As we have seen above, depth of field can vary greatly from photo to photo. Technically, the depth of field is determined by the amount of subject matter that stays in focus both in front of and behind the point of focus. Therefore, before you make changes to your depth of field, you first need to be able to focus the lens of your camera.

Selecting the Focus Mode

There are two focus modes in the EOS Digital Rebel: Auto Focus (AF), Manual Focus (MF). While manual focusing enables you to be very precise with your point of focus, there are times when you need to lock down your focus quickly. Shooting in the Auto Focus mode allows you to do that.

The focus mode control switch is located on the lens barrel (figure 1). When in the AF (auto focus) mode, the camera detects the subject through a seven-point pattern within the frame. The shooter has the option of selecting which point will detect the subject for proper focusing.

We decided to use the MF (manual focus) mode for this lesson.

To activate the manual focus on the Rebel, locate the focus control switch on the lens and switch the setting to MF (manual focus) (figure 2).

Controlling Depth of Field

Once you know how to set your focus within your frame, you can start taking pictures, but keep in mind that if the shooting mode of the camera is set to P ("Program", or automatic), you will not be able to control your depth of field, as the camera will make automatic settings depending on the availability of light. In order to control your depth of field, you need to manually adjust your aperture and shutter speeds.

Aperture and Shutter Speed Control

When the EOS Rebel is in the M (manual exposure) mode, you can quickly and easily change the shutter speed settings with the Selection dial, and press and hold the +/- button to easily change the aperture settings.

To set the camera to manual exposure, turn the mode selection dial until the "M" is next to the indicator mark (figure 3).

Shutter Speed

In the manual mode, simply use the selection dial on the top of the camera to choose the shutter speed you wish (figure 4). The selected shutter speed setting will appear on the left of the display on the back of the camera.

Aperture

To set the aperture press and hold the +/- button and turn the selection dial on the top of the camera to select the desired aperture (figure 5). The selected aperture setting will appear in the center of the display on the back of the camera.

The most important thing to remember is that your aperture setting controls your depth of field. The smaller your aperture number, or "f/stop", the shorter your depth of field will be.

Once the aperture is set, the shutter speed can be configured to accommodate the proper exposure. One of the nice things about digital cameras is that, if you take a shot and it's either too light or too dark, you can immediately make adjustments to the shutter speed until you render the proper exposure. Likewise, if you want to maintain a certain shutter speed, you can make adjustments to the aperture to render the proper exposure.

To demonstrate different depths of field, we took some portrait shots in a shaded area in front of a picket fence at different aperture settings. In the camera, we set the ISO to its lowest setting (100), set the Image Quality to Large 1, and set the White Balance to the "sun" icon to match the color temperature of daylight.

We then began by setting the aperture to f/22 for a long depth of field (figure 6). We then took a few shots at different shutter speeds until we arrived at a good exposure (figures 7 and 8).

In the result shot, notice how the picket fence in the background is just as focused as the subject. This is because our depth of field (f/22) is deep, allowing everything to stay in focus from front to back. Remember, the higher the aperture number, the deeper the depth of field.

Next, we wanted to achieve a shorter depth of field by setting the aperture to f/5.6 (figure 9). We adjusted the shutter speed to accommodate a good exposure, focused on the model, and took another shot (figure 10).

In the result shot, notice how the picket fence in the background is now somewhat fuzzy. This is because the focus area, or depth of field, is limited to the area around the model. The advantage to a limited depth of field is that the viewer's attention is drawn to whatever is in focus. In this case, it happens to be our model.

Next, we have a quick demonstration of focus and how the eye is drawn to objects in focus. The next couple of images were shot with the aperture set to 5.6 for a short depth of field.

We focused on a blade of grass a couple of feet from the camera and took a shot (figure 11).

With the model and fence out of focus, our attention is drawn to the blade of grass in the foreground, whereas in previous shots, the grass is barely noticeable.

Next, we changed the focus to the back fence and took a shot (figure 12).

Again, notice where the eye is drawn now: past the model to the crisp lines of the fence. You can see how determining your point of focus is crucial when shooting with a limited depth of field.

Using a Telephoto Lens

Next, we decided to substitute our zoom lens for a telephoto lens. With a longer focal length than standard or wide-angle lenses, the telephoto lens enables you to minimize the amount of background in your shot. This helps to keep the perspective of your subject looking natural. It also allows you to create the crop you want and to focus more on your subject. As with a traditional SLR camera, these interchangeable lenses are easy to switch out.

After attaching the lens, we maintained the distance between the camera and the model for comparative purposes (figure 13). Then we refocused on the model, positioned the camera to render an interesting composition, and took shots at both f/22 (figure 14) and f/5.6 (figure 15).

Again, notice the difference between the two settings. Figure 14 (shot at f/22) has everything in focus while Figure 15 (shot at f/5.6) focuses primarily on the subject. And with the tighter crop afforded to us by the telephoto lens, the attention is drawn even more toward the subject.