LENSES

Note: The information in this section refers mainly to the 35mm format.

The Zoom lens is a lens in which the focal length can be varied over a specific range depending on the design of the lens.
Typical ranges that can be obtained are 20-35mm, 28-80mm, 35-200mm, 70-300mm, 28-300mm, 300-1200mm, and many others.

Advantages of zoom lenses are:
The image size is continuously variable throughout the range, compared to that of fixed focal length lenses.
This gives the photographer the abilty to frame the subject without other objects intruding into the frame, thus reducing the need to crop the final print.
Less risk of missing that important picture because you were changing lenses.
Fewer lenses to carry in your kit.
Useful for special effects by utilising 'perspective control' techniques.
Many zooms have macro facility, which means you don't have to carry special lenses for close-up work.

The main disadvantages are:
The widest apertures tend to be 1 - 11/2 stops less than an equivalent fixed lens.
They tend to have a lower image contrast, and sometimes poorer definition, particularly with the cheaper models, but modern designs are tending to eliminate these problems.
Zoom lenses are usually heavier than the equivalent fixed lens within the range.
The maximum aperture can be 1/2 - 1 stop less from minimum to maximum focal length, so exposure should be adjusted if the camera is not TTL metered, or if using a non-dedicated flashgun.
The depth of field varies throughout the zoom range, being greatest at the shortest focal length and least at the longest focal length.

Typical 28-80mm, f3.3-5.6, Zoom lens.

70-300mm, f4.5-5.6, Zoom lens.

All the perspective and depth of field effects discussed with wide angle and telephoto lenses, apply equally to zoom lenses, depending on the focal length being used.
The zoom lens gives the photographer much greater control over the image, as he can continuously vary the effects of perspective and depth of field, depending on his viewpoint and the focal length and aperture of the lens.
Where possible a tripod should be used, as camera movement is magnified towards the longer focal length, especially in the 200mm plus ranges,and can cause images to be blurred.
A monopod can be also be useful, especially in crowded areas.
If the camera has to be hand held then a fast shutter speed should be used.
A good guide to the slowest practical hand held shutter speed, is to use the reciprocal of the focal length i.e for a 300mm focal length lens, 1/300th sec (1/250 on most cameras), a 500mm, 1/500th and so on.
If using slower shutter speeds then the camera should be steadied on something solid, such as a wall or fence, to provide stability.

These pictures were both taken from the same position with a 28-80 mm zoom lens.
The one on the left was taken at the 28mm and the one on the right at 80mm.

These photographs were taken with a 70-300mm zoom lens, from the same position as the previous set.
It is often said that a long focus lens compresses perspective.
This is not strictly correct, the compression effect is an illusion caused by the narrow field of view of the lens Click here for more info.
.

Conclusion,
The zoom lens gives the photographer absolute freedom and flexibility in creativity and composition of the image.
The ability to use a continually variable focal length allows for precise framing of the image, and when combined with the effects of perspective control and variations in depth of field, it provides the photographer with a very useful creative tool.
It is a useful lens for portraiture, especially in the range, 75 - 200mm, as the photographer can utilize the short DoF to put the background out of focus and place the emphasis on the subject, whereas the shorter end of the range with it's greater DoF makes it ideal for landscape work.
The illusion of image compression at the longer end of the range can often create interesting or unusual images.
Some interesting effects can be obtained by setting a long exposure (say 1/2 -1sec), and small aperture, and operating the zoom control during the exposure, preferably with a tripod, but hand held shots can create some unexpected and creative effects as well.

Disadvantges,
The resolution of some of cheaper zooms may not be as good as a fixed lens in the same focal range, also the maximum aperture may well be less.
The zoom lens is usually heavier than an equivalent fixed lens.
Thje maximum aperture usually decreases through the range from the wide angle end to the telephoto end.
To avoid blur caused by camera movement at the longer focal length, a tripod is often required.