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There are two main factors that must be considered when selecting the most appropriate lens for a particular application. The focal length and the type of iris control.
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Choosing the Right Lens
There are two main mounting standards. They both have a 2.54mm thread and look the same.
CS-mount. The distance between the sensor and the lens should be 12.5mm.
C-mount. The distance between the sensor and the lens should be 17.5mm. A 5mm spacer (C/CS adapter ring) can be used to convert a C-mount lens to a CS-mount lens.
There are two main factors that must be considered when selecting the most appropriate lens for a particular application. The focal length and the type of iris control.

Focal Length
The focal length of a lens determines the field of view at particular distances. A small focal length gives a wide-angle view. A large focal length gives a narrow telephoto view. Wide-angle lenses have a better depth of field than telephoto lenses. This means that you can focus both close to the camera as well as at a distance. Telephoto lenses require a more precise focus adjustment.

The focal length of the lens requires careful selection to ensure that the correct area is in view and that the degree of detail is acceptable. A rule of thumb is that to be able to “see” a person on a monitor they should represent at least 10% of the screen height. To “see” in this context means to be able to able to decide that it is a person. For purposes of being able to identify that person requires them to be at least 30% of the screen height and preferably greater than 60%.

There are three main types of lenses:
Monofocal. The focal length is fixed.
Variable focal zoom. This is a design of lens that has a limited range of manual focal length adjustment. Whenever the focal length is altered, the lens has to be refocused.
Zoom. The focal length can be adjusted over a range. The image stays in focus when the focal length is adjusted. The zoom may be manual or motorised.

The focal length of the lens in combination with the size of the sensor gives the viewing angle. The lens must make an image large enough for the sensor. The larger the sensor, the more expensive the lens.

The angle of view is given by the following formulae:

Horizontal Angle of view = 2 x tan-1 (w/2f)

Vertical Angle of View = 2 x tan-1 (h/2f)

Where, f is the focal length and w and h are respectively the width and height of the sensor. The angle of view for a given focal length varies according to the sensor size.

The relationship between sensor size, image size, the distance of the object from the camera and the focal length of the lens is given below:

f = w x D/W

where f is the focal length, D is the distance to the object, W is the width of the object and w is the width of the sensor [w=3.6mm, 1/4” sensor, w=4.8mm, 1/3” sensor]. The aspect ratio, Horizontal Size/Vertical Size, is 4/3.

Lens Calculator
 
Camera image sensor:
3.6mm (1/4")
4.8mm (1/3")
 
  Width: Distance to the scene:  
Height:
Focal Length   (mm)
Iris
The role of the iris is to adjust the amount of light passing through to the lens. There are three different types of irises on lenses;

Manual Iris. On a manual iris lens the iris is fixed or adjusted by manually turning a ring.
DC Auto-Iris. The Iris is connected to the output of the camera and is automatically adjusted by the Digital Signal Processor (DSP) in the camera.
Video Auto Iris. This is sometimes referred to as a servo lens. The iris is driven by the video signal from the camera (the voltage if the video signal is proportional to the amount of light on the imaging device).

Auto iris lenses are recommended for outdoor applications. The iris automatically adjusts the amount of light reaching the imaging sensor for optimum results and also protects the sensor from too strong light.

A small iris diameter reduces the amount of light and gives a better depth of field (the image is in focus over a greater distance). A large iris diameter will give a better image under low light conditions.

F-Number
The size of the aperture is known as the “f-number” of the lens. This is the ratio of the mechanical components of the lens and is defined as:

f-number = focal length/effective diameter

The effective diameter is related to the size of the front lens and is not the actual diameter. The f-number is a measure of the amount of light that the lens will pass to the imaging device. The smaller the number then the larger is the aperture.

The table below shows the amount of light falling on a lens that is passed to the sensor, as a function of f-number.

Light passed by f-numbers
f-number f0 f1.2 f1.4 f1.7 f2.0 f2.8 f4.0 f5.6
% of light passed 20% 14.14% 10% 7.07% 5% 2.5% 1.25% 0.625%
For a reference guide to "Typical Illumination Levels" and "Scene Reflectance", please refer to Capturing Images.
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