What makes a lens "normal," and how does sensor size affect it?
Asked 1/18/2012
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People often call a 50mm lens a "normal" lens, but I’m trying to understand what that actually means. Does a lens have to be exactly 50mm to count as normal, or is there a range? Does the definition depend on the camera’s sensor/film size and crop factor? Also, how does a normal lens relate to terms like wide-angle, telephoto, prime, zoom, and "standard" lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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A normal lens is one who's focal-length is equal to the diagonal of the sensor or film. This is said to give a natural perspective similar to that of a single human eye.
On a full-frame DSLR, it is usually a 50mm lens. On a cropped-sensor (APS-C) DSLR, a normal lens falls around 35mm but from 30 to 55mm, it would still be considered normal. For Four-Thirds and Micro Four-Thirds, you would use a 25mm. Usually most manufacturers make sure to have one bright prime that corresponds to the normal focal-length for the sensor-size.
Originally by user1620. Source · Licensed CC BY-SA 4.0
user1620
14y ago
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Generated from our catalog & community — verify before relying on it.
A "normal" lens is generally one whose focal length is about equal to the diagonal of the camera’s sensor or film frame. In practice, that means a field of view that looks natural rather than obviously wide or telephoto.
So 50mm is only "normal" on full-frame/35mm format. On APS-C, normal is closer to about 30–35mm. On Four Thirds / Micro Four Thirds, it’s about 25mm. The exact number is not magical, and there’s some leeway rather than one perfect focal length.
The key idea is that focal length must be considered relative to sensor size: the same lens can be normal on one format, wide on a larger format, or telephoto on a smaller one.
Related terms:
- wide-angle: shorter than normal
- telephoto: longer than normal
- prime: fixed focal length
- zoom: variable focal length
- standard lens: often used loosely to mean a normal lens, or a standard zoom that includes the normal range
So, a lens does not need to be exactly 50mm to be normal; it needs to be near the normal focal length for the format being used.
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