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Infobank

Lenses: Focusing and depth-of-field

Most EF lenses have a small switch on the side marked AF/MF, or AF/M. AF is short for automatic focusing, and is probably the position you use most, if not all, of the time. Move to MF and you are in manual focusing mode. Here you need to use the focusing ring on the lens to bring the subject into focus.

Autofocusing is fast and accurate in most situations − certainly faster than most photographers can focus manually. So what is the point of manual focusing? Well, AF can become sluggish, or even stop, in low light conditions. Here, manual focusing may be your only option.

But manual focusing has others uses. Focus lock is a feature of the one-shot AF shooting mode. Partial pressure on the shutter button locks the focus, letting you focus on one part of the subject before recomposing the image in the viewfinder. But exposure is also locked at the same time. If you want to lock the focus without the exposure, use autofocus to set the lens, then flick the AF/MF switch to MF. The focus will not change as you recompose the image and press the shutter button again.

Most EF lenses have a switch on the barrel that lets you change from autofocusing to manual focusing.

Depth-of-field

Very occasionally, you will come across an EF lens that has a depth-of-field scale. There is one on the original EF50mm f/1.8 lens, though not on the Mark II version. You will not see a depth-of-field scale on many zoom lenses, because they become too complex.

A depth-of-field scale sits on either side of the focus index mark. The distance range encompassed by the two numbers representing the aperture set on the lens is the depth-of-field − the near and far limits of apparent sharpness in a photograph. So, in the above example (EF50mm f/1.8 lens), with the lens set to f/11, the depth-of-field extends from about 1.3m to about 1.8m.

If you want depth-of-field figures for any lens at any aperture and distance, you can use one of the many calculators available on the internet (try http://www.dofmaster.com/dofjs.html). Or if you own a Palm handheld organiser, you can buy the excellent and inexpensive Focus+ software and carry all this information in your pocket.

Infrared focusing

The depth-of-field scale usually incorporates a small red dot or line to the right of the focus index. This mark is there for users of infrared film. Infrared light comes to a focus in a slightly different plane to visible light. This means that if you focus visually, the infrared image will be out-of-focus. The trick is to focus normally first, switch to manual focus, and then turn the focusing ring a little to the right (anticlockwise) so that the focusing distance originally opposite the normal focusing index is opposite the red dot or line.

Full-time manual focusing

Many EF lenses can be focused manually while still in autofocus mode. This is called full-time manual focusing (FTM). It means that you can autofocus normally, and then use the focusing ring to fine-tune the focus − refocusing on the eyes when the camera has focused on the nose, for example. Full-time manual focusing is particularly useful for wildlife photography with telephoto lenses, where the camera may focus on rocks or a tree branch instead of the main subject.

There are actually two types of full-time manual focus. With one, turning the manual focusing ring sends an electronic signal to the lens motor, which then refocuses the lens. This system, more correctly called electronic manual focusing (E-M), is found mostly in the more expensive prime lenses and uses battery power. The second system provides a direct link between the focusing ring and the focusing mechanism. This does not require battery power.

The following list shows some of the EF lenses with E-M or FTM. However, it is easy to check your own lenses. If the focusing ring turns without changing the focus when the lens switch is set to ‘AF’, you don’t have full-time manual focusing. In this case, the focusing ring will only become active when the lens switch is set to ‘MF’.