Using depth of field scales on lenses
Using depth of field scales on lenses | ||
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You may find that your camera lens carries a depth of field scale, next to its scale of subject distances. The scale gives you a rough guide to the limits of depth of field and is useful if you are 'zone focusing' – presetting distance when there is no time to judge focus and depth of field visually. Scales also show how you can gain bonus depth of field in shooting distant scenes. For example, if it is focused on infinity (losing half your depth of field 'over the horizon'), read off the nearest subject distance sharp. This is called the 'hyperfocal distance' for the f-number you are using. Change your focus to this setting and depth of field will extend from half the distance through to the horizon. Depth of field is also exploited in some cheap cameras with simple symbols for setting lens focus. Typically a silhouette of mountains sets the lens to its hyperfocal distance; a 'group of people' symbol means 3.5 metres; while a 'single head' is 2 metres. Provided the lens has a small working aperture, these zones overlap in depth of field. So users stand a good chance of getting in-focus pictures as long as they make the correct choice of symbol. Remember that depth of field limits don't occur as abruptly as the figures suggest – sharpness deteriorates gradually. Much depends too on what you regard as a permissible 'circle of confusion'. If you intend to make big enlargements, your standard of sharpness on film must be higher, and this automatically means less depth of field. Even if your camera allows you to observe depth of field effects on a focusing screen you should work well within the limits of what looks sharp, or you may be disappointed with the final print. When using a camera giving a large-format image you can anticipate less enlargement than, say, with a 35 mm camera. But although this makes a larger circle of confusion permissible, the long focal length lens needed by the large camera to include the same angle of view has a much stronger and opposite influence. In practice the smaller your camera the greater your depth of field. However you also have to bear in mind the effect of the grain pattern of the chemically recorded image when a smaller film is enlarged. The same applies to the number of pixels used by a digital camera. Since final sharpness in your photograph is influenced by these factors as well as depth of field, the small camera is deprived of some of its advantages. | ||