Zoom Lenses for Digital Cameras

If you have looked at any specifications for digital cameras or their descriptions you will have probably noticed that the amount of zoom they are capable of is stated as a number of times. For example a lens may be described as 3x or 6x. This is also referred to as the zoom ratio. This provides you with a useful rule of thumb when you are assessing a camera's zoom capabilities. You should be aware though that one 3x zoom digital camera can offer a different amount of zoom to another camera that is also described as having 3x zoom.

The Basics of Calculating the Number of Times or Zoom Ratio

All cameras have something called a focal length. This is true even for a camera without a zoom lens. The focal length is the distance from the optical center of the lens to the camera's focal point. For digital cameras the focal point is located on the camera's sensor. For traditional film cameras the focal point is found on the film itself.

For a camera without a zoom lens the focal length is said to be fixed as the lens never changes position. Therefore the distance or focal length between the lens and sensor never changes.

When using a zoom lens the position of the lens changes as you zoom in and out. As you zoom in the focal length increases. As you zoom out the focal length decreases. Therefore the focal length of a camera with a zoom lens is given as a range of two figures. For example 35 - 105mm. The focal length is always quoted in millimeters. The 35 mm represents the focal length when the zoom is not being used. The 105mm is the focal length when the zoom is fully extended.

By dividing the longer focal length (105mm) by the shorter one (35mm) we can work out that this camera has a 3x zoom lens.

Why a Zoom Lens Can Differ Between Digital Cameras

It is not unusual to take two cameras that are both described as having the same zoom ratio, but with one camera actually being able to zoom in close than the other. The reason for this is that one camera may offer a wider picture angle. This means that it will have a shorter focal length than the other camera. Cameras offering a wider picture angles are said to have a wide angle lens. There are advantages to having a wider picture angle. This allows you to squeeze more into the picture for shots such as landscapes and groups of people. The disadvantage is that the lens will not zoom in as close for you at the other end or the spectrum.

A lens is considered to be a wide angle lens if it is 30mm or under. Therefore an example of a 3x zoom lens with a wide angle is 28mm - 84mm. As you can see when the zoom is fully extended its focal length is 84mm. The camera with the focal length mentioned earlier of 35 - 105mm can zoom in closer even though both cameras have a 3x zoom lens.

An Example

The Nikon Coolpix 5400 is considered a wide angle digital camera. Its focal length ranges from 28-116mm. The Canon Powershot G6 is not and has a focal length that ranges from 35 - 140mm. If we calculate the zoom ratio for each camera we get 116 / 28 = 4.1x and 140 / 35 = 4x. Despite the fact that the Nikon Coolpix 5400 has a slightly bigger zoom ratio the Canon Powershot G6 can get you in closer to the subject you are shooting.

A Further Point on Focal Lengths

Further confusion can arise as the focal length for digital cameras is also affected by the size of the CCD or image sensor. Therefore the best way of comparing zoom lenses is to use the 35mm equivalents, as used in this article. 35mm equivalents are almost always quoted in a digital camera specification.

Zoom Length

For some time now the standard zoom ratio for a lens on a digital camera has been 3x. Lately there has been the introduction of significantly longer zoom lenses. The longest currently is 12x. If exceptionally long lenses sound great then there are a couple of worthwhile considerations. First extended zoom increases the likelihood of camera shake. This can lead to blurred images. This can be rectified by the use of a tripod. Increased pincushioning can occur too. This is where the edges of a shot can suffer from a mild form of distortion with the edges appearing to curve slightly. Images can also be darker as less light travels through the lens to the CCD. The use of flash or longer shutter speeds may be necessary.

Digital Zoom

It is normal to see two zoom ratios quoted for a camera. One is for optical zoom already discussed the second is for digital zoom. Digital zoom is completely different to optical zoom. Rather than using the lens to zoom, digital zoom is carried out by software inside the camera. In a nutshell the software enlarges part of the image to give the impression of zooming in. An element of guesswork is involved in the process and leads to a decrease in quality. Therefore optical zoom is considered far superior to digital zoom.

On a camera offering both optical and digital zoom you can use both together. For example a camera with 10x optical zoom and a 4x digital zoom can give a combined total of 40x zoom.

Lens Converters

An increasing number of digital cameras allow you to fit converters. These increase the cameras capacity to zoom in or use a wider picture angle. Lens converters are sold separately and are not supplied with a camera.

Lens Quality

In some specifications you may see a lens described as having a number of elements or groups. As a rough guide the more elements and groups a lens has the higher quality image it is likely to produce. This doesn't always follow as the quality of the lens depends on a number of factors, but it is worth noting.

Likewise if a lens is described as aspheric this is likely to lead to increased quality.

Where UA is used in the description of a lens you will find that although the lens can perform the same functions as other lenses it is likely to be smaller. This is very useful in small digital cameras.

How Much Zoom Do You Need? No Zoom
No zoom used - Focal length 37mm
Minolta Dimage Xg

3x Optical Zoom
3x optical zoom - Focal length 111mm
Minolta Dimage Xg

6x Optical Zoom
6x optical zoom - Focal length 234mm
Fuji Finepix 3500

8x Optical Zoom
8x optical zoom - Focal length 280mm
Nikon Coolpix 8700

10x Optical Zoom
10x optical zoom - Focal length 380mm
Olympus C-765

12x Optical Zoom
12x optical zoom - Focal length 420mm
Konica Minolta Dimage Z3

5x Digital Zoom
5x digital zoom
Canon Powershot A310

40x Combined Zoom
40x combined optical and digital zoom
Olympus C-765

Further information on zoom: Smart Zoom

Digital Camera Zoom

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