When a lens is focused on an object, theoretically, the whole plane passing through the object and perpendicular to the optical axis should be in focus. Practically, objects slightly in front of and behind the object in focus will also appear sharp. This “extra” depth of sharpness is called depth of field.
A wide depth of field might be an undesired feature, as it is, for example, when we want an object we are photographing to be isolated from the foreground and the background. This is very characteristic when taking portrait shots with a telephoto lens, where the depth of field is very narrow.
in CCTV, however, we often want the opposite effect. We want to have as many objects in focus as possible, no matter where the real focusing plane is.
The depth of eld depends on the focal length of the lens, the F-stop, and the format size of the lens (2/3”, 1/2”, etc.). A general rule is the shorter the focal length, the wider the depth of field the higher the F-stop, the wider the depth of field, and the smaller the lens format, the wider the
depth of field.
The depth of field effect is explained by the so-called permissible circles of confusion. The permissible circle of confusion is a projected circle of the depth of field area. If the smallest picture element (pixel) of the imaging chip is equal to or bigger than the permissible circle of confusion, then it is obvious that we cannot see details smaller than that circle. In other words, all objects and their details that appear within the circle will look equally sharp, since that is the actual size of the pixels. From this it is clear that the size of the permissible circles of confusion for a CCTV camera is determined by the pixel size of the imaging chip – in other words, the chip resolution.
It may now be understood why some short focal length lenses in CCTV, such as 2.6 or 3.5 mm, do not have a focusing ring at all but only an iris adjustment. This is because even with the lowest F-stop for that lens (be it 1.4 or 1.8) the depth of field is so wide that it actually shows sharp images from a couple of centimeters in front of the lens up to infinity. There is literally no need for focusing.
the depth of field is an effect of which we should be very aware, especially when adjusting the so-called back-focus. If the back-focus is not adjusted properly, and a camera is installed at daylight (that is when the auto iris of the lens closes the iris as much as possible, due to excessive light), the depth of field will produce sharpness even in areas that are not really in focus.
Practical experience shows that depth of field applied in this way (when the back-focus is not done correctly) is the biggest source of frustration for a 24-hour operating system. The reason is obvious: at night, when the iris opens due to a low light level (providing the AI functions properly), the depth of field narrows down and shows the images out of focus even if they were in focus during the day. When an operator complains to the installer or service people, not knowing the cause of such a problem, he or she usually gets the service to visit during the daytime. Obviously, the problem will not be there then, thanks to a wide depth of field that reappears “inexplicably” at nighttime.
The moral of the above is that the back-focus adjustment should be done when the iris is fully opened. The easiest way to have the iris opened is when low light levels reach it, either at the end of the day (or at night) or by artificially reducing the daylight with external neutral density filters (usually placed in front of the lens objective). All this is in order to reduce the depth of field and consequently make back-focus adjustment easier and more accurate
Quite often, when cameras with infrared lights are used another effect is present. Because of the extremely long wavelength of the infrared light (compared to normal light) and the lesser angle of refraction, we get the focused image plane slightly behind the imaging chip. Refer to the heading Lenses as Optical Elements for further explanation of this phenomenon. If an image is sharp at day, then at nighttime objects of the same distance will be out of focus. This might be a quite noticeable and unwanted effect. In order to minimize it, a lens should be designed with a special compensation for infrared viewing (some manufacturers have special glass lenses for this purpose). Another practical and common solution would be to have the camera back-focused at night with an infrared light on, in which case the depth of field is minimal but the objects are in focus. At day, the depth of field will increase the sharpness to a wider area, compensating for the difference between the infrared and the normal light focus.
The last note I would like to make is about the vari-focal lenses and the back-focus adjustment with them Namely, vari-focal lenses started to prevail with fixed cameras lately, despite the vari-focal concept being one of the lowest optical quality when compared to manual focal length lenses, or even zoom lenses. The fact is that they offer the easiest method to decide the angle of view on the spot, during the installation process. But it is also a fact that such vari-focal lenses have typically two optical elements groups which are positioned until the desired angle of view becomes sharp, then this is screw-locked and such an image stays fixed with that choice of view. Back-focus for such lenses is not of great importance. There is one exception to this rule, and this is the Pentax’s design of “Vari-focal+” series of lenses, where in fact the “vari-focus+” is a manual zoom lens design, so that by correct back-focusing, a sharpen image, stays sharp for all the angles of view such a lens offers