DEPTH OF FIELD
by Bill Diller

For anyone familiar with photography, the phrase 'depth of field' has special meaning. Simply put, it means the area in a photograph that is in focus. A narrow depth of field will isolate a particular subject, while a greater depth of field will have more of the subject in focus.

In landscape photography it's generally desirable to have as much of the scene as possible in acceptable focus. The opposite extreme would be to take a certain part of a scene, such as a single flower, and set it apart from its surroundings.

There are many variables in achieving these effects. The two that matter most has to do with the lens used, and how light is allowed to reach the film plane or sensor.

Photographic lenses are broadly categorized three ways: wide angle, normal and telephoto. A normal lens is described as giving a view similar to what you would see naturally. In essence, if you look through the viewfinder of a camera, and then take your eye away from it, the perspective changes very little. In 35mm photography, lenses of approximately 50mm to 60mm would meet these criteria. Lenses of 35mm or less are considered wide angle lenses. If you get down to a focal length of 18mm or less, you begin to get distortion. These lenses are called fish-eye lenses, and give a wider than 180 degree view. Conversely, lenses of greater focal length than normal are considered telephoto lenses, a category that goes from 85mm up to 2000mm, and beyond.

A fundamental rule is: the lower the number, the wider the focal length of the lens, the greater the depth of field it can provide.

If you've ever watched a sporting event on television, you've undoubtedly seen people carrying cameras with extremely large lenses. Some of these people also carry an additional camera with a much smaller lens, because the 'super' telephoto lenses won't focus up close. If the action comes near them, they switch to the camera with the smaller lens. If you were to see a side by side comparison between shots taken with the two lenses, you would get a good idea of what depth of field means.

A long lens, while good for close-up shots at long distances, has a disadvantage if you desire a greater depth of field. While the wide angle lens, in addition to giving a broader view, also has the advantage of providing a greater depth of field.

The second variable associated with depth of field has to do with how the camera is adjusted. There are two basic settings used to determine correct exposure: shutter speed and aperture. Both settings control the same thing, but in different ways. A camera works by allowing light to enter through an opening. When this light hits a piece of film, or in the case of digital photography, when it hits the sensor, a scene is recorded. This is accomplished by the opening and closing of a screen, or shutter, in front of the film plane or sensor.

Shutter speed determines the length of time the screen is open. Speeds are recorded in fractions of a second, such as 1/2000, 1/500, 1/125 and so on. Cameras are also capable of having the shutter open for longer periods of time, up to 30 seconds and beyond.

The aperture setting determines how wide the screen will open. Settings for aperture are recorded as f stops. In this case the lower the number, such as f 1.4 or f 2.8 mean the shutter screen is open wider, while higher numbers, such as f 11 or f 22, mean the shutter screen is less open. When you hear someone say their shutter was stopped down, it means they were shooting at a higher number f stop. Conversely, when they say they took a shot wide open, it means they were using a smaller number f stop. Sounds complicated, and backward, but it's really very simple. Envision the screen as a flat surface with a round opening. As you rotate the f stop dial from lower numbers to higher numbers, the opening becomes smaller.

Together the shutter speed and aperture setting determine exposure.

What does this have to do with depth of field?

The combination of lens length and shutter openings are the determining factors in how much of a scene in any particular photograph will be in focus. Shorter focal length lenses, set to smaller or higher numbered, apertures, give the maximum depth of field. Conversely, long focal length lenses set to more open, or smaller numbered, apertures, give the least depth of field. The photographer determines which lens/shutter opening combination will work to give the desired effect.

To complicate matters, with any lens you can change the depth of field by changing the shutter/aperture relationship. It sounds harder than it is. To put it in practice, focus on a particular object and take a meter reading with the f stop set to the smallest number, such as f 1.4 or f 2.8. Note what shutter speed is determined by the light meter. Let's say the proper exposure is determined to be f 1.4 at 1/500 seconds. If you stop down to f 11, the meter will determine the correct shutter speed to be 1/15 of a second, a combination that will give you a much greater depth of field than the original setting.

Using a wider angle lens will increase the depth of field. If you took the same scene, using the same settings, with different focal length lenses, the depth of field will be greatest the wider (lower number) the focal length of the lens.

Basically, it comes down to this: No matter what focal length lens you use, the higher number f stop, the greater the depth of field.

You can see tremendous differences by experimenting with different lens/aperture combinations, and depth of field can make or break a photograph. But of course, like beauty, a good photo is in the eye of the beholder.