Hikvision Focal Length DifferencesWithout a shadow of a doubt, the absolute most frequently asked question we receive is regarding focal lengths, and what viewing angles different focal lengths will offer. For good reason, too - it's a very important decision in the system spec process, and can greatly affect image quality. You wouldn't want to use a wide-angled lens to view a number plate from 50m away because you'd have no hope of reading it, and so it's imperative you pick the right focal length for the job at hand.
The focal length (measured in mm) refers to the distance from the camera lens and the image sensor whilst the image is in focus. This directly dictates the viewing angle a camera will deliver, and thus the degree of zoom the camera will have.
Hikvision cameras with a fixed focal length come in four standard focal length options, and these approximate to the below viewing angles:
The common misconception is that a wider viewing angle will always be better - you can see more, right? The problem there is that objects of interest such as a face or a number plate will appear smaller in the scene and take up less pixels-per-inch, providing a less detailed and smaller image of what you're actually interested in seeing. So, it's best to just use only the viewing angle you need to cover what you want to monitor.
- 2.8mm = 90° horizontal field of view
- 4mm = 60° horizontal field of view
- 6mm = 45° horizontal field of view
- 12mm = 25° horizontal field of view
We created a video recently to show the differences between these focal lengths on our YouTube channel
Below are some stills from the above video showcasing these different focal lengths. All were taken from a DS-2CD2342WD-I mounted via a DS-1273ZJ-130-TRL wall-mount bracket recording at 4MP resolution. I tried to keep the brown wall on the right-hand edge of the cameras view, with the goal of keeping the pavement level.
Click on the image to view a larger, full-sized image, or right-click it and select "Open image in new tab"
In this instance, the ideal camera is the 6mm lens.
We have a write-up in our forum on a great method you can use to estimate viewing angles yourself with nothing other than your own hands.
There are also Varifocal camera options. Varifocal cameras allow you to manually or remotely adjust the focal length and thus zoom of the camera within a certain range. In the most popular Hikvision cameras, this typically ranges from a 2.8mm - 12mm lens, and so you'd be able to set the camera to any viewing angle from approximately 90° - 25°. It's worth double checking the datasheet, of course.
Some PTZ cameras have a much, much wider range of focal lengths, allowing a great level of optical zoom. The DS-2DF8223I-AEL for example has a focal length range of 5.9mm - 135.7mm, offering 23x optical zoom and thus a 59.8° - 3.0° FOV.
Hopefully that helps clear up some confusion - let me know if you need any further clarification!