Focal length is the length of the lens. Common focal length ranges go from 16mm-200mm. There are lenses that go up to 800mm and beyond but for primary usage something in the 16-200mm range is usually sufficient. The top of the line Canon Zoom lenses cover this range. In the Canon line up 3 lenses are needed to cover the range: 16-35mm, 24-70mm and 70-200mm. There are multiple versions of each but all of them are excellent high performing lenses.
The lenses are engineered so that the maximum amount of glass in the lens is being used when it is being used on a full frame camera. When these lenses are used on a cropped sensor camera not as much of the lens is used due to the smaller sensor size. This creates the same effect as cropping the image which is why camera manufacturers talk about the cropped field of view. On Canon cameras it is typically a factor of 1.6x which means that if you have a 16mm lens on a cropped sensor camera you would need a 25.6mm lens on a full frame camera to duplicate the field of view. Clear as mud?
There are lenses that are made specifically for cropped sensor cameras and while those lenses aren’t compatible with full frame cameras they are typically made to have focal lengths that are equivalent to similar full frame lenses. For example Canon has a 10-22mm lens that can only be used on cropped sensor cameras. The crop factor for Canon’s sensors is 1.6x. So 10-22 on a cropped sensor will produce the exact same image as a 16-35mm on a Full Frame camera. This can be useful when it comes to needing more reach. For example, a 200mm lens on a cropped camera will give you the equivalent of 320mm. Below are examples of various focal lengths throughout the 16-200mm range using a full frame camera. For these photos the camera was fixed to a tripod, all camera settings were kept the same, the only thing that changed was the focal length and the lens.
As you can see there is an enormous difference between 16mm and 200mm. A slight amount of distortion is also evident in the 16mm shot. These shots are intended to give you an idea of what field of view to expect with each focal length.
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