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Required resolution

The traditional way of defining requirements for resolution of an analog CCTV system has been by specifying what percentage of the full screen the observed object occupies. Different surveillance objectives require different percentages.

For example, detecting the presence of a person in a scene could require that the person occupies 10% of the view. Recognizing a known person, however, could require that the person occupies 50%, and further identifying that person could require 120% or more.

Examples from Stockholm Subway using AXIS 225FD Network Camera

   
  20%    40%    140%
 Too small
for recognition
   Face turned away
from camera
   Face partly turned away
from camera

Today, network video cameras offer a wide range of available resolutions. Using the percentage requirements is no longer practical, and pixels are now used when specifying resolution requirements. For a detailed discussion on resolution requirements for identification, recognition and detection, see the Perfect pixel count tutorial.

Other criteria are valid for objects such as license plates, where typical recommendations are that the height of letters should be represented by 15 pixels (corresponding to about 200 pixels/m) to ensure legibility.

It is also important to take legal and regulatory requirements into account when determining the resolution needed in order to be able to use camera footage as evidence in court.

Finding a camera to match resolution requirements

The resolution of a captured scene is determined by the camera resolution and the size of the scene. For example, if you are using a camera that delivers 4CIF (704 x 576 pixels) resolution, you can cover a scene that is, at most, 1.4 m wide, if the linear resolution is 500 pixels/m or more. You will need to select a camera and lens that will allow the field of view to match the scene size at the desired distance between the camera and the scene.

Camera model Focal length Horizontal
resolution
Maximum
distance
Maximum
scene width
AXIS P1357 2.8 – 8 mm 2592 pixels 9 m 5.2 m
AXIS P3354 12mm 3.3 – 12 mm 1280 pixels 6 m 2.6 m
AXIS Q1755 5.1 – 51 mm  1920 pixels 41 m 3.8 m
AXIS Q6042-E 3.3 - 119 mm 736 pixels 50 m 1.5 m
AXIS Q6044 4.4 – 132 mm 1280 pixels 67 m 2.6 m
Table 2: Maximum distance for identification (500 pixels/m horizontal linear resolution,
80 pixels face width) for some Axis cameras 

 

Axis Lens Calculators and the Axis Product Selector are useful tools that help finding a suitable camera and focal length. For advanced users, a pixel and distance calculator spreadsheet is also available.

Higher camera resolution means fewer cameras and better overview

Since the maximum size of a scene covered at a given resolution only depends on the camera resolution, cameras with higher resolution can cover larger areas. For example, if your 7 m wide scene requires five cameras delivering 4CIF resolution, these can be replaced by two cameras at 1080p HDTV resolution (1920 x 1080 pixels). Also, a camera with higher resolution can be used to give a better overview, by covering a larger scene while maintaining the required linear resolution.

Resolution example
Cameras with higher resolution can cover larger areas

Depth of field

The larger the depth of field is, the larger the area where persons or objects are in focus. With a large depth of field, your chances of identification increase. Depth of field is determined by the iris opening, the focal length and the distance to the camera.

The depth of field increases with smaller iris openings. This means that good lighting conditions can help increase depth of field. The P-Iris feature of some Axis cameras will adjust the iris to optimize depth of field for different lighting conditions. You can learn more about the P-Iris from the P-Iris white paper:

Using shorter focal lengths will also increase depth of field. Using cameras with higher resolutions will let you capture the scene using shorter focal lengths, while maintaining resolution requirements.

Distortion

Most lenses exhibit distortion. Often this is in the form of barrel distortion. Barrel distortion is caused by lens magnification being smaller on the edges of the field-of-view compared to the center of the image. The effect is that objects that are near the edge appear closer to the center compared to an undistorted image. Objects of the same size will cover fewer pixels when they are near the edge, compared to what they would cover if they were closer to the center. This means that objects that are near the edge of the field-of-view need to be closer to the camera in order to fulfill requirements on minimum resolution.

The effect of barrel distortion is often much more pronounced at short focal lengths, making wide angle lenses less suited for identification purposes.


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