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.
It can be argued that for a given composition, focal length does not affect depth of field significantly. Imagine taking a head-and-shoulders portrait of somebody. If the photographer took one picture and then changed to a lens of twice the focal length, he/she would have to move back and double the distance from the subject to achieve the same composition. The decrease in depth of field due to change in focal length is matched by a corresponding increase due to change in distance.
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 way 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. 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.
A demonstration tool for calculating depth of field is provided. Click: depth of field calculator
For a discussion of equivalence of different sensor formats, see: Equivalence
Depth of field – detail
For details of the mathematics, see: Wikipedia
When the subject distance is much less than the hyperfocal distance and the magnification is small (i.e. not macro), an approximation to depth of field is given by:
Where N is f/number, c is acceptable circle of confusion (mm), m is magnification (relative size of image on the sensor):
Where f is focal length, s is distance to subject (using consistent units). Hence if (f << s ):
Effect of f/number. From the expressions above, we can see that the effect of f/number (N) is straightforward. DOF is proportional to the f/number set on the lens – doubling the f/number doubles the DOF.
Effect of Focal Length. Considering the first expression above for DOF, if we take a given camera (c) and set up, for example, a head and shoulders shot, thus defining a fixed magnification (m), the DOF depends only on f/number (N). This gives rise to the observation mentioned above that for a fixed composition, the DOF is independent of the focal length. However, this is something of a half-truth – as when we change focal length, we have to adjust subject distance to get back to the same magnification, which restores the same DOF.
The second expression for DOF perhaps makes this more clear. Depth of field increases with distance to subject – and decreases with increases in focal length. If these 2 factors increase at the same rate, the effects balance out and we get no change in depth of field.
Effect of Sensor Size. To understand the effect of sensor size, we have to consider the “acceptable circle of confusion” (c) in the expressions above. Consider photographing a single, very small point of light. If we focus exactly on that point, the image created on the sensor will also be a tiny point. If we then focus slightly in front or behind the point of light, the image on the sensor will be a small, blurred circle. The question is, at what amount of out-of-focus will we start to notice the blurring..? The “acceptable circle of confusion” is the largest blurred circle on the sensor that, when viewed in normal conditions in the final print, will still be perceived as a point. Normal conditions is sometimes defined as viewing an 8” by 10” print at 25 centimetres, i.e. hand held.
The acceptable circle of confusion depends on the size of the sensor. If we start with the (physically small) image on the sensor and create an 8” by 10” print, the amount of magnification involved depends on the relative size of the sensor and the print. If the sensor were 1″ wide, the image would be magnified 10 times to fill a 10” print. If our small point of light was slightly out-of-focus and was recorded as a small blurred circle on the sensor, the blurred circle will be 10 times bigger on the print. So we can see that the larger the sensor, the less magnification is needed when printing and hence the more blur we can have on the sensor image before it becomes visible in the print. The acceptable circle of confusion is therefore proportional to the size of the sensor. Examples of acceptable circle of confusion for different sized sensors are:
- 0.029mm for 35mm sensor
- 0.018mm for APS-C Canon
- 0.015mm for Four Thirds
Note from the expressions above for DOF that if we set up a studio shot with a given subject distance (s), focal length (f) and f/number (N), the DOF is proportional to the acceptable circle of confusion (c), i.e. the sensor size. So a larger sensor gives a larger DOF.
However, consider setting up a full-frame 35mm camera and a Four-Thirds camera side by side, with the same f/number, and choosing lenses to get the same field of view, perhaps a head-and-shoulders shot. If we choose a standard 50mm lens on the full frame camera, we would need a 25mm lens on the Four-Thirds camera to reproduce the same field of view on its smaller sensor. The effect of the smaller sensor on the Four-Thirds camera would be to halve the DOF – but the focal length on the Four-Thirds camera will also be halved. The overall effect is a doubling of the DOF for the Four-Thirds camera, due to the use of a wider lens.
It is frequently said that a characteristic of cameras with small sensors is that they have large DOF. This again is a half-truth. The direct effect of reducing the size of sensor is to reduce the DOF. However, as described above, cameras with small sensors are generally used with lenses of short(er) focal lengths which, for a given composition, increases the DOF.
Thin lens equation
As a reminder, here is the standard thin lens equation:
1/s + 1/s’ = 1/f
Where s = subject distance, s’ = image distance, f = focal length