NEXT MEETING … Thurs. 26th Sept 2013
A new season begins with cheese, wine and an illustrated talk by Club Members Adrian & Pam White with some of their amazing travel photography.
New or potential members always welcome – and you can register your interest straightaway by emailing email@example.com
From near to infinity
Depth of Field (DoF) describes how far in front of the main subject, and how far behind it, things are acceptably sharp. Sometimes you want very large DoF. For example landscapes may need a foreground object such as a tree stump in sharp focus while keeping the distant horizon equally sharp. At other times you may need a very shallow DoF so that perhaps one flower is sharply in focus with others either closer or further away nicely blurred.
- In summary, for landscapes with maximum DoF, use a wide angle lens and fairly small aperture such as f/11 or even smaller. For minimum DoF, use a medium telephoto at its full aperture focused as close as possible to the camera.
Small cameras with their small sensors have a much wider DoF than 35mm or APC-S cameras. Very large format cameras have an extremely small DoF which is one reason why professionals use complex tilt-and-shift lenses. Non-zoom lenses normally have a depth of field scale marked on the focus ring that help by showing large apertures such as f/2.8 have a narrow DoF while smaller ones (f/16, f/22, etc) are much wider.
For details about DoF theory, online calculators, and free software for making charts, see www . dofmaster . com. This web site explains about the Circle of Confusion (CoC) and Hyperfocal Distance (HD). Another excellent reference is http://en.wikipedia.org/wiki/Depth_of_field . The CoC for 35mm format cameras (e.g. Canon 5D) is normally taken to be 0.030mm and 0.019mm for APS-C cameras (e.g. Canon 60D, 600D). The CoC is determined by the human eye’s resolution when looking at a print at normal viewing distances; if you intend to crop heavily or print a very large poster, you would need to use a smaller CoC. The CoC for a APS-C sensor is only some 4 or 5 times the pixel spacing so there would be no point in using a CoC of less than say 0.010mm where diffraction effects will also limit the sharpness that can be obtained. For small compacts with their very short focal lengths, small sensors and tiny pixel spacing, the CoC can be as small as 0.004mm or about one tenth of the diameter of human hair.
For a more gentle introduction to Depth of Field concepts, you may prefer to look at http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
The Hyperfocal Distance is calculated using the actual focal length of the lens, the camera’s CoC and the selected aperture. This is why many photographers prefer to use Av mode. The importance of HD is that if the lens is focused at this distance, then everything beyond this all the way to the far distance will be acceptably sharp. Furthermore, the DoF will extend to halfway in front of the HD.
- A lens focused at its Hyperfocal Distance will be in focus from half that distance to infinity
For example, a lens focused at its Hyperfocal Distance of 16 metres will show everything from 8 metres to infinity in focus. If you want a tree stump 1.5 metre away to be in focus, you will need HD to be no more than 3 metres which for a APS-C DSLR 50mm lens means using f/45: this is impractical due to diffraction effects so using a wider angle shorter focal length 30mm lens means you only need f/16 – a much more practical setting.A less recognised fact is that any lens focused at or close to infinity will show everything beyond the Hyperfocal Distance as in focus.
Knowing HD can also help to get a shallow DoF, e.g. portraits or when you need to separate the subject from a cluttered background. However do note that the term “Depth of Field” relates to acceptable sharpness so if you want to throw background or foreground objects significantly out of focus, they may need to be several times further away than this calculated “Depth of Field”. The Japanese have a term for the aesthetic quality of this out-of-focus effect – “bokeh” http://en.wikipedia.org/wiki/Bokeh
Experience is the best guide to controlling DoF, plus using the DoF preview button on your SLR when the illumination is bright enough. However a rule of thumb applies when your lens is focussed at or less than 1/10th of HD:-
- focus distance 1/10th of HD, the DoF will be 1/10th of your focus distance in front & behind.
- focus distance 1/20th of HD, the DoF will be 1/20th of your focus distance, and so on.
Remembering all this can be tricky so some people print out a hyperfocal chart tailored for each of their lenses. Alternatively, if you pride yourself on mental arithmetic, there is a convenient way of remembering the Hyperfocal Distances for APS-C zoom lens(es).
Most zoom lenses have focal length markings which fit the following sequence:
The Hyperfocal Distances for an aperture f/8 and CoC = 0.019mm matching these form a simple and well known geometric progression where each term is twice the one preceeding it…
2, 4, 8, 16, 32, 64, 128, 256, … metres.
So you just need to remember the combined sequence…
2@18, 4@24, 8@35, 16@50, 32@70, 64@100, 128@135, 256@200 at f/8
For other apertures, e.g. for f/16 just multiply the HD by 8/16 (i.e. half) or for f/4 multiply by 8/4 (i.e. two).
For other cameras, the same sequence can be used but it is necessary to choose a different reference aperture. For example, a full frame SLR camera with CoC = 0.030mm follows the sequence at f/5.0 although it might be easier to approximate this to f/5.6.
If anyone is interested, we have a very simple compact and unique slide rule that gives hyperfocal distances and depth of field for any camera and any lens focal length/aperture combination. It is extremely quick and easy to use and is a great learning tool, being of more practical use than anything you might find on the web or elsewhere.
I hope this hasn’t confused you, and I’m always pleased to answer questions or queries.