Understanding depth of field and plane of focus

Of the three variables that control exposure (shutter speed, film speed, and aperture), aperture is the least understood by beginning photographers. In an effort to explain how your camera’s aperture setting works, and how it affects your photography, I’ve put together the following explanation.

Aperture and quantity of light

The aperture setting is a number that represents the size of the diaphragm opening your lens. It is preceded by “f/”. These numbers appear in a common sequence: f/2.8, f/4, f/5.6, f/8, f/11, f/16/, f/22. The lower the number, the larger the opening in the aperture, and more light goes through the lens. The difference between each number, called a “stop”, represents doubling or halving the amount of light. Going from f/5.6 to f/8 (stated as “going down one stop”) means half as much light. f/4 to f/2.8 means twice as much light. f/16 to f/8 is two stops, or four times the light, and so on.

Plane of focus

When you focus on a point with your camera, you’re determining the ‘plane of focus.’ From this point in every direction on a plane parallel to your camera, subjects will be in focus. Nearly all lenses have a minimum distance you can focus (called, logically, “minimum focal distance” or “MFD”). On the other end of the spectrum is “infinity focus”. This represents the point where the lens doesn’t differentiate further and further distant points of focus. When you focus your lens on a subject, you’re determining a point between MFD and infinity.

Depth of field

Depth of field, sometimes called zone of focus, is the amount of your scene in front of, and behind your point of focus, that is in focus. The width of this zone of focus, “the depth of field”, is determined by your aperture number. The higher the number, the wider the depth of field and more of your scene is in focus. A picture taken at f/4 will have less in focus than the same picture at f/16.

Distance and depth of field

As the point of focus gets closer to the camera, the depth of field gets smaller. Macro photographers know that depth of field is challenging when your subject is very small… the antenna of an ant can be in focus and it’s body out of focus. Conversely, as the focus point gets farther away, depth of field expands.

Depth of field and lens length

Related to the distance aspect explained, the length of the lens dramatically affects depth of field. Wide angle lenses (those having a length shorter than a normal lens) will have more depth of field at an aperture setting, and the wider the lens, the wider the depth of field. In fact, extreme wide angle lenses almost don’t need to be focused at many aperture settings because the depth of field is so deep. This attribute is called hyperfocal distance.

Hyperfocal distance

Hyperfocal distance is the minimum distance required to make the picture everything in focus, out to infinity. This distance is based on the length of the lens and a given aperture setting, and it’s useful to know when you want to render the whole scene sharp without focusing.

Zone focusing

Related to the hyperfocal distance, some photographers (especially “street photographers”) employ zone focusing. They prefocus their camera at a certain distance (say, 10’ or 3 meters) and then select a depth of field that will give them a wide zone of focus on either side of that point. For example, with a 50mm lens focused at 3 meters and an aperture setting of f/8, you might have a zone of focus of 2 meters to 4. The photographer then needs only worry about subject and composition, and they can be confident the subject will be in focus.

Depth of field and camera sensors

Visualize the shape of an hourglass, with your subject on one end and your camera’s sensor (or film plane) on the other. In the middle, at the narrowest point, is your lens. As the depth of field around your subject increases with aperture number, the zone of focus on the other side – at the camera sensor – widens as well. Normally this change is inconsequential, but if your sensor is dirty, the specks of dust will appear in your picture at wider depths of fields (higher f/ numbers).

Key points about depth of field:

  1. Depth of field, the amount of your scene in focus, increases as the f/stop number gets bigger. f/4 has less in focus than f/16.
  2. The closer something is, the shallower the depth of field, even at the same f/stop number.
  3. Wider lenses have inherently more depth of field than longer lenses. f/4 with a 20mm lens offers far greater depth of field than f/4 with a 100mm lens.

In future articles, I’ll give detailed explanations about shutter speed and film/sensor speed, and how these three characteristics work together to determine exposure.

Shoot more.

Article originally published on Startphoto on July 19, 2013.

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