Image quality

Basic camera settings

The opening or aperture of a lens, also known as the iris, greatly affects the amount of light reaching the sensor. The f-number of the lens is the quotient of the focal length of the lens and the diameter of the opening. For example, a 50 mm lens with a 25 mm aperture would have an f- number of 2.0, as 50/25=2. The higher the f-number, the smaller the opening will be, and vice versa. A lower f-number means that more light will reach the sensor.

The aperture also affects the depth of field, that is, how much of the scene that is in focus at the same time. A wide open lens will have a very shallow depth of field. Objects slightly closer to or further from the camera than the set focus point will be out of focus. By increasing the f-number (thus closing the aperture), the depth of field increases, and the objects can be brought back into focus.

Figure 2: The aperture also affects the depth of field, that is, how much of the scene that can be in focus at the same time. 

Another camera setting directly connected to the amount of light available in the scene is the shutter speed. This is the amount of time that the shutter is opened for, allowing light to enter and hit the sensor and create an image, for example 1/50th of a second. When there is more light available, the shutter does not need to stay open for as long, so faster shutter speeds are possible. As the light decreases, the shutter speed needs to be slower, to allow the sensor more time to get enough light to form an image. When the shutter speed is very slow, anything moving in the scene will appear blurred in the image, as the object’s position changes during the capture. This is called motion blur, and has a negative effect on both image quality and usability of video.

Many cameras employ an internal boost of the image signal, called gain. To enable image capture in low light without affecting the shutter speed or the depth of field, the weak sensor signal can be electronically amplified, resulting in a brighter image. A side-effect of this is that tiny imperfections in the image are also amplified and are reproduced as image noise. This noise degrades image quality and generally requires more bandwidth for the video stream. Image noise also increases with rising temperatures, so adding active cooling to your camera can sometimes be useful.

Axis cameras automatically adjust the aperture, shutter speed and gain to produce an image that is always correctly exposed. It is also possible to specify a priority setting to favor either low noise or low motion blur, depending on your requirements.

 

Figure 3: Noise degrades image quality and generally increases the bandwidth needed for the video stream.

Advanced camera settings