There are several types of digital camera lenses and terminology associated with them. The focal lengths of lenses are described in terms of the 35mm equivalent. It’s important to note that just because your camera is digital doesn’t mean your zoom lens should be!
Fixed focal length lens
The camera lens does not change at all. With a fixed focal lens the only way you can get a close up is to move closer to your subject. But don’t get too close ot your subject’s features are likely to become distorted.
Types of zoom Lenses
Optical zoom lens: a true zoom
An optical zoom is a true zoom. Its focal length changes and the zoom mechanism itself extends and retracts so an image is magnified by the lens itself. An optical zoom produces the best quality images.
Digital zoom lens
A digital zoom is not a true zoom. It is a simulated zoom that enlarges the central portion of an image in the camera. The actual length of the lens does not change. Digital zoom produce images with reduced visual quality.
Interchangeable and converter lenses
Single Lens Reflex digital cameras (DSLR) accept a wide range of interchangeable lenses. Some lenses used with a 35mm film SLR may be compatible with the same brand DSLR, though features such as auto-focus may not work if the lens in old.
You can get decent consumer level lenses for reasonable prices, however high quality professional lens can cost $1000 or more. There are specialized lenses for any photographic scene or subject.
Add-on converter lenses are available for many compact (non-dslr) digital cameras. Converter lens are relatively inexpensive and extend the flexibility and capability of a built-in lens by allowing you to attach another lens.
Related reading: extended optical zoom