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Depth of Field

This is one of a series of articles exploring how the modern digital camera works.

Depth of field and focal plane are one and the same thing.

The focal plane is the area of the image, (the depth into the scene), that is in focus - the depth of field.

The depth of field in a photograph is determined by the use of the f/stop.

The larger the aperture, the lesser the focal plane; the smaller the aperture the greater the focal plane.

Not only is the f/stop used to help control exposure, it can also be used for artistry  - the exposure being taken care of with the other two elements of reciprocity.

In the image opposite, the focal plane can easily be seen in the tarmac.

This was shot at f/4 giving a very shallow depth of field.  

Because the shutter was wide open allowing in a lot of light, I was able to shoot at 1/80th sec because the ISO had been set to 100 to get crisp detail with the minimum of noise.

Conversely, setting the aperture to f/10 will allow the focal plane to encompass both the fore and background.

Again the other elements of reciprocity were altered to ensure good exposure.

In this case the ISO to 100 to reduce noise and the shutter release at 1/100th sec.

So what else does a wide aperture give us?

In a word, Bokeh.  

Taken from the Japanese word Boke, loosely translated as blur, Bokeh refers to the blurred background caused by wide aperture photography.

Bokeh is achieved by using a wide aperture, a long focal length or standing close to the subject, typically a combination of all three.

Wide Aperture:               The  wider the aperture the greater the blur in the background.

Focal length:                    With telephoto lenses, the longer the length of the lens, the more blurring will occur.


Distance from subject:    The closer you are to your subject the more the background will be blurred.

Bokeh can really enhance a photograph, by making the subject stand out and or appear as 3-dimensional.  But as in all things there is both good and bad Bokeh.

Below are two examples of Bokeh making the subject appear 3-Dimensional.

Both long focal lengths and close to the camera.

While both these shots were taken with a long focal length of 300mm and a 5.6 aperture, the shot on the left has a more pleasing background because the camera was closer to the subject, with a more distant background.

The shot on the right was further from the camera with a closer background.  The more defined background simply detracts from the image.

When to use an open and closed Aperture:

Large Aperture

Small Aperture