Andy Harbin Photography
Capturing the beauty of The Cotswolds and surrounding areas

Some would have you believe that you cannot understand depth of field without first understanding 'Circle of Confusion'. Whilst the mathematics behind depth of field requires the circle of confusion figure for your camera / format, no prior knowledge is required. However, if you are interested in this subject click here to read the article.

Precise focus is only possible at one particular distance. As we come away from that point, elements of the image will become progressively more out of focus. In reality however more of the image (in front of and beyond the point of precise focus) will 'appear' sharp due to the limitations of the human eye. This area of 'acceptable focus' is called the depth of field.

Before proceding, a word of caution. Those just starting out in photography may well be tempted to stop their lens down to the smallest aperture to achieve the sharpest focus. Unfortunately this isn't the case as an effect called diffraction starts to creep in at apertures smaller than around f16 (for cameras with an APS-C sensor) and starts to blur the image! You may think this is very limiting but a basic wide angle zoom lens (set to say 20mm focal length) and an aperture of f11 will give you sharp focus from less than 1m away to infinity. What more could you want!

The actual depth of field is governed by a number of elements, namely Lens Focal Length, Aperture, Object Distance (Point of Focus) and the infamous Circle of Confusion for the camera used. The diagram below shows how the depth of field is affected by Aperture and Object Distance, given a lens of focal length 50mm and a Circle of Confusion of 0.019 (Canon EOS 400D).

Looking at the graph we can see that for an aperture of F2, focussing on an object 50m away we have a nearside focus limit of 28.42m and a farside focus limit of 207.67m. This gives a depth of field of 179.25m.

The graph also shows how the depth of field increases as the aperture is reduced (f number increases). At f32 the range increases to; nearside = 3.8m, farside = infinity but beware of diffraction! A PDF table for the graph above can be found by clicking here

Formulae:

Sould you want to try things for yourself, the formulae are given below. For information on Hyperfocal Distance, click here