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Focal length is an actual length, expressed in millimeters (it corresponds to the distance between the optical center of the lens and the film plane, though you need not worry about that).
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The lower this number, the less zoomed in you are.
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Though we will see later that it is not exactly true, as an approximation, you can zoom with your feet: walking 10 meters closer to your subject or adding 5mm to your focal length will result in the same image
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A modern compact like the Canon S90 has focal lengths between 6.0 and 22.5mm, yet the same values on a lens for a FX DSLR like the Canon 5D would be unbearably wide and totally unusable.
The culprit is what we call the crop factor
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The focal length is a physical property of a lens, but the resulting angle of view, which is what we are really interested in, depends on another factor: sensor size. The bigger the sensor, the wider the angle of view for the same focal length. In order to convert angles of view between different formats, we use the crop factor, which is a ratio between the standard 35mm film area and the actual sensor size.
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Remember how a bit earlier, I said you could zoom with your feet? Well, it’s not quite true. The reason is that perspective will change. One effect of using a long focal length is that it will compress perspective, making everything appear to be on the same plane. Wide angle, on the other hand, will exagerate depth, sometimes to extreme lengths. This is why landscape photographers like to use ultra-wide lenses.
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Ultra-wide angle – 14-24mm: They are pretty specialized lenses as they will tend to exaggerate perspective to levels which can easily be disturbing. Our eyes are not used to such wide angles of view, and they will look unnatural, which can be used for artistic purposes. Landscape and architecture photographers love these focals as they will create a lot of depth and emphasize perspective.
Wide angle – 24-35mm: Wide enough to show a lot of context, but not so wide that they look unnatural, they were used a lot by photojournalists. It is a good “default” focal range, which explains why most kit lenses include them (18-xx lenses on DX DSLRs, for instance).
Normal – 40-75mm: What exact length a normal lens should be has been subject to a lot of debate, but it is estimated to be around 45mm. This is an angle of view which looks very natural and “inoffensive”, neither too wide nor too tele. It also corresponds more or less to the focal length we actually perceive (though due to peripheral vision, our eyes have an estimated 22mm focal). Street photographers love these lengths.
Mild tele – 85-105mm: This is prime portrait category: long enough to isolate the face and create separation from the background (through shallow depth of field, more on this in another lesson) but short enough that you can still be within communicating distance from your subject.
Medium tele – 120-300mm: Just like wide angle, this is very polyvalent focal length which can be used in most genre to isolate details and simplify compositions. For landscape work, remember about the “perspective flattening” effect.
Long and exotic tele – 300-800mm: Those are specialized lenses for wildlife and sport photographers who need to get close to their subjects but can’t physically move. They are complex and very expensive lenses, and their angle of view is so narrow that it won’t be of much use to most photographers. Tripods and fat wallets are often required.