When you are walking through a camera installation site and deciding what cameras to put where, keep in mind that your eyes do not perceive an image in the same size as a camera will. Most standard cameras have a ‘fixed’ 3.6mm lens. This will give a camera about a 90 degree field of view. Compare that to the average human field of view of approximately 22mm with 120 degrees of view.
The focal width and the focal height of a camera are in the shape of a “V”. Combined they are actually more like a pyramid (increasing height and width). The further away from the camera you look the wider and taller the image, and as a rule, the smaller an object within that field of view appears. The zoom of your lens will determine how wide or narrow your ‘pyramid’ field of view is.
To determine exactly what lens you need, you must first have two pieces of information. First is the distance away that you want to view. Second is the height or width that you need to capture in the image at that distance.
Here is an example. Picture a straight driveway that is 100 feet in length and 30 feet wide that goes from the garage door to the road. Let’s say the best place to mount the camera is above the garage door and you need to see as much of the drive way as possible. Using the DSC lens calculator, choose the correct chip size of the camera you are considering (usually 1/3 inch), the distance from the camera, and the lens millimeter size. In this case let’s start close to the camera (10ft) and use a standard lens (3.6mm). With these settings we have a width of about 13 feet and a height of about 7 feet. Therefore, if a person is standing 10ft away from the camera they would take up a majority of the image head to toe which would provide great facial recognition. However, that is far short of the 30 feet needed to see the entire width of the driveway. In fact, the entire width of the driveway would not be in the field of view until the distance of about 23 feet. Using the same camera – the field of view at a distance of 100 feet away from the camera (for example, a mailbox at the end of the driveway) would see well beyond the driveway width by about 50 feet on each side for a total width of about 133 feet and 75 feet tall. The same person at this distance would be no more than 10% of the image height. Not an ideal situation for facial recognition.
Note: The practical solution to an application like the above would require more than one camera. We would recommend one camera with a fixed camera lens (2.8 or 3.6mm), perhaps at the corner of the building, to provide an overview of everything going on in the yard and good facial recognition close up. A higher millimeter varifocal camera would be used to keep an eye on the mailbox – the exact lens size determined by how far away the mailbox is from the house.
Once you have determined the required lens size using the DSC lens calculator, then you can choose a camera that incorporates the size you need. Generally you will select either a fixed wide angle lens for close-up or panoramic views, or a varifocal lens for longer distances. For example, if you need a 7mm lens to get the detail required, then a 4-9mm varifocal lens would be a great choice. For detail at larger distances, try a varifocal lens that goes up to 60 or 100mm. Varifocal lenses are great because you can adjust the focal distance at install time to capture the best field of view.