Depth of Field
An Explanation by Derek Godridge LRPS AFIAP
It’s the amount of the picture which is acceptably sharp to our eyes, both in front and behind the point of focus.
WHY does it matter?
e.g. For Portraits you only want a shallow DOF so the viewer concentrates only on the area in focus. It’s a great way of directing your viewer’s eye.
e.g. For Landscapes you want much more of the picture to be in focus to enable the viewer to see the foreground and background. Again you are inviting the viewer to travel around and through the picture.
HOW do you achieve it?
The main way is by varying the Aperture of the lens on your camera.
e.g. A Large Aperture has a SMALL “f” number but a LARGE DOF – say f32 e.g. A Small Aperture has a LARGE “f” number but a SMALL DOF – say f2.8
This MAY be confusing so just think that a LARGE number gives you a LARGE DOF whilst a SMALL number gives you a SMALL DOF.
Where to focus ?
We might think that the best place to focus is on the distant part of the picture to make sure those distant cliffs or mountains are in focus, but in fact we are then “wasting” a lot of DOF.
Why ? The way lenses work is that wherever we focus, approximately only 1/3 of the available area is sharp in front of that focal point and 2/3 is sharp BEHIND that focal point. So if we are focussing on those distant mountains we may not be getting the foreground in focus – even WITH that large aperture number (say f32)
The Answer ? Well just focus about 1/3 of the distance into the scene in order to maximise the parts of the picture which will be acceptably SHARP. Even with a portrait and with the correct f stop = say f5.6, if you focus on the eye then probably the end of the nose and the back of the head will be sharp.
HOW do the various Camera shooting MODES work with DOF ?
The main ones for us are “A” and “S” modes which stand for ‘A’perture or ‘S’hutter speed priority e.g. In “A” mode your camera is programmed to allow you to change the Aperture setting and then it automatically changes the Shutter speed to give you a correct exposure. In “S” mode whenever you change the shutter speed the camera will alter the aperture to give the correct exposure
However, many new photographers start out with the more automated modes such as those shown “People”, “Landscape” or “Sport”. This is when the camera decides what aperture/shutter speed combinations to use dependent upon which mode you have selected.
Just REMEMBER ....
If there is little available light and you have selected a LARGE f number (small aperture) that the resultant shutter speed the camera selects may not be fast enough to prevent CAMERA SHAKE.
Imaging a night time landscape shot with a tiny aperture and hence slow shutter speed and expecting to get the image sharp when you are hand holding the camera! So always check the selected shutter speed to ensure you can get a sharp image OR increase the ISO or use a TRIPOD. The camera may even be flashing to warn you!
Other ways of affecting Depth Of Field
By using different lenses OR zooming your lens in or out
e.g A Wide Angle lens will tend to give greater DOF than a Telephoto Lens
In environmental portraiture we may wish to include more of the background surroundings and hence use a 28mm lens whilst if we wish to make a portrait of the same person a longer lens will help to isolate the individual from the background and hence an 85mm or 105mm would be ideal.
OR Get Closer !