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3.2.3 Lens mount standards
When changing a lens, it is also important to know what type of lens mount the network camera
has. There are two main standards used on network cameras: CS-mount and C-mount. They both
have a 1-inch thread and they look the same. What differs is the distance from the lenses to the
sensor when fitted on the camera:
>
CS-mount.
The distance between the sensor and the lens should be 12.5 mm.
>
C-mount.
The distance between the sensor and the lens should be 17.526 mm.
It is possible to mount a C-mount lens to a CS-mount camera body by using a 5 mm spacer (C/CS
adapter ring). If it is impossible to focus a camera, it is likely that the wrong type of lens is used.
3.2.4 F-number and exposure
In low-light situations, particularly in indoor environments, an important factor to look for in a
network camera is the lens’ light-gathering ability. This can be determined by the lens’ f-num-
ber, also known as f-stop. An f-number defines how much light can pass through a lens.
An f-number is the ratio of the lens’ focal length to the diameter of the aperture or iris diameter;
that is, f-number = focal length/aperture.
The smaller the f-number (either short focal length relative to the aperture, or large aperture
relative to the focal length), the better the lens’ light gathering ability; i.e. more light can pass
through the lens to the image sensor. In low-light situations, a smaller f-number generally pro-
duces a better image quality. (There may be some sensors, however, that may not be able to take
advantage of a lower f-number in low-light situations due to the way they are designed.)
A higher f-number, on the other hand, increases the depth of field, which is explained in section
3.2.6. A lens with a lower f-number is normally more expensive than a lens with a higher
f-number.
F-numbers are often written as F/x. The slash indicates division. An F/4 means the iris diameter
is equal to the focal length divided by 4; so if a camera has an 8 mm lens, light must pass
through an iris opening that is 2 mm in diameter.
While lenses with automatically adjustable iris (DC-iris) have a range of f-numbers, often only
the maximum light gathering end of the range (smallest f-number) is specified.
A lens’ light-gathering ability or f-number, and the exposure time (i.e., the length of time an
image sensor is exposed to light) are the two main elements that control how much light an
image sensor receives. A third element, the gain, is an amplifier that is used to make the image
brighter. However, increasing the gain also increases the level of noise (graininess) in an image,
so adjusting the exposure time or iris opening is preferred.
CAMERA ELEMENTS - CHAPTER 3
