Selecting the
Correct Lens
The quality of the lens is vital in determining the quality of the final image. The basic optical characteristics of a lens are the focal length (in zoom lenses the focal length range) or angle of coverage, and the aperture range. We’ll look at these two characteristics together with a third important characteristic, the format.
Focal Length (F)
The focal length represents the distance from the optical centre of the lens to the point at which the image is in sharpest focus and where the
The focal length determines the angle of coverage of the lens. The longer the focal length, the narrower the angle of coverage and the shorter the focal length, the wider the angle of coverage.
One particular angle of coverage produces a picture very similar to that seen by the human eye. A lens giving this angle is called a ‘normal’ lens. The focal length of a normal lens is approximately equal to the diagonal of the picture area. For example, for a 2/3 inch camera, the normal lens would have a focal length of 16mm. A 1/2 inch camera would have a normal lens of 12.5mm and a 1/3 inch camera, an 8mm normal lens.
A focal length shorter than a normal
focal length produces a wide angle |
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view. A longer focal length |
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produces a telephoto image. |
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Some lenses have variable |
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focal lengths and are called |
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zoom lenses. |
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d | F |
Basic Lens | FocalPlane |
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F= Focal Length
d= Lens Diameter or Aperture
f= Lens Stop Expressed as a ratio of Focal Length to Aperture
f = F
d
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