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 ...

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