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." (Explanation of focal length from -http://www.trustedreviews.com/opinions/digital-photography-tutorial-focal-length
I have a Nikon D5000 and used a DX VR 18-105 mm f3.5-5.6 zoom lense to obtain the following pictures to illustrate the different focal lengths. As you look through the pictures you can see how using a longer focal length reduces the width of the picture and condenses the image.
First image with focal length at 18mm, f11 @ 1/1000 sec
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First image with focal length at 18mm |
Second image with focal length at 24mm, f11 @ 1/1000 sec
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Image with focal length at 24mm |
Third image with focal length at 35mm, f11 @ 1/1250 sec
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Image with focal length at 35mm |
Fourth image with focal length at 50mm, f11 @ 1/1250 sec
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Image with focal length at 50mm |
Fifth image with focal length at 75mm, f11 @ 1/1250 sec
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Image with focal length at 75mm |
Fifth image with focal length at 75mm, f22 @ 1/400 sec
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Image with focal length at 105mm |