 # 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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