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What is Focal Length?

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Pretty much every compact digital camera on the market has a zoom lens, and of course there is a wide range of zoom lenses available for all digital SLRs, as well as a variety of non-zoom lenses with different focal lengths. A zoom lens is one that has a variable focal length. In this tutorial, we’ll take a look at what focal length means, and some of the effects you can achieve by varying it.

The focal length of a lens is an expression of its magnification power, and is usually stated in millimetres. If you look on the front of your camera, usually inscribed around the front of the lens you’ll find the focal length range. (If it’s not there, take a look in your manual on the specifications page. It’ll be there somewhere.)

For most 3x zoom digital compacts with 1/2.5in sensors the focal length range will be in the order of 5.8-17.4mm. Sometimes written next to this is the equivalent focal length on a 35mm camera. For 3x zooms this will usually be around 35-105mm.

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The reason for quoting both the real and the equivalent focal length is simply that more people are familiar with the sizes of 35mm lenses, so they know that 35mm is wide angle and suitable for panoramic shots, or that 105mm is a short telephoto, suitable for portraits and medium-range subjects.

The reason the real and equivalent focal lengths are different is a bit more complicated. Basically it is because digital camera sensors are a lot smaller than a frame of 35mm film, and are fitted much closer to the lens than the film would be. Compact camera sensors of the 1/2.5in type are usually about 5.76 x 4.29mm, although exact sizes vary between different manufacturers, while a 35mm film negative is 36 x 24mm. Most digital SLRs use what is known as an APS-C sized sensor, which is around 21.5 x 14.4mm.

Because there are several different sizes of sensor, it is more usual for compact camera zoom lenses to be rated in terms of their magnification power, such as 3x, 4x, 10x etc. This relates directly to the focal length. A lens with a range of focal length from 5.8mm to 17.4mm is called a 3x zoom, because 17.4 = 3 x 5.8.

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This means that when using lenses designed and calibrated for 35mm film SLRs on a digital SLR, a multiplication factor has to be applied to arrive at the correct focal length. For APS-C sensors this conversion factor is 1:1.5, so a lens that has a focal length of 100mm when used with 35mm film would have an apparent focal length of 150mm when used with a digital SLR using an APS-C sensor.
In the most basic terms, the focal length of a lens is the distance from the mid-point of the lens to the point at which light rays parallel to the centre-line of the lens are focused, as in the diagram below.

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While in older simpler camera lenses a 200mm lens would literally be 20cm long, modern optical systems use multiple lenses working in combination, which means that the light path can be shortened while still maintaining the same effective focal length. As a result quite powerful telephoto and zoom lenses can be relatively compact.

As for what constitutes wide-angle and telephoto, these are relative terms. On a 35mm film SLR with a standard viewfinder, a 50mm lens produces approximately the same view as a human eye, and is traditionally the standard lens for this type of camera. Generally anything longer than this is considered a telephoto, while anything shorter is considered wide angle. On a digital camera it’s a bit harder to define, but for APS-C format DSLRs the cut-off point is approximately 35mm.

The shorter the focal length, the more the lens bends the light rays, and to cut a lot of mathematics short, the wider its angle of view, so a lens with a focal length of 28mm can fit a lot more of a scene into the frame than can a lens with a focal length of 200mm. On the other hand, the narrower field of view of the longer lens means that distant objects appear larger in the frame.

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