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If we will use crop lenses only on crop bodies, why do we call crop lenses for example "10mm" when they are effectively "16mm" (with a crop factor of 1.6 in this case)?

I am familiar with the history and the difference between a full-frame and a crop body. My question was about why we call these lenses different in focal lengths since they both yield the same field of view in degrees (the crop lens on a crop body, and the FF lens on a FF body). I know the lens doesn't actually turn into a longer focal length optically on a crop body, but since the result is the same, why not just label it that way?

marked as duplicate by Michael C, null, Caleb, Philip Kendall, Dan Wolfgang Apr 16 '16 at 10:46

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Because a 10 mm lens is a 10 mm lens.

Crop factor has nothing to do with the real mm of a lens.

Crop factor is the same as if you take your Photoshop and crop the center of a photo.

Take a look at this answer: Do I use the crop factor in calculating aperture size and area?

The crop factor equivalent is to give you "an idea" if you have being using a 35 mm film camera on how your framing will look compared to that size.

A "crop" lens is diferent vs a full frame lens, becouse if you use it on a full frame camera you will have a vignetting. "Simply" because it is cheaper to project a smaller image. Smaller glass, less weight, etc.

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The 35mm film camera format has been with us since 1924 when the German Leica was introduced. The image size (format size) measures 24mm height by 36mm length. Now digital cameras are replacing film cameras. Most were built to house a digital imaging chip that has the same format size. These are called full frame cameras. As technology marches on it has become possible to shrink the digital image sensor size. The newer compact digital cameras sport an image sensor that measures 16mm height by 24mm length.

The crop factor, sometimes called the magnification factor is derived by dividing the diagonal measures. The full frame diagonal is about 45mm. The compact digital frame diagonal is 30mm. Thus 45 ÷ 30 = 1.5. This is the crop factor. What does the crop factor tells us? OK 1/1.5 = 0.66, in other words, the compact digital is 66% of the size of the full frame.

If we mount a lens with a focal length about equal to the diagonal measure, the angle-of-view delivered is 45⁰ with the camera held horizontal. In other words a 45mm lens mounted on a 35mm format is said to deliver a “normal” view. Same is true if a 30mm is mounted on a compact digital. Special note: While a 45mm delivers a “normal” view on a 35mm camera, lens makers have traditionally rounder this value up to 50mm. Thus a 50mm lens is considered normal for the full frame.

The full frame became so popular that most photographers became super familiar with how it performed with most lenses. Thus it became the gold standard and this is why some think we need a crop factor. The truth is, if you have never used a 35mm full frame, the most often quoted crop factor is useless.

The fact is, the focal length engraved on the lens remains factual regardless. Better you become familiar with how lenses preform on your camera, regardless of its chip size. As the future unfolds, image sensors will shrink and you will have even more difficulty trying to us a crop factor.

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    Actually, the earliest digital imaging sensor were not 35mm sized, especially those offered in consumer products. They were smaller. Thus the earliest lenses designed to throw an image circle the size of an APS-C sized sensor were referred to as digital lenses. – Michael C Apr 15 '16 at 23:34
  • True but the full frame 35mm yielded a truckload of good but used lenses that were already designed plus the market place had a zillion used but good lenses that were available. Thus the full frame became the gold standard. – Alan Marcus Apr 15 '16 at 23:40
  • +1 for the idea that full frame is just a somewhat arbitrary reference point that is quite meaningless if you have never used it. – null Apr 16 '16 at 0:31

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