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Video Camera Techniques, 2nd Edition by Gerald Millerson

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Focusing — The Principles

Strictly speaking, a lens only gives an absolutely sharp image for subjects at the distance to which it is focused (the focused plane). In practice however, things that are nearer or further away may still look reasonably sharp. The depth of this ‘sharpness zone’, can vary considerably; from a few centimeters to infinity (a)! Outside this zone, which is called the depth of field, subjects appear increasingly soft-focused.

What affects the depth of field?

The depth of field changes whenever you alter –

•  The focused distance – i.e. how far away you are focused.

•  The lens’s focal length – i.e. the zoom setting (hence the lens angle).

•  The lens aperture – i.e. the f-stop you are working at.

Change any of these, and ...

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