Focusing

The focal plane: This is the point where all the light rays converge, forming a sharp image. A lens has only one plane, parallel to the film, sharply in focus, with a circle of confusion (CoC) of about 1/50 mm. The CoC is the area of the film, illuminated by light from outside the camera, that appears on a print as a point - on either side of the focal plane, the CoC gets larger, becoming unsharp. More planes in the scene may appear sharp, but this depends on the depth of field, which is determined by the focal length and size of aperture (see page 50 in Photograph Australia). Objects with deep dimensions extend beyond the plane of focus, hence a suitable depth of field needs to be chosen to keep as much of the object in focus as possible.

Critical focusing varies between lenses. Extreme to medium wide-angle lenses are comparatively easy to focus, having, by their optical nature, a great depth of field. In landscape and city photography, there will be objects in the foreground and background that you want in focus. This requires depth of field management with selective focus: focusing on the midground with the aperture set small should allow the entire image to be in focus.

When you use standard to medium long lenses - 50 mm to 150 mm - care in focusing is important. At around 100 mm and beyond, the point of focus you choose can make or break an image. Wildlife photography often requires the use of extreme telephoto lenses - 400 mm to 600 mm - so understanding where and how to manage the focus is vital. My approach with longer lenses is to fix the point focus on an animal's head (if the subject is small and some distance away) or its eye (if the subject is close).

My 600 mm lens was set at a maximum aperture of f4 and selective focus was made on the yellow and blue sail on the right. The sail on the left and the background are out of focus due to depth of field.

Still with aperture set at f4, selective focus was made on the red and mauve sail on the left. The sail on the right and the background are out of focus due to depth of field.

Still with aperture set at f4, selective focus was made on the background. Now the sails are both out of focus due to depth of field.

I focused about midway between the sails and the buildings and set my aperture to f22 (the minimum aperture and therefore the greatest depth of field). This time both sails and the background are in focus.

Focus lock - When you compose a picture using auto focus, but don't want to focus on the centre of interest, you will need to use focus lock. First activate the focus by half-depressing the shutter button, which locks the auto focus on a selected area.

Then, while still half-depressing the shutter, adjust the composition. The focus will not be affected. Once the picture is re-composed, depress the shutter fully to make the exposure. An alternative, of course, is manual focus.