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How To: Camera Basics, by Mike Farley LRPS
Using Aperture Creatively

All lenses have a characteristic known as depth of field, in which objects in front of and behind the point of focus appear acceptably sharp. The exact amount depends on a combination of the distance of the focus point from the lens, the focal length of the lens and the aperture set. Depth of field increases with short focal lengths, small apertures and distance. It decreases with long focal lengths, wide apertures and short focussing distance. Thus a wide-angle lens will exhibit good depth of field even at wide apertures, whereas a telephoto will have a shallower one despite using a small aperture.

A landscape photographer will normally want to get everything in the picture sharp, so will set a small aperture. A wide aperture, particularly on a long lens, is useful for rendering an unwanted background out of focus so that it does not detract from the subject. Another use is differential focussing where part of the image is rendered sharp and is contrasted by another part of the scene at a different distance, usually in the background, which is out of focus but still recognisable.

One area where problems can occur is in macro work, where even at small apertures depth of field can often be measured in millimetres. Unless the subject itself has a narrow plane, some areas will inevitably be out of focus and some care is necessary that this does not occur in a way that will distract from the final image. When it comes to composition, there are no hard or fast rules a wide aperture for a macro shot can concentrate the viewer’s attention on the part of the subject that the photographer intends.

Extensive depth of field works best with digital compacts as the sensors in these cameras are so small that they have lenses with short focal lengths. This results in a large depth of field at all apertures and focal lengths. DSLRs often have depth of field preview facility, which is achieved by stopping down the lens to the chosen aperture to allow the user to gauge the depth of field effect. The benefit is somewhat abated as the viewfinder goes very dark making it difficult to gauge the effect. This does not happen with the newer hybrid type since the signal to the electronic viewfinder is boosted to compensate.

One further reason for varying the aperture is that very few lenses perform at their best when wide open. It is normally best close the aperture by a couple of stops to improve image quality, which means that many zoom lenses perform best between f8 and f11.

Finally, it should also be remembered that there is a direct correlation between aperture and shutter speed. If a small aperture is set, the shutter speed will have to be slower, whereas a wide aperture means that shutter speed can decrease. This ensures that amount of light entering the lens to make a correct exposure remains the same.

The use of a long telephoto lens shot at its maximum aperture of f/4 has rendered the distracting background out of focus, which focuses the viewer’s attention on the kestrel.
A wide-angle lens and small aperture (f/10) has ensured a wide depth of field, so that the image is sharp from back to front.