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# P1FL - distortion called curvilinear Straight lines may...

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Art Photography I   PGY1401C Focal Length and Apertures   We categorize lenses by  focal length .  Focal length is the distance  from the optical center of the lens, when focused at infinity, to the film  plane.  It is usually expressed in millimeters. Normal  focal length equals the diagonal measurement, in mm, of one  piece of film.  The larger the film size the longer the focal length  required to reproduce normal perspective.  This normal focal length  tries to match the view we have with our eyes, about 45 degrees.  For  35mm film, the normal focal length is  50mm Wide angle  lenses give a wider than normal view.  They have focal  lengths of  < 50mm .  They will usually make your subject recede and  the foreground will lengthen.  Wide angle lenses have an inherent

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Unformatted text preview: distortion called curvilinear . Straight lines may curve and parallel lines may converge. Telephoto lenses have longer focal lengths, 50mm > . They bring your subject closer to you in viewpoint and tend to compress distances between objects. This is a type of distortion called compression . You will notice a narrower angle of view also. Aperture numbers are ratios relating the focal length to the actual diameter of the diaphragm opening. The formula is: f# = Focal Length / Diaphragm Diameter Example: a 100mm lens set at an opening of 25mm has an f# of 4 (100/25 = 4). Lens with larger aperture openings are called fast. Smaller apertures sizes are called slow , capturing less light....
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