Steve Bavister (ePHOTOzine) describes depth-of-field in the following way ” when you cast your eye over a scene everything seems more or less equally sharp, but sometimes in the finished shot only part of the subject appears acceptably sharp. This zone of sharpness is called the depth-of-field and it extends in front of and behind the point that you actually forcus on”.
He goes on to write the size of the zone is determined by three key factors – the aperture of the lens, the focal length of the lens used, and the distance you are from the subject.
The way he summerises the practical use of depth-of-field is :’
To maximise depth-of-field and have as much sharp as possible use a wide angled lens set to a small aperture and stand back from your subject.
To minimise depth-of-field with only a small zone of the scene sharp use a telephoto lens, set a large aperture and get closer to your subject.
These are 3 images depicting depth-of field I found on the web
The following are three images I created to illustrate depth-of-field(1) of a semi blurred background to isolate the subject (2) a male model standing in front of water 2 fountains in the distance houses and mountains and clouds all in focus to have a large depth-of field (3)and a very limited depth of field of a close up of an orchid with the centre being sharp .
(1) I focused on the pink flowers and throwing out the background isolates the subject better but still keeps it within a recognisable environment.
(2) Focusing on the male model and a small aperture the subject in the foreground is sharp as is the landscape in the middle and houses and clouds in the distance.
(3)In the picture of the orchid the depth of field is really limited with the centre of the orchid very sharp and the background petals softer.