Depth of field refers to the distance behind and in front of the point of focus that appears acceptably sharp. This distance is inversely proportional to the aperture, as illustrated below.
And demonstrated in photographs…
So far, so simple. However, depth of field is complicated by the concept of acceptably sharp. Because there is no clearly defined point at which an object goes from being in-focus to out-of-focus, we instead talk about a circle of confusion to define a how much a point needs to blurred to be classified as out-of-focus. Essentially this is the point at which a blurred spot will still be perceived as a spot by the human eye—any more blurring and we would see it as out-of-focus. The size of this circle is affected by:
- camera type (sensor size)
- focal length
- print size
- viewing distance from the final image.
To get an idea of this concept of the circle of confusion, take a look at the image of the vase and two cups below.
Clearly the cups are out-of-focus, leading the viewer’s eye straight towards the vase in the middle. This diagram shows how the light falls on the camera sensor.
For the cups in both the foreground and background, the light imprint on the sensor is clearly bigger than the circle of confusion as both objects are out of focus. But as we stop the lens down, like in the photographs at the top of the article we would see the cups gradually creep into focus until all three items are sharp to the viewer.