can read more about this mode in the next section.
✓ Not sure which aperture setting you need to produce the depth of field you want? Good news: Your camera offers depth-of-field preview, which enables you to see in advance how the aperture affects the focus zone. See the section labeled "Checking depth of field" for details on how to use this feature.
Every lens can be characterized by its focal length, or in the case of a zoom lens, the range of focal lengths it offers. Measured in millimeters, focal length determines the camera's angle of view, the apparent size and distance of objects in the scene, and depth of field. According to photography tradition, a focal length of about 50mm is a "normal" lens. Most point-and-shoot cameras feature this focal length, which is a medium-range lens that works well for the type of snapshots that users of those kinds of cameras are likely to shoot.
A lens with a focal length under 35mm is typically known as a wide-angle lens because at that focal length, the camera has a wide angle of view and produces a long depth of field, making it good for landscape photography. A short focal length also has the effect of making objects seem smaller and farther away. At the other end of the spectrum, a lens with a focal length longer than about 80mm is considered a telephoto lens and often referred to as a long lens. With a long lens, angle of view narrows, depth of field decreases, and faraway subjects appear closer and larger, which is ideal for wildlife and sports photographers.
Note, however, that the focal lengths stated here and elsewhere in the book are so-called 35mm equivalent focal lengths. Here's the deal: For reasons that aren't really important, when you put a standard lens on most digital cameras, including your Rebel XS/1000D, the available frame area is reduced, as if you took a picture on a camera that uses 35mm film negatives (the kind you've probably been using for years) and then cropped it.
This so-called crop factor, sometimes also called the magnification factor, varies depending on the digital camera, which is why the photo industry adopted the 35mm-equivalent measuring stick as a standard. With your camera, the cropping factor is roughly 1.6. So the 18-55mm kit lens sold with the Rebel XS/1000D, for example, actually captures the approximate area you would get from a 27-82mm lens on a 35mm film camera. In the figure here, for example, the red outline indicates the image area that results from the 1.6 crop factor.
Note that although the area the lens can capture changes when you move a lens from a 35mm film camera to a digital body, depth of field isn't affected, nor are the spatial relationships between objects in the frame. So when lens shopping, you gauge those two characteristics of the lens by looking at the stated focal length — no digital-to-film conversion math is required.
✓ The extent to which background focus shifts as you adjust depth of field also is affected by the distance between the subject and the background. For increased background blurring, move the subject farther in front of the background.
✓ If you adjust aperture to affect depth of field, be sure to always keep an eye on shutter speed as well. To maintain the same exposure, shutter speed must change in tandem with aperture, and you may encounter a situation where the shutter speed is too slow to permit hand-holding of the camera. Lenses that offer optical image stabilization do enable most people to handhold the camera at slower shutter speeds than non-stabilized lenses, but double-check your results just to be sure. You can also consider raising the ISO setting to make the image sensor more reactive to light, but remember that higher ISO settings can produce noise. (Chapter 5 has details.)
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