In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) is the ratio of the lens’s focal length to the diameter of the aperture or iris diameter. It is a dimensionless number that is a quantitative measure of lens speed. The f-number N is given by 
where is the focal length, and is the diameter of the aperture or iris diameter. For example, if a lens’s focal length is 10 mm and aperture diameter is 5 mm, the f-number is 2 and the aperture diameter is f/2.
Ignoring differences in light transmission performance, a lens with a greater f-number, the smaller the lens opening (f16 – narrow aperture), projects darker images. The brightness of the projected image (illuminance) relative to the brightness of the scene in the lens’s field of view (luminance) reduces with the square of the f-number. Doubling the f-number reduces the relative brightness by a factor of four. To maintain the same exposure when doubling the f-number, the exposure time would need to be four times as long.
Some lenses change the size of the aperture stop and thus the aperture or iris 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. There is also another precise iris control called P-Iris. It is an automatic system that involves a P-Iris lens and specialized software that optimize image quality. The system is designed to address the shortcomings of an auto-iris lens. A lens with a lower f-number is normally more expensive than a lens with a higher f-number. Most popular video security systems like Axis, Sony, RedLeaf, etc. apply these technologies in their cameras.
Exposure is the light’s amount that reaches to the sensor of camera. The longer the exposure time, the more light an image sensor takes. Bright environments require shorter exposure time, while low-light situation require longer exposure time. Note that when the exposure time is increased, motion blur is also increased, and increasing the iris opening has the downside of decreasing the depth of field.
Exposure Value (EV) is a number that represents a combination of two fundamental camera settings, speed of camera shutter and f-number, such that whole combinations that yield the same exposure have the same EV value (for any fixed scene luminance).
Although all camera settings with the same EV nominally give the same exposure, do not necessarily have the same picture. The f-number (relative aperture) determines the depth of field, and the shutter speed ( exposure time) determines the value of motion blur.
Exposure value is:
where
- N is the relative aperture (f-number)
- t is the exposure time in seconds
When deciding on the exposure, a shorter exposure time is recommended when fast movement or when a high frame rate is required. Image quality in poor lighting conditions will be improved by longer exposure time, but it may increase motion blur and lower the total frame rate since a longer time is required to expose each frame. In some network cameras, an automatic exposure setting means the frame rate will increase or decrease with the available light amount. It is only as the light level decreases that artificial light or prioritized frame rate or image quality is important.
Note that if more light than necessary can reach to device, the image will be over-exposed, and as a result, a white-out image is recorded. On the other hand, if insufficient light could reach the image capturing device, the image will not form properly, and will be dark (i.e., under-exposed). An over- or under- exposed image contains less details. Therefore, the right amount of light that can reach the image capturing device must be controlled a camera lens.
EV=0 corresponds to the exposure time of 1s and a relative aperture of f/1.0. If the EV is known, it can be used to select combinations of exposure time and f-number.
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