Lens is a key component of any camera. Due to its complex design it allows the camera to record the actual image. Lens parameters determine the quality of recorded videos. As mentioned before, the field of view depends on the focal length of the camera. It is a distance between the optical centre of the lens, in which the light is focused and the image sensor, assuming that the focus is set to infinity (Fig. 1). The value is expressed in millimetres.
1 - Lens
2 - Image focus (image sensor)
3 - Light beam
4 - Focal length
The light beam passing the lens reach a photosensitive element - an image sensor. The light beams reaching the opposite sides of the image sensor form the lines defining the field of view of the camera. (Fig. 2). Unlike the photo technique, where the field of view of the camera is included between the opposite corners of the image, in CCTV techniques, the field of view is defined in relation to the angle of view.
1 - Recorded image. The dotted lines indicate the camera field of view.
2 - Field of view (horizontal)
3 - Lens
4 - Focal length
5 - Image sensor diagonal
As shown in the diagram, the field of view (2) of CCTV camera depends not only on the distance between the lens and the image sensor, i.e. focal length (4) but also on the size of the image sensor (5). The larger the focal length, the lower the field of view. The larger the photosensitive element, the higher the field of view. Remember, that the lens is designed for use with a specific image sensor size. The same lens can have a different fields of view if used in different cameras.
For comparison, the image from APTI-27V2-2812 camera with a 94° field of view (Fig. 3), and an image from M5-V1 camera with a 150° field of view (Fig. 4).
Fig. 3. Screenshot from APTI-27V2-2812 camera
Fig. 4. Screenshot from M5-V1 camera
The first image is a screenshot from a camera with a commonly used field of view. Both on the straight ceiling beam and the floor boards, a slight curvature can be observed. The image is close to natural. The screenshot from the second camera shows an image, which looks unnatural (fish-eye phenomena), however, it has a wider field of view, outside the scale used in the test room. Wide angle cameras are a good alternative to rotary camera due to a wide field of view.
A direct correlation between the field of view and the focal length of the lens, image sensor size and other parameters can be observed. Installation height and required field of view must be considered when selecting the CCTV camera. We can use different equations, tables and calculators to select a suitable camera. All cameras in Delta offer has been tested and all relevant parameters are available on the website of each product. Our employees, based on basic installation parameters and field of view will help you select the best camera for your application.