Depth of Field is one of the most important concepts for the photographer to understand – whatever style of photography they enjoy, landscape or people, macro or architectural, wildlife or still life.
Depth of field is the amount of the image that appears to be in acceptable focus. Although the lens can only precisely focus the image at one particular point, the loss of focus on parts of the image away from this point is gradual, and so only becomes apparent to the eyes at some distance from this point of precise focus. The amount of the image that appears sharp is therefore the depth of field.
Several factors control depth of field:
- Lens aperture
- Distance of the scene from the camera
- Focal length of lens
- Size of the camera sensor
While these all need to be understood for the photographer to make the most of creative control of the depth of field in their images, changing the lens aperture or the distance of the scene to the camera may make the most obvious difference to the photographer seeking more creative control from their SLR camera.
Depth Of Field Is Greatly Controlled By Lens Aperture
The size of the aperture (the hole that light passes through in the lens before it reaches the camera’s sensor) is one of the key factors controlling depth of field. Apertures and f numbers are considered in a related article, but in simple terms the smaller the f number, the larger the aperture (large hole for the light to pass through) and larger the f number the smaller the aperture (small hole for the light to pass through) see Fig 2 for an illustration showing this.
When light has a small aperture (hole) to pass through (eg f22), the light entering the lens is forced to be more parallel as it passes through the aperture. This results in the point at which they are acceptably focussed to be spread over a wider distance of sensor to lens, and so the depth of field appears large. When light has a large hole to pass through (eg f2.8), the rays can come from a greater variety of angles, and so as they pass through the image sensor they diverge again quite rapidly. This results in the point at which they are acceptably focussed to be spread over a much narrower distance of sensor to lens, and so the depth of field appears small. See fig 1 for an illustration showing this.
Depth Of Field Is Affected By Distance From Subject
The closer the camera is to the nearest part of the image, the smaller the depth of field. This is due to an effect called magnification, but in simple terms the camera can only focus on a small range of distances, and this effect is not linear. So if the nearest point focussed on is very close eg 1m, then an object 10m away is at 10x the distance of the nearest point, and so is more out of focus, which gives a smaller depth of field. However, if the nearest point is some distance away eg 50m, then an object at 100m, is only 2x as far away, and so is more in focus, hence a larger depth of field.
A simple way of increasing depth of field is therefore to move away from the nearest part of the image. Or to blur the background intentionally (less depth of field) move closer to the subject!
Length Of Lens Appears To Affects Depth Of Field
The longer the lens, the shallower the depth of field appears to be. It does not actually change the true depth of field, as proved by this Luminous Landscape article, but by changing the perspective, the resultant effect appears to be greater depth of field.
Size Of Sensor Influences Depth Of Field
The smaller the sensor the greater the depth of field. This is why most compact cameras are unable to utilise the Av setting to its greatest advantage – namely that of limiting depth of field to selectively blur the background to emphasise the subject or object in the foreground.
This article is intended as a simple guide to understanding depth of field. Anyone wanting a more technical explanation with lots of formulas may find this Wikipedia article useful.