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 Image quality and lenses

Image quality and lenses

The most common reason for poor image quality is insufficient light. With too low light levels, the colors will be dull and the images blurry.

The light level is measured in lux. Strong sunlight is approximately 100,000 lux; full day light is 10,000 lux and candle light is 1 lux. Usually at least 200 lux is required to capture good quality images.

Bright areas should be avoided, as images might become over-exposed and objects appear too dark. This problem typically arises when trying to capture an object in back light. A camera adjusts the exposure to obtain good average light level for the image, but the contrast of colors between the object and the background influences the exposure. To avoid this problem, small dark objects should be displayed on a dark background to achieve correct contrast and color.

In addition, if you compare the images from different cameras you will find that the quality differs considerably between different brands. These quality variations depend on several factors and are especially noticeable in low light conditions.

Determining factors
The major factors effecting image quality for cameras can be defined as follows:

Lens
Optical filter
Image sensor
Camera's Digital Signal Processor (DSP)
Compression standard and implementation

Varifocal lenses
A good lens is important for achieving high image quality. Good basic lenses can be found from a few manufacturers, but note that the basic lenses sourced from these suppliers should use glass (not plastic).

Infrared
An optical filter is placed between the lens and the image sensor. Its main purpose is to remove the invisible infrared (IR) light. All image sensors are sensitive to IR light; without this IR-cut filter the images tend to look very bad in some environments, e.g. in rooms lit with normal light bulbs.

Night cameras are not fitted with this filter as it reduces sensitivity. The filter is invariably a piece of glass, but it may also be built into the lens. In more expensive cameras an optical low pass filter is added. These reduce the amount of false color that can occur when the scene contains 'thin' details. See more on Day & Night functionality and the role of IR-cut filters.

Image sensors
Two main image sensor technologies are currently available: CCDs and CMOS sensors. CCDs are more sensitive in low light conditions and give better dynamics than CMOS sensors. CMOS sensors, on the other hand, provide alternative camera solutions that are both smaller and cheaper.

The raw image data from an image sensor must be processed in order to achieve a good image, i.e. the color must be trimmed, the edges must be sharpened, the exposure must be controlled, and the white balance must be adjusted. These functions are all driven by the Digital Signal Processor (DSP).

The image compression should be made in the hardware to achieve a high frame rate. It is important that the compression is made without losing the color levels and the details in the images.
 


A basic mono focal lens


 



A high quality varifocal lens

 


An optical IR-cut filter

 

 


A 1/3-inch CCD
 


A 1/4-inch CMOS sensor

The comparison chart above displays the relative sensitivity and wavelength for different sensors. Generally, black and white sensors are more sensitive than color sensors; CCDs are more sensitive than CMOS sensors. See also a comparison between CCDs and CMOS sensors.

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