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Figure 1-20 Subject Coverage Compared to the Lens Focal Length
How to Use Lenses

Photography (Basic) - Introduction to photography and other graphic techniques
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Figure 1-21.–Angle of field. closely  resembles  the  central  vision  coverage  of  the human eye. Wide-angle lenses have a large or wide angle of field Long focal-length lenses (often called telephotolenses) have a narrow angle of field (fig. 1-21). Angle  of  View.–Angle of view determines the coverage of a lens with a particular size of film, with the lens-to-film distance remaining unchanged. Angle of view is an angle with the intercept point at the lens and its sides matching the corners of the film. The angle of view (fig. 1-22) of a normal focal- length lens with a given film size can approach but never exceed the angle of field of the lens. Any lens recording an angle greater than 55 degrees with a given film size has  a  short-focal  length  and  is  called  a  wide-angle  lens. Any lens with an angle of view less than 45 degrees with a given film size has a longer focal length. Lens Diaphragm The diaphragm of a lens is an opening in the lens that allows light to pass through it to expose the film (or other recording medium). This opening can be made larger or smaller to allow more or less light to pass through the lens. When the diaphragm opening is very Figure 1-22.–Angle of view. large, only the object that the lens is focused on is in sharp focus. As the diaphragm opening is reduced (made smaller), more objects in a scene, both in front and behind the point of focus become sharper. The lens diaphragm is used in conjunction with the shutter of the camera to control the amount of light to expose the film. PERSPECTIVE The human eyes see objects in three dimensions, but a lens reproduces a view in two dimensions. The missing dimension, depth, is suggested by the relative size and position of the various objects in a picture. Perspective, which is the relationship of objects in a photograph, affects the naturalness of a picture. Good perspective represents  objects  as  they  actually  appear  to  human eyes. Since wide-angle lenses take in a greater area, most photographers  use  them  to  photograph  in  tight  quarters. And they use long-focus (long focal length) lenses to bring distant objects closer. This is fine, but it is only part of the story. Lenses of different focal length are also used  to  control  perspective. Perspective is NOT dependent on the focal length of the lens. It is a function of camera-to-subject distance. But a choice of lenses of a different focal length does enable you to get the desired image size at the selected distance for best perspective. For example, suppose you come across a placid farm scene. A rustic rail fence is in the foreground, and a cow is munching on a haystack in the field. The cow and her lunch are 100 feet behind the fence; you are 10 feet in front of the fence. The fence is essential to your picture and you use a 50mm lens. The result! The cow is 110 feet from the camera and is too small in relation to the fence. Your picture is a flop. Now change your perspective. Back up 40 feet from the fence 1-17







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