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The "mm" marking represents the focal length of the lens as measured from the optical center of the lens to the film plane when the lens is focused at infinity. If the lens is a zoom, focal length is expressed as a range such as 28 - 80mm, which means it can be infinitely varied from one length to the other. Most zoom lenses accomplish this in part by varying the position of the optical center. The focal length or zoom range is typically indicated on the lens barrel and/or the area surrounding the front lens element.

Lenses that accept accessory filters are almost never marked with a filter diameter or series size on the lens, though screw-in filters have the diameter indicated in millimeters on the filter rim. Since many lenses are stored with a filter in place, this might easily be mistaken for a part of the lens itself, particularly with screw-in filters, which normally sit flush with the front diameter of the lens barrel.

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15y ago
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13y ago

It denotes the diameter of the lens in millimeters (mm).

*incorrect answer*

"mm" denotes the "focal length" of the lens... the distance the lens must be from the "film plane", or "sensor", to focus on infinity.

the diameter of the lens is the "f-stop". ie: f2.8.

the f-stop is a ratio of the focal lenth, to the diameter of the front element, and denotes how much light passes through the lens in a given amount of time.

focal length applies to the "zoom" of the lens, or, the size of the image on folm, whereas the fstop, is used to control exposure, and/or to limit of expand depth of fierld.

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Q: What does the mm mean on a camera lens?
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