Focal Length (mm)
The focal length (measured in “mm”) of a lens determines how far the camera can see. The most common focal length on security cameras is 3.6mm, which will allow for roughly a 72 degree field of view (FOV described below). This particular focal length works well for residential or small office surveillance applications.
If you can only learn one thing about security camera lenses, you should learn that the bigger the focal length, the farther away it can focus. The smaller the focal length, the wider the field of view.
CCTV Lens Focal Length
Field of View (FOV)
Field of View is the area, in width, that the lens will allow you to see. If you hear the term “wide angle” then it will have a smaller lens, like 2.8mm. This means just what it sounds like, the picture will be wider. With a smaller focal length details are also more difficult to see. If you are capturing video in a normal sized room, maybe 20x20ft, then a wide angle lens will work just fine. If you need to capture a person’s face, then you’ll want to consider something with a larger focal length, like 16mm for example.
CCTV Lens Field of View
Vari Focal Length Example
Monofocal vs. Varifocal
Monofocal lenses or “fixed” lenses can only focus at a specific distance… “mono” –> one –> one length! They will have one focal length, like 16mm or 3.6mm. They are generally cheaper than Varifocal lenses, which will allow you to adjust the focal length manually. Varifocal lenses will usually have a range, like 2.5-6mm or 5-50mm. Zoom lenses are similar to varifocal, except the focal length will adjust electronically, thus they are generally more expensive.
Zoom CCTV lenses are bit more expensive because, well, they’re better. They can automatically adjust focus to increase the definition on a given object.
Manual vs. Auto Iris
This is very similar to the choice between Monofocal and Varifocal, in that one is fixed and the other “varies”, and thus, is more expensive. The iris has to do with how much light is allowed to filter in through the lens. A manual or “fixed” iris lens is one you’d use if you are in an area where the lighting does not change, like an office. An auto iris lens will automatically adjust for light variations. You’ll probably need an auto-iris lens for outdoor surveillance.
Also important for various lighting conditions, is the camera’s LUX rating.
Here, we’re referring to the CCD image sensor of the camera. You’ll notice that most CCTV cameras list either 2/3”, 1/2”, 1/3”, or 1/4” for their CCD image sensor. This will affect your field of view. The larger the image sensor, the larger the field of view the camera has. For example, 2/3” will see more than a 1/3” image sensor will. Now, all of the above just applies to the camera, not the lens. For the lens, what is important to note is that whatever the lenses’ image format, say “1/3 inch image format”, is the maximum size it will allow. So for example, for a camera with a 1/2” CCD imaging device you would want a lens of 1/2” or bigger, or else there will be a ring on the outside blocking the outside of the picture.
CCTV Lens Camera Format
Aperture Rate (F-Stop)
Aperture, measured in F numbers, is the camera’s ability to gather light. The lower this F number is, the larger the lens aperture will be, the more open your lens, and the better it will be at allowing light to pass through and onto the CCD image sensor. For example, if you see the lens has a low aperture of F1.2, then it will work well in low lit situations. Consider your eye, how your pupil will dilate or contract depending on how much light there is. If you’re in a particularly dark area, your pupil will dilate to allow more light so that you can see better.
C or CS Mount
When mounting a lens to a CCTV security camera, you’ll have C or CS mount to choose from. Both the lens and the camera will specify which they are. Many cameras say “C/CS mount”, which means it will allow for either.
The difference between the two is the distance between the image sensor and the lens….
C Mount – 17.5mm
CS Mount – 12.5mm
Tip: Not having the right matching lens is a common cause for not having sharp images.
Video Drive or Direct Drive
The video and direct drive refers to auto iris lenses. To reiterate from above, auto iris lenses allow for variations in lighting conditions. Video drive lenses contain amplifier circuitry that will allow the iris control to operate. Direct drive lenses will require the camera to contain the amplifier circuitry. Be sure to check the product options because some Direct Drive lenses can include an adapter that will convert it to a Video Drive auto iris lens.