Varifocal lenses are lenses with variable focal length (and thus angle of view), which can be adjusted by moving groups of optical elements with respect to each other inside the lens. The variable focal length allows for multiple combinations of working distances and magnifications, offering several different configurations with a single lens.
The convenience of variable focal length comes at the cost of complexity and some compromises on image quality, weight, dimensions and aperture. Varifocal lenses can suffer of loss of image resolution at their maximum aperture, especially at the extremes of their focal length range. This effect is evident in the corners of the image, when displayed in a large format or high resolution. The greater the range of focal length a lens offers, the more exaggerated these compromises must become.