Calculating The Required Lens In Millimeters

Focal Length

Focal length is the distance from the "principal point" of a lens to its focal point, expressed in millimeters (mm).

Focal length controls the magnification of the image captured and the field of view.

Focal length can be fixed or variable - see "Varifocal & Zoom Lenses."

The chart below covers the basic information about focal length:

Formula Table	1/3"	2/3"	1/2"	1/4"
H = Horizontal Width (in feet)	F= 4.8 x OD       H	F= 8.8 x OD       H	F= 6.4 x OD       H	F= 3.6 x OD       H
V = Vertical Height (in feet)	F= 3.6 x OD       V	F= 6.6 x OD        V	F= 4.8 x OD       V	F= 2.7 x OD       V
OD = Object Distance from lens and viewing area (in feet)	H= 4.8 x OD       F	H= 8.8 x OD       F	H= 6.4 x OD       F	H= 3.6 x OD       F
F = Required Focal Length (in millimeters)	V= 3.6 x OD       F	V= 6.6 x OD       F	V= 4.8 x OD       F	V= 2.7 x OD       F

Example

Using a 1/3" camera, you want to see a 12 foot horizontal field of view from 25 feet away.

 4.8 x 25 = 10 mm lens (1/3" format)
      12

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