Lens and Depth of Field
Lenses are characterised by their focal length and the maximum size of the aperture, measured as an “f/number”.
The focal length of a lens is the distance between the centre of the lens and the plane of the camera’s sensor when the lens is focussed on a distant object.
|28mm or less||35 to 50mm||85mm or more|
|Landscapes||General purpose||Portraits, nature|
The focal length determines the angle of view of a lens. For a standard SLR camera, a lens with focal length of about 35mm to 50mm gives a view that seems most “natural”. Lenses with shorter focal lengths give a wide field of view, “wide-angle”, and objects in the middle distance appear to be smaller / further away. In the other direction, long “telephoto” lenses have a narrow field of view and objects appear closer – like looking through binoculars.
The f/number expresses the lens aperture as a fraction of its focal length. For example if a lens is described as “50mm f/2”, then the maximum aperture is 50/2, i.e. 25mm.
Most lenses have a variable aperture and, as the lens is “stopped down”, the f/number continues to describe the size of the aperture as a fraction of its focal length. For example, if the width of the aperture is set to an eighth of the focal length, the aperture is called “f/8”. Notice that as the size of the aperture is reduced, the f/number gets bigger; this is a little counter-intuitive until one gets used to it.
Depth of field
When you focus the camera on any subject, there is always a band, stretching from in front of the subject to behind, in which all objects are in acceptably-sharp focus. This band of good focus is the “depth of field”. It is affected by lens aperture, focal length and distance to the subject as in this table:
|Shallow depth of field||Deep depth of field|
|Telephoto lens||Wide-angle lens|
|Subject close to camera||Subject in the distance|
|Wide aperture (low f/number)||Small aperture (high f/number)|
The pictures below show the effect of changing the f/number. The scene was taken with a standard 50mm lens at f/1.2 (first photo) and then f/16 (second photo), both focussed on the sign.
An interesting if somewhat technical concept is that of hyperfocal distance. The idea here is to imagine taking a photo of a landscape and focussing on the distant horizon. In this scenario, the band of acceptable focus will stretch from the middle distance to a point well passed the horizon. Our depth of field will extend beyond the farthest limits of the scene. So we can conceive of a distance at which we might focus the lens so that the far end of the depth of field just extends to the horizon. This is the hyperfocal distance.
For example, if I take a 50mm lens at f/8 and focus on the far horizon, the depth of field will extend from 9.75 metres in front of me to infinity (and beyond) – Buzz Lightyear would be proud. However, if I now focus at 9.9 metres, the depth of field will extend from 4.9 metres to (just) infinity (see footnote). So I get much more of the foreground in focus while still keeping the far end of the depth of field at infinity. It is clearly inconvenient to have to calculate such things but we can make use of the concept by focussing in the middle distance and using the camera’s depth of field preview button to check that the depth of field extends far enough into the distance.
Note that this concept should be used with care. When talking about depth of field, we are referring to the area of acceptable focus, not critical focus. In the example above, if it is important to get the horizon as sharp as possible, we should focus on the horizon.
More on depth of field
A demonstration tool for calculating depth of field is provided: Depth of field calculator
A more technical description of depth of field is here: Depth of field
Footnote - hyperfocal distance. When the lens is focussed at the hyperfocal distance, the near focus distance will be half the hyperfocal distance.