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Article: Photographic lenses. (usage tips and focal length technique)
- Article from:
- PSA Journal
- Article date:
- July 1, 1992
- Author:
CopyrightCOPYRIGHT 1992 Photographic Society of America, Inc. This material is published under license from the publisher through the Gale Group, Farmington Hills, Michigan. All inquiries regarding rights should be directed to the Gale Group. (Hide copyright information)
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The focal length needed to cover a given negative can be determined by applying the Pythagorean theorem which states, the square of the length of the hypotenuse of a right triangle is equal to the sum of the squares of the lengths of the other two sides, or A squared plus B squared equals C squared.
If a lens is to cover a 4x5 inch negative then A = 4, B = 5.4 squared = 4x4 = 16, 5 squared = 5x5 = 25, 16 + 25 = 41. The square root of 41 = 6.4. The hypotenuse, or in this case the diagonal of the negative = 6.4 inches.
Lens focal length are always expressed in metric measure. There are 25.4 millimeters to the U.S. inch so 25.4 x 6.4 = 162.56 millimeters. ...