Making your aperture selection work for you

The terms F-stop and aperture are used interchangeably to describe the same thing: how much the lens opens to let light hit the camera's sensor. This is the second part of the relationship that reads shutter speed + f-stop = exposure.

In simple terms, the correct combination of shutter speed and lens opening produces the correct exposure. From a purely technical standpoint, you need a value of X for a proper exposure; if your shutter speed and aperture add up to X, you've got a good exposure.

Achieving proper exposure is a minimal accomplishment for the creative photographer, particularly with today's sophisticated digital cameras. The creative part of the process comes when the photographer decides on a certain combination of shutter speed and aperture that achieves proper exposure and also takes advantage of the nuances the two controls offer. Shutter speeds offer the ability to freeze or blur motion, and f-stops offer the ability to control depth of field.

Depth of field is an issue that many novice photographers have trouble grasping. Even though the photograph is a two-dimensional image, the scene depicted within it represents a three-dimensional world. The apparent distance from foreground to background is the image's depth of field. The range of apparent sharpness from foreground to background is determined by the size of the lens opening (the aperture or f-stop). Use a large lens opening, and the range of sharpness is minimal. This is described as "shallow depth of field" because only a small range of foreground to background area will be sharply focused. Use a small lens opening, and the range of apparent sharpness will be much greater.

Keeping the entire image area sharp from foreground to background depends entirely on your choice of aperture setting. The smaller the lens opening, the sharper the photograph's focus is from foreground to background.

Aperture sizes are determined by dividing the focal length of the lens by a particular lens opening. As the lens openings get smaller, the f-stop number increases, hence larger f-stop numbers translate to smaller lens openings.

For example, most high-end lenses offer great light-gathering capabilities. These lenses frequently offer a maximum lens opening of f2.8. As you close down the size of the lens opening (an action referred to as "stopping down") the lens opening gets smaller, whereas its f-stop designation gets larger. A normal sequence goes f2.8, f4, f5.6, f8, f11, f16, f22. Math aficionados will likely recognize this as a logarithmic progression.

Understanding that a larger lens opening results in a shallower depth of field, and a smaller lens opening provides greater depth of field, provides the photographer with a useful set of tools for creative control.

If you want the entire scene sharp from foreground to background, choose a small lens opening (large f-stop number, such as f11 or f16). Figure 5-2 illustrates what I mean.

If you're trying to photograph a subject within a cluttered scene and want to isolate your subject from everything else, go with a large lens opening and focus precisely on your subject to throw the background out of focus.

There are a couple of focusing options that you can use, zone and selective focusing, and they are described in the following two sections.

Zone focusing

This technique calls for you to choose a fast enough shutter speed to avoid camera shake, while choosing the smallest possible f-stop to maximize depth of field. Then you set the lens focus point to an approximate mid-point distance representing how far away you think your subject might be.

Thanks to the depth of field created from the small lens opening, you'll have a zone both before and behind your focusing point that will be in acceptably sharp focus.

I use zone focusing a lot at parties, proms, and other get-togethers where a lot of people are interacting. Because I'm usually employed to get candid shots, I don't want to tip off my subjects that I'm about to take their photo. Instead, I set my camera up with a fairly wide-angle lens, choose an aperture around f8, and pre-focus my camera to a distance of about five feet so that everything from three feet to 10 feet is in focus. As I walk around, I just point my camera and trip the shutter. This system works much faster than hoping my camera's auto-focus system can acquire a subject and lock focus before people realize I'm trying to take their picture.

Figure 5-2: To achieve maximum depth of field, I used a 1/90 shutter speed and an f9.5 aperture.

© 2004 Dan Simon

Figure 5-2: To achieve maximum depth of field, I used a 1/90 shutter speed and an f9.5 aperture.

© 2004 Dan Simon

H"^4' You can find information on a more extreme form of zone focusing using " a hyper-focal point in Chapter 11.

Selective focus

Selective focus is a popular technique for portrait photography and it revolves around minimizing depth of field in order to focus the eye on a specific subject.

Several factors are involved in making selective focus work properly:

1. Pick a longer focal length. Some lenses have more inherent depth of field than others. Longer lenses, such as telephotos, have much less inherent depth of field than wide-angle lenses.

2. Set your aperture to its widest possible opening to reduce depth of field.

3. Focus as tightly as you can while still creating an effective composition. The closer your focusing distance, the shorter the area of sharp focus, which maximizes the selective focus effect. Figure 5-3 shows two examples of selective focus.

Figure 5-3: Two images of the same scene reveal how selective focus works: The stop sign is the focusing point of the image on the left, whereas in the image on the right, the focusing point is the large cog in the foreground.

© 2004 Dan Simon

Figure 5-3: Two images of the same scene reveal how selective focus works: The stop sign is the focusing point of the image on the left, whereas in the image on the right, the focusing point is the large cog in the foreground.

© 2004 Dan Simon

Lenses have sweet spots. For most camera lenses, the sweet spot is two f-stops past the maximum aperture. Although it's not a hard and fast rule to try and hit this particular aperture, it's useful to have a reason for deviating from it.

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Digital Cameras For Beginners

Digital Cameras For Beginners

Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.

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