| Aperture and Depth of Field - how it works |
First, 2 comments: 1 - Depth of Field is not depth of focus: - There is only one plane of focus, at the distance where the lens is focused. (surprisingly enough). Any other part of the image, is increasingly blurred, the further away it is from the plane of focus. (Strictly speaking the plane of focus is not a plane, but part of the surface of a sphere whose radius is the distance of the focus point from the camera) 2 - There is no hard boundary between ‘within dof’ and ‘out of dof’: - As stated above, the further away an object is from the plane of focus, the more blurred it is. Depth of Field is simply defined as the region within which the level of blur will be undetectable to most people under normal viewing conditions. Does that mean Depth of Field is arbitrary? Essentially, yes, however there are conventions, which define how blurred an object can become before, it is perceived as blurred by the viewer. These conventions are all based on ‘Circles of Confusion’, which I will explain below. Confused by Circles of Confusion? |
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In the diagram above, the upright red arrow is the subject. The inverted red arrow is the image of the subject focussed by the grey lens onto the film plane (Solid vertical line). The yellow triangles show the cone of light leaving the upper tip of the subject, spreading out to hit the lens, and focussed (through the blue aperture) onto the film plane. On the other side of the film plane, the cone spreads out again. The green triangles show exactly the same thing for the light from the lower tip of the subject. In fact, these cones of light exist for all points on the subject visible from the lens. It is clear that if the image is focussed perfectly on the film plane, then an infinitely small point on the film plane represents each infinitely small point on the subject However, if the precise point of focus lies in front of, or behind the film plane, (in this case, the film plane is represented by one of the two dotted vertical lines). Then it is apparent from the diagram that each point on the subject is represented by a circle on the film plane. This is the so-called circle of confusion. How big can a circle of confusion get, before it is visible? Obviously different people will have different abilities to detect ‘blurryness’. Also viewing conditions will have an effect. For example, the further away the viewer is from an image, the harder it is to detect blur. Therefore conventions exist which define maximum circle size. One convention states that the circle of confusion must be smaller than 1/1730 of the size of the diagonal of the image. This is the convention I have used for my calculations. The larger the image, the larger the circle is allowed to be. This is possible because larger images tend to be viewed at longer distances. Try looking close up at the size of the dots on a billboard poster to see how this works. Interestingly there are 2000 pixels across the diagonal of my Olympus C2000 image. This means that, using this convention, the circle of confusion must be slightly smaller than a single pixel. Another convention used in 35mm photography states that the circles must not be larger than 0.03mm on the negative. This is slightly larger than the 1/1730 rule, which would give a maximum size on a 35mm negative of around 0.025mm Why does Aperture affect depth of field? If we duplicate the diagram above, but this time make the aperture (space between the blue lines at the lens plane) smaller we get: |
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In this case, the aperture restricts the diameter of the cones of light hitting the film plane. This means that, for the same subject distance and focus point, the circles of confusion are smaller. Some examples of depth of field In the following 10 photographs (not intended to win any awards J ), the camera was set up on a tripod, and the aperture was adjusted from F2.8 to F11. The first set of 5 was at a lens to subject distance of around 23 cm. The second set were at around 38 cm. The subject was supposed to be the coin, but I think the camera focussed slightly closer than this. In both cases, the lens
was set to full telephoto (105mm in 35mm equivalent) |