How Far Can I See With My Cameras?
Pixels Per Foot: a complicated answer to a simple question.
In an open field of view, a security camera can technically see forever. How far away from a camera you can get usable video is another matter. Whether or not a person is readily identifiable or just a featureless blob on your video depends on the size of the subject on the recorded image.
The security industry has a standard term to measure the size of an object on recorded video: Pixels Per Foot (PPF). Pixels Per Foot is a measurement of quality of the final video produced based on the size the area the video is recording.
Pixels Per Foot depends on the distance from the camera, the focal length of the camera lens and the resolution of the camera. An easy way to think about it is how many pixels a license plate (a US license plate is about 1 foot wide) would take up at a certain distance. If a license plate is 100 pixels wide in a recorded image, then the scene at that distance is 100 Pixels Per Foot.
How many Pixels Per Foot do I need?
The security industry uses three classifications for Pixels Per Foot recommendations:
- Recognition; being able to identify features and provide positive identification of a subject. Facial features, logos on clothing and license plate numbers for example. Professional security system designers want 60 PPF or higher during the day or 80 to 90 PPF at night for recognition
- Classification; being able to tell general details of a subject. Gender, race/ethnicity, color and type of clothes, make and model of a car for example. Professional security system designers want about 40 PPF or higher during the day or 60 PPF at night for classification
- Detection; being able to see that some activity is going on. A car has entered the parking lot or a person is in your yard for example. Professional security system designers want about 20 PPF or higher during the day or 40 PPF at night for recognition
These diagrams show the relationship between PPF and focal length:
HD420 Camera - 4MP @ 3.6mm lens (70° Field of View)
HD820 Camera - 4MP @ 12mm lens (23° Field of View)
How far can my AvertX camera see?
Here are the maximum effective distances of a few popular AvertX cameras based on the above Pixels Per Foot recommendations:
Note: These distances are to the actual camera location, which takes into account the distance from the base of the wall and the installation height (a2 + b2 = c2).
|Camera and Focal Length||Recognition (60PPF)||Classification (40PPF)||Detection (20PPF)|
|HD320 / HD420
|3.6mm (70° FOV)||30ft||45ft||90ft|
|HD820 / HD920
|@ 2.8mm (80° FOV)||23ft||35ft||70ft|
|@ 5mm (50° FOV)||42ft||63ft||125ft|
|@ 12mm (23° FOV)||100ft||150ft||300ft|
|@ 3mm (80° FOV)||25ft||38ft||75ft|
|@ 6mm (45° FOV)||50ft||75ft||150ft|
|@ 9mm (30° FOV)||75ft||113ft||225ft|
If you don’t see your camera on this list, you can calculate the maximum effective distance using an online camera calculator/planner. Several are available online and can be found with a web search. You will need to know the resolution, sensor size, and lens focal length of your camera, which can be found on our support downloads page.
How do I maximize the PPF resolution of my cameras?
To maximize the PPF of your cameras, place cameras at choke points and/or transaction points that people are likely to use on your property. Some examples of choke points are doors, gates, hallways and driveways. Some examples of transaction points are cash registers or front desks. Placing a camera either very close to or zoomed in on these places will maximize your chances of getting an image you can use for positive identification of a subject.
Remember: most of the time you only need a single high resolution image for identification.
What if I need to see more detail out further?
If you need to get a usable image at a distance further than your camera will allow, you have a few options:
- Use a camera with a varifocal lens and zoom in. Zooming in the camera will enlarge objects at a distance and therefore increase the ppf on your image at the expense of the camera’s field of view. HD820 and HD920 cameras have varifocal lenses and are perfect for these situations.
- Add more cameras. If your camera doesn’t have a varifocal lens or you would like to maintain a wider view of a particular area, adding another camera closer to choke points and transaction points can provide the additional detail that you need.
Does this guarantee I’ll get a good image?
These guidelines are meant as a guideline of what your camera is capable of under optimal conditions. Other factors such as motion, angle, lighting conditions, glare, motion and weather will also affect your camera’s ability to capture a clean image.
If you have any questions about Pixels Per Foot or how to choose the optimal camera for your installation, please contact our Pro Team.