Control the Range of Focus

EXTEND YOUR CREATIVITY

There are various creative techniques that will make your images more interesting. Controlling the range of focus is one of the most elementary, but it can be a little confusing.

Depth of Field

Depth of Field is the area of apparent sharpness in front of and behind the focusing point of the lens. Depth of field does not extend equally either side of the focussing point but rather extends farther behind it than it does in front.

Although a lens can only focus on one subject at a time, the range of apparent focus, or sharpness, can be extended or reduced by adjusting the size of the hole through which light passes. This hole is known as the lens aperture. If the aperture is large, f2.8 for example, there will be less apparent focus than if the aperture is set to f16 (the bigger the number, the smaller the hole).

Many cameras have a special button or custom function setting that will allow you to see the effect of depth of field by closing down the lens to your chosen aperture. However, it’s not always easy to see the effect through the viewfinder.


Depth of Field and Depth of Focus Explained


Depth of Focus… is the distance by which the lens can be moved towards, or away from, the film when the subject is in focus without producing noticeable confusion and blurring of images on the final print. In practice some latitude is permissible in focusing a camera accurately, because the human eye does not notice a slight confusion in the image on a print.

When a distant object [A] casts a sharp image on film the image of a closer object will not be sharply in focus for it will be cast sharply behind the film at point B1. On the film itself the image of B will take the form of a blurred circle of confusion [A1]. But as the diaphragm of the lens is ‘stopped down’, i.e. made smaller in diameter, the circle of confusion will grow smaller and the depth of field will thus be increased. The smaller the diaphragm stop [aperture] the more sharp will the image of all objects, near and far, appear on the film.

Depth of Focus is sometimes called Depth of Field but this is wrong because, though the two are related, they are not the same thing. Depth of focus, as we have seen, is inside the camera whereas depth of field is outside it.

Eric de Maré (1910-2002)
Photography, (out of print)

Wide-angle lenses

A lens’s angle-of-view also affects the range of apparent sharpness. Wider lenses, or wider focal lengths on a zoom lens, will extend the range. A landscape shot, properly captured with a 20mm wide-angle lens set to a small aperture, will appear to be sharp from the foreground to the horizon.

Longer focal lengths

The sharpness range can be greatly reduced with longer focal lengths and larger apertures – an effect that can be used to isolate a subject from a distracting background or draw attention to the subject by blurring what is in front of it and what is behind it (selective focus).

Macro lens

The perceived range of focus with any lens decreases the closer the subject is to the camera. This very shallow range of focus presents unique difficulties when using a lens designed for the photography of small subjects. The camera should be perfectly steady and the subject still.

Summary

In more simple terms, the aperture setting, the lens’s focal length and the shooting distance, all work together to affect the appearance of sharpness in your images.

Hyperfocal Distance

You achieve hyperfocal distance with any lens/aperture combination by focusing on the nearest point to the film plane that keeps the farthest point of the scene (infinity) in focus. This technique is useful when using a wide-angle lens for landscape photography. Hyperfocal distance is thought by many to maximise depth of field. Maximum depth of field can mean a tripod will be necessary to keep the camera perfectly steady. Built-in stabilisation technology will help too.

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