Zooms

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.

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