The first of a new set of tools to be offered by CPN for Canon photographers is an online depth-of-field calculator that allows you to input your camera, aperture, focal length and focus distance and get an almost instant calculation for your required near and far limits of focus, total depth-of-field and hyperfocal distance. But why is this useful?
Depth-of-field is the area of apparent or acceptable sharpness in an image and can also be described as the range of distance within a scene that will record as sharp. It is one of the main creative controls a photographer has in that you can choose a shallow depth-of-field to isolate a subject – such as a person or a flower - from a background or use a wide depth-of-field to keep many parts of a landscape in focus.
When calculating depth-of-field several variables need to be taken into account. One of these is the ‘Circle of Confusion’. The simplest way to explain the circle of confusion is to say that it represents the largest blurred circle that can still be clearly defined by the human eye from a given distance. This figure will depend on certain other factors such as the eyesight of the viewer, the conditions under which the circle is viewed and the size the display medium is enlarged from the original. To create a constant certain assumptions are made. These assumptions are that the eyesight of the viewer is able to determine resolution of around five line pairs per millimetre, it is viewed from around 25cm and the viewing size is approximately a 10x8inch print.
Using these figures, you can calculate circle of confusion figures for each of the Canon digital camera sensor sizes which provide figures of: Full-frame cameras: 0.03; APS-H format cameras: 0.023; and APS-C format cameras: 0.019.
To use the Canon depth-of-field calculator you simply select the name of your EOS camera from the drop down list, enter in your intended focal length, select the aperture from the drop down list, and put in your focus distance. After filling in all of the input information the calculator will quickly tell you your near limit of focus, far limit of focus, total depth-of-field, and the hyperfocal distance (the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp).
You can find out much more about depth-of-field with Canon cameras, and all of the variables involved, in the CPN Infobank.