Optics. Video camera lens.
Optics. Video camera lens
Over the past few years, several generations of video cameras have changed — vidicons are a thing of the past, digital signal processing and numerous other electronic tricks have become the norm. However, one thing has remained unchanged — the lens. The set of glasses that create an image on a light-sensitive device has not changed almost since the time of Galileo, who created a two-lens telescope. Since then, the methods for calculating lenses have changed, but the design of the lens has remained largely the same. And it is often the lens parameters that limit the capabilities of the video surveillance system.
Geometrical optics (ideal lens)
If you remember, in the school physics course the «ideal lens» was always considered — infinitely thin, but of infinitely large diameter. It has one single parameter — the focal length.
Angle of view
This is really the main parameter — it is what determines the angle of view of the camera with such a lens. The longer the focal length — the smaller the angle of view. But the more distant targets can be confidently distinguished on the monitor screen. Figure 1 — an illustration in the style of «school optics». The main thing that can be taken away from it (Fig. 2) — is that the greater the distance to the object of observation, the longer the focal length should be. And the larger the camera matrix, the longer the focal length, too.
Resolution and required angle of view
Often, when designing a system, one task is set — to see as much as possible, the field of view as wide as possible, so that no enemy slips past the camera. However, there is a problem — the more objects are visible in the frame, the smaller each of them is, the more difficult it is to distinguish them. Therefore, the recommendations of the British Ministry of Internal Affairs (Fig. 3) say that for confident identification of a familiar person, he should be approximately half the frame in height. More precisely, it is recommended that the horizontal field of view be 5 meters. For the most common cameras (with a 1/3 matrix), this means that the focal length in millimeters should be equal to the distance to the person in meters — very easy to remember! If you need to identify a person at a distance of 50 meters — you need a 50 mm lens. Or vice versa — with a 4 mm lens you can confidently identify a person at a distance of up to 4 meters. Frankly speaking, the «recommendations» are given with a reserve — for bad weather, for the fact that the video signal will be recorded on a tape recorder, for the fact that the person does not want to be identified — in short, in fact, when directly observing a person on a monitor screen, it is quite possible to recognize him at a distance 2 times greater than the «recommended», but the reserve, as we know, does not pull the pocket — if you want to be sure of a reliable result, listen to the «recommendations». Moreover, for identifying a stranger (from a photograph), a twice as close-up is recommended.
Recalculation for other conditions
Now that you have remembered the basic recommendation, you can easily recalculate for a different matrix size or other requirements for distinguishability if necessary. The field of view (as can be seen from Figure 1) is directly proportional to the distance to the object, directly proportional to the size of the camera target and inversely proportional to the focal length:
(field of view) = (matrix size) * (distance to object)/(focal length).
For example, for a camera with a 1/2″ matrix, the focal length should be one and a half times greater than for a 1/3″ matrix. Or, for example, for «person detection» (the required field of view is 4 times larger than for «identification») with a 1/3 camera, you can use a lens four times shorter — that is, at a distance of 50 meters, you can use a 50/4 = 12 mm lens.