How to Calculate the Field of View of a Camera Lens

Written by andrew kraft
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In photography, the field of view (FOV) is the angle between the rays extending from the centre of the lens to the edges of the camera's view for a given dimension: horizontal, vertical or diagonal. It indicates how wide of an image a camera can capture. Field of view is dependent upon the lens' focal length, as well as the dimensions of the film (or of the image sensor if a digital SLR camera is used). The rest is high school trigonometry.

Skill level:
Easy

Things you need

• Focal length of the lens
• Dimensions of the film or the image sensor

Instructions

1. 1

Find the focal length of your camera's lens. The focal length is the distance from the centre of the lens to the film or image sensor. You should be able to find this measurement expressed in millimetres (mm) somewhere on the barrel of the lens. Lenses that can be zoomed in and out, such as telephoto lenses, will have a range of focal lengths indicated with a dash separating the minimum and maximum focal lengths.

2. 2

Find the dimensions of the film, or of the image sensor if you are using a digital camera. For 35 mm film, the most popular film size, the actual image dimensions are 36 mm wide by 24 mm high. For digital cameras, refer to the camera's documentation to find the size of the image sensor.

3. 3

Use the following formula to calculate the field of view for the desired dimension:

FOV = 2atan(0.5w/f)

Where "w" is the width of the film/image sensor, "f" is the focal length, and "atan" indicates the inverse tangent.

For example, with 35 mm film (36 mm X 24 mm) and a lens with a 20 mm focal length,

FOV = 2atan(0.5 X 36mm/20mm) = 2atan(0.9) = 84 degrees horizontally.

To calculate the vertical or diagonal field of view, substitute the height or diagonal length of the film for "w."

Tips and warnings

• To calculate the inverse tangent, it's easiest to just use a calculator that has that function available.
• If you are using a digital SLR camera that is compatible with regular film lenses, you will need to multiply the lens' focal length by the camera's focal length multiplier, which can be found in the camera's documentation. This number compensates for the difference between the size of the camera's image sensor and the size of 35 mm film.

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