A varifocal lens is a lens with a manually adjustable focal length. Devices of this type are used in cameras (equipped exclusively with lenses of this type) and video cameras (they can be either with a constant focal length or with a variable one). The variofocal lens consists of a single optical system, the components of which are mechanically moved relative to each other, resulting in a smooth change (adjustment) of the focal length and, accordingly, the image scale in the focal length range. However, the sharpness of the object’s aiming and the relative aperture do not change.
Varifocal lens with manual adjustment allows you to change the focal length twice, which makes it possible to adjust the viewing angle of the camera to the optimal image. Remote-controlled devices can change the focal length from 6 to 50 times. Such lenses are used for video surveillance.
Let's see what are the advantages and disadvantages of such a device as a varifocal lens.
The advantages of the device include the fact that it allows you to adjust the viewing angle directly on the spot when installing the camcorder. Well, of course, after installing the camera, it is possible to adjust the focal length. On this, the advantages of such devices end. Now we turn to the consideration of shortcomings.
A camera with a varifocal lens needs to be set up, which is not always convenient, since you have to constantly carry additional equipment with you (test monitor, battery, etc.). The lens setting is quite complicated, it is necessary to determine the optimal ratio of focal length and sharpness, and the procedure is performed with special filters (very expensive) that darken the picture. So, the first drawback is the complicated setup.
The variofocal lens has a worse throughput (aperture) compared to fixed lenses. This is especially noticeable on devices with a wide range (5-50), the image quality at dusk will be disgusting. Low aperture is the second drawback.
The third disadvantage of the varifocal lens is that over time it is able to defocus from mechanical or thermal influences. As a result, after a few months, additional configuration will be required.
The fourth drawback is the price of such a lens, it is much more expensive than a fixed one.
To summarize, let’s say that varifocal lenses, despite their shortcomings, are very popular and are in great demand than fixed ones. However, it is worth noting that the review given in this article applies only to cameras designed for video surveillance, because only they have a choice of fixed or custom lenses. Cameras and camcorders use exclusively customizable optics.