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Understanding Digital Camera Lens Specifications

The lens is a critical component of the camera. Digital camera lenses are small and need to be accurately manufactured. They contribute a lot to the cost of the camera so specifications vary considerably. There are also a number of different performance parameters that can be considered:

Zoom Range
The most obvious lens parameter is zoom range. It is very convenient to be able to take wide angle and telephoto shots with the same camera. 3X optical zoom is good, but fairly expensive. 2X also gives useful range but is not usually so good if you like talking pictures of distant object (wildlife and sports for example). Cameras with fixed focus lenses tend to be easier to use and smaller. Some cameras also have a digital zoom feature. All this does is enlarge the size of the picture (and crop it) after it is taken. It does not capture more detail. Digital zoom can be used to extend the optical zoom range, but always check the optical zoom range, this is what really counts. Try the zoom in the store where you buy the camera to see if you like the range.

Minimum focal length
In addition to the range of zoom, check out the minimum focal length (how wide you can zoom out). Some digital cameras do not have good wide angle operation because it is hard to make a wide angle lens for a CCD sensor. Check the zoom range - less than 35mm is acceptable, 28mm is good (both measured in 35mm equivalent focal length, see below).

Maximum F-stop
If you care about taking pictures in low light look at the maximum F-stop your camera can support. There is an iris in the lens and this is a measurement of the maximum it can open up, it affects the brightness of the image. F/4 is not bad for digital camera lenses, f/2.8 is good. Maximum F-stop can also vary a bit with zoom. High F-stop zoom lenses have very large lenses at the front and would be heavier. F-stop also affects depth of field, the range of distances for which the subject will be in focus.

To get a general impression of light gathering performance look at the size of the lens at the front. Some cameras have tiny lenses - this means that less light enters the lens and therefore the camera does not perform so well in low light level.

Close ups
Some cameras can focus very close to the lens and be used as microscopes (it is called 'macro' mode). If you like to take pictures of people and landscape this does not matter. But if you like to take close-up subjects (like flowers and plants) it is best to check this. Focus closer than 50cm (0.5m) is good.

Zoom lens scales
Lens focal lengths are measured in mm. The higher the number the more the telephoto effect. The range is usually marked on the front of the lens and appears in the specification sheet of the camera.

Since the CCD (the device used to collect the light) in the digital camera is much smaller than a film camera then all the lenses and their focal lengths are smaller too. A digital camera with a 6-15mm lens and a certain size CCD has exactly the same range as a regular film camera with a 28-72mm zoom lens. To further complicate the issue the sizes of CCDs used in cameras are not all the same.

When everyone used 35mm film cameras it was easy to know the type of lens as long as you knew the scale. To make it easier with digital cameras many manufactures quote their zoom lenses in "35mm equivalent" sizes, which is more convenient. In this case the '35mm' refers to the width of the film used in traditional cameras. The following table shows typical values:

Conventional 35mm
film camera
Typical digital camera Range
<20mm < 4.3mm Very Wide angle
21-35mm 4.5-7.5mm Wide angle
50mm 10.7mm Normal - similar to what your eye would see
70-200mm or more 15-43mm Telephoto

Use of different zoom settings has a strong effect on perspective in the picture. Wide-angle lenses distort pictures of people faces and tend to make them look fatter with larger noses!

Sometimes lenses are quoted as 2X or 3X. This refers to the range, so a 25-50mm would be 2X and 25-75mm would be 3X. This measurement is used more often for video cameras. When buying a camera keep in mind that X values only yell you part of the story. You should also look at the range covered; a 70-140mm lens would be 2X, but would not be able to photograph close subjects, a 35-70mm lens is also 2X but would offer very different performance.

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