LYCOS RETRIEVER 
Lens: Distances
built 621 days ago
Stand an arm's length from one side of the lens; have a friend stand an arm's length from the other side. Look at your friend's face through the lens. Have your friend bring his or her face closer to the lens as you back away, keeping the same two-arms' length distance between the two of you. Then reverse this procedure: You step closer to the lens while he or she moves away. Notice how his or her face appears; ask your friend how your face appears.
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A "normal" or "standard" camera lens is one that produces prints with no apparent wide angle or telephoto distortion. In other words, when viewed at a standard distance, a print taken with a normal lens will appear to have no unusual perspective. For a camera taking 35mm film, a 50mm lens is considered normal.
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If two thin lenses are separated by some distance d, the distance from the second lens to the focal point of the combined lenses is called the back focal length (BFL). This is given by:
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Distances