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Depth of Field (part 2)
Bokeh
Bokeh, from the Japanese word for 'blur' (pronounced as two discrete syllables: ‘bo’ and ‘ke’ as in kettle), refers to the area of an image that is intentionally thrown out of focus by the use of a large aperture. As strange as it may sound, the quality of the bokeh can be important to the overall quality of the image. Every lens produces its own characteristic bokeh, and one of the factors that influences this is the shape of the aperture in the lens. In an ideal world, the aperture would be perfectly circular, however, this would be very expensive to manufacture. Therefore, lens companies approximate a circular aperture by using curved diaphragm blades - the more blades, the closer the approximation to a true circle. On inexpensive lenses, five blades are quite common (the lens in the picture below has six blades). Higher quality lenses will often have eight or more to give a visually more pleasing effect.
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The reason the shape of the aperture is important is that the circle of confusion created by objects out of focus takes on the shape of the aperture in the image. Most of the time this will not be noticeable as all the circles of confusion overlap to create a smooth blur. However if you have bright out-of-focus points of light against a black background, the aperture shape will become clearly visible. The picture shown on the right clearly demonstrates this. You can see this was taken with a lens with a five blade aperture diaphragm as the point light sources show up resembling a pentagon (albeit one with curved sides).
Picture courtesy of Canon and taken with a Canon 50mm f/1.8 lens. |
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Circle of Confusion
In photography, a circle of confusion is a spot formed on the image sensor caused by the incoming rays from an object not being correctly focused. Consider a tiny bright spot five metres in front of the camera. If your camera focuses on this spot, it will create a small spot on the sensor. Another bright spot nearer to, or further away from the lens may not be in focus - this will form a blurred circle on the image sensor.
This blurred circle is called the circle of confusion, and from the illustration above you can see that as objects become nearer or further from the camera's focal distance, the circle of confusion gets bigger.
The depth-of-field is the region where the circle of confusion is less than the resolution of the image sensor. Once the circle of confusion gets bigger than a single pixel, it starts to overlap with other pixels and that part of the image starts to look out of focus. Think of this another way - if a large region of the photograph is out of focus and the circle of confusion at a given point covers 50000 nearby pixels (easily possible) – it probably means that any given pixel is going to be a blend of 50000 different circles of confusion and therefore you get a very smooth blend of colours. In the picture below the circle of confusion (you can clearly see one near the top a quarter of the way in from the left caused by a slightly brighter point in the background) covered about 150,000 pixels on the sensor.
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Depth of Field on Compact Cameras
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Control over the depth of field on a compact camera works in the same way as on a DSLR, however, it is not possible to get the really shallow depth of field images that are so easy on the DSLR. This is because the depth of field is dependent on the actual focal length of the lens and not the 35mm equivalent focal length, and for most compact cameras, the focal length is very small. Typically a compact camera with a 3x zoom may have a focal length range of six to 18mm which is not conducive to getting those classic shallow DoF portrait shots. |
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To create a very shallow depth of field, use the telephoto end of the zoom range and get as close to your subject as possible, while trying to maximise the distance between them and the background. Sometimes it is worth trying the macro mode and being a little more creative. The picture above was taken with a Fuji compact camera in macro mode. I stood no more than about four feet from the subject.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"Great photography is about depth of feeling, not depth of field."
- Peter Adams
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