Zoom lenses have the ability to change their focal length within a designed range. Typically the range of focal lengths is indicated as for example 24-70 mm, indicating that any focal length between the limits can be chosen. The focal length of a zoom lens is usually changed by rotating a control ring on the lens (‘zoom ring’) although some lenses use a push-pull control where the focusing ring is pushed forward or pulled toward the camera (instead of rotating)
While flexible, zoom lenses tend to have some weaknesses:
- zooms are bulkier than primes.
- zooms tend to be less sensitive to light, necessitating longer a shutter speed or higher sensitivity.
- zooms can not be optimised for every focal length in the range. Therefore zooms tend to be at their best at some focal length, while others are less optimal.
- therefor zooms tend to not have the same level of image quality as a comparable prime lens.