the Next Step: Depth of Field

Depth of Field

Depth of Field is the area of sharpness in a photograph.  This is the area before and after the focus point in the scene that remains in sharp focus.

Depth of Field is controlled by three items:  1. The aperture setting.  2. The focal length of the lens.  3. The point of focus.

We know the aperture controls the amount of light entering the lens and being absorbed by the digital camera’s sensor.  Aperture settings are indicated by the letter “f”; f/1.8, f/2.0, f/2.8 … f/22, f/32, etc.  A small aperture is signified by a large number.  A large aperture is signified by a small number.  A small aperture could be f/22 or f/32.  A large aperture could be f/1.8 or f/2.0.  An aperture setting of f/2 allows in considerably more light than an aperture setting of f/22.  The more light that is allowed to hit the sensor, the smaller the Depth of Field.  The less light that hits the sensor, the greater the Depth of Field.

The focal length of the lens is the distance from the optical center of the lens to the camera’s imaging sensor when the lens is focused at infinity.   Focal lengths are designated by a measurement in millimeters. 

Lenses with a focal length of 4-16mm are considered to be “fish-eye” lenses.  They produce a distorted angle of view up to 180 degrees.  Lenses with a focal length of 16-35mm are called “wide angle” lenses, typically 108 degrees to 63 degrees.  A “standard” lens has a focal length of 35-50mm.  This is very similar to what the human eye sees.   With an angle of view of 8 degrees to 3 degrees, “telephoto” lenses have a focal length of 70-800mm. 

The shorter the focal length of the lens, the wider the angle of view.  The longer the focal length, the narrower the angle of view.   

Shorter focal lengths make the image appear small or far away from the camera.  The larger the focal length, the closer the image will appear in the photograph.

The shorter the focal length the more exaggerated the perspective will be, making the foreground and background appear further apart.  Just the opposite is true with a long focal length.  Items in the foreground and the background appear to be closer.

 

f/5.6, 1/10 sec, 70mm

The closer the camera is to the subject, the more the Depth of Field is reduced.  An example of this is represented by the image of a flower.  The blossoming flower is in focus, but the leaves and shrubbery in the background are out of focus.  The relatively large aperture (f/5.6) coupled with a medium focal length (70mm) produces a narrower Depth of Field.  This is an example of “macro” photography.  Macro photography automatically reduces the Depth of Field.  The photograph of the trees and water (small aperture, small focal length) exhibits a large Depth of Field.  The grass and the two trees in the foreground remain in focus as well as the trees behind the fountain in the background.

f/25, 1/125 sec, 40mm

Telephoto lenses with a small aperture are often used to capture images of birds and wildlife, not only to maintain a distance from the subject, but also when the photographer wants to blur the background but keep the bird in focus.  This image of a Western Kingbird placed the bird 75 feet from the camera.  F/8 is usually a medium aperture.  But the grassy hill in the background was another 125 feet from the bird, rendering the background blurry because of the long distance from the subject.

f/8, 1/160 sec, 300mm

  A telephoto lens can make the two objects appear to be much closer to each other than they really are.  There are numerous stories of paparazzi, under court order to not come within 75 feet of an individual or celebrity.  A telephoto lens can make the image of the paparazzi seem close to the celebrity.    

The point of focus is the specific spot of the scene that is clearly in focus.  This is the main or primary subject of the photographic image.  The point of focus can be a person, an animal, a tree, a car.  The photographer chooses the subject carefully and places that subject in focus.  Depending on the aperture and the focal length, the scene before the subject (the foreground, below the red line) and / or the scene after (the background, above the red line) the subject may be purposely out of focus.  The Depth of Field does not change abruptly before and after the point of focus.   

F7.6, 1/400 sec, 21mm

But, gradually the focus begins to lose sharpness.  This area that gradually is out of focus is called the “circle of confusion”.  The circle of confusion can be exaggerated by the size of the printed image.

f/4.0, 1.6X crop factor

To capture wildlife, photographers often use a telephoto lens and a tripod when only the animal or bird is wanted to be in focus and the foreground and/or background slightly out of focus.  A larger aperture and a short focal length will have a short Depth of Field, often tenths or hundredths of an inch.