Now the field of view 'a' is the angle which the camera can 'see' - a person's field of view is approximately 100 degrees (3D), or 120 degrees (2D), or 180 degrees (monochromatic peripheral vision). The focal length 'f' is the distance from the lens assembly to the detector, and the other element in the equation is the size of the sensor 'd'.

I can remove the current CS lens, and I can see the sensor, which measures approximately 5mm x 7mm. A standard 1/2" sensor has the dimensions: 4.8mm x 6.4mm, s it must be a 1/2" sensor that I have. Note that if you can't measure the sensor, you can always work it out with the camera supplier's data sheet - you need to know the horizontal angle of view, and the focal length of the fitted lens - put the numbers into the calculator at the bottom of the page to find the nearest sensor size.

There is a relationship between the angle of view 'a', focal length 'f' and size of the CCD 'd' - pythagoras can help here, and from school, I know that tangent is opposite over adjacent, so I can work out the missing sizes with the equations shown below.

I want a field of view 'a' of about 100 degrees, and now I know the sensor width 'd' (6.4mm), so I can work out the required focal length 'f', which is 2.68 mm.

Maplin do a 2.9mm CS lens, which works out at 95.6 degrees. The resulting image can be seen below:

The circle round the outside of the image, I believe, is due to the lens being suitable for a 1/3" sensor, not a 1/2" sensor - so I need to find a ~2.5mm CS lens suitable for a 1/2" sensor.

To make things easier, I've included a handy-dandy Javascript calculator - just 'calculate' the missing item.

Calculation CCD Size

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