Depth Of Field Explained

Underline

Diagram showing depth of field range from a given aperture setting.
Camera set to f22 for 1.6 seconds, target focus point the centre pencil.
Camera set to f3.5 for 1/30th of a second, target focus point the centre pencil.
Camera set to f22 for 1/80th of a second using auto flash.
Camera set to f3.5 for 1/80th of a second using auto flash.
Older SLR lenses had a depth of field calculator etched on the barrel so you could work out the "In Focus" length at a given aperture.
Here for example ƒ16 would give a DOF range from 1.5 to about 3.5 meters with the focus point at 2 meters.

What is depth of field?

Simply put it is the distance in front of and beyond the subject that appears to be in focus!

When a subject is focused upon it is obviously at a set distance from the camera, this then gives a distance to the main point of the image known as the image focal distance.
However as the image is rarely a wafer thin object it contains depth which may be a point in front of and behind the image.
Depending on a combination of parameters for the camera this can determine how much of the chosen object is in focus, this is known as "Depth of field"
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Some Examples:

Here are four different example pictures, (albeit not very good ones) to help explain the depth of field effect in different circumstances. In all cases the target focus point was the centre, (white) pencil, the camera was mounted on a tripod to keep the same area of picture.
  1. Camera set to ƒ22 for 1.6 seconds notice nearly all the pencils are in focus.
  2. Camera set to ƒ3.5 for 1/30th of a second, in this case only the centre pencil is of reasonable focus quality.
  3. Camera set to ƒ22 for 1/80th of a second using auto flash, again nearly all in focus but slightly under exposed.
  4. Camera set to ƒ3.5 for 1/80th of a second using auto flash, better exposure but lacking focus.

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Older Style Lenses:

On older style lenses there used to be a scale where the depth of field at a given distance at a certain aperture setting could be calculated from a scale etched on the lens barrel.
At any aperture a certain amount of the subject is in focus in front and behind the main focal point, this is known as depth of field.
The smaller the aperture the greater the depth of field. Also lenses with a short focal length such as wide angle lenses have a greater depth of field.
A depth of field scale is etched onto the lens barrel, the example shows a distance setting of 2m and an aperture setting of ƒ/5.6, the subjects located within the 5.6 mark (between the 4 and 8) at both sides of the index mark, (roughly 1.8m in the front range, 2.4m in the rear range giving a depth of field of 0.6 Meters) are sharp and clear.

Note: An aperture setting of ƒ16 would give a greater depth of field, (1.5m to 3.2m approx.) as the aperture is smaller.

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Depth Of Field Scale Etched On The Lens Barrel.



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