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What is the meaning of the term "cropped sensor"?

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See also What are the different sensor sizes used in DSLRs?, and also cropped-sensor. –  mattdm Jan 18 '12 at 11:53

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up vote 5 down vote accepted

Basically you just record a part of the image, hence crop...

A full frame sensor is 36mmx24mm in size, a cropped sensor, in the case of Canon APS-C approximately 27,9mmx18,6mm.

Lenses are typically designed to throw an image circle with a diameter of 56mm to cover old wet film - or the equally sized full frame sensor. A crop sensor that sits the same distance away, covers a smaller area of the image circle and hence records only a portion of the image circle thrown - i.e. it crops the image thrown by the lens.

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Why 56mm and not 43.3? –  mattdm Jan 18 '12 at 12:13
    
Also, what is "wet film"? –  mattdm Jan 18 '12 at 12:15
    
Because lenses tend to darken towards corners known as vignetting, hence the image circle needs to be larger than the diagonal of the sensor. Wet film = old analog film. –  DetlevCM Jan 18 '12 at 12:25
    
And (sorry for so many questions!) since the image circle is cropped to a rectangle by film or a full frame sensor, and needs to cut out that vignetting, why don't we call that a cropped sensor too? –  mattdm Jan 18 '12 at 12:25
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With digital full frame sensors and the remaining terminology retained, we just accepted anything that is smaller to be a "crop sensor". At the same time, camera manufacturers also needed a way to describe their smaller sensors and decided "crop" with respect to the 35mm film "standard" was a good way to do so and the term since stuck. –  DetlevCM Jan 18 '12 at 12:35

A "crop" sensor is a sensor (usually in a DLSR) that is smaller than the standard 35mm film size. 35mm sensors are referred to as "full frame". Images produced from a "crop" sensor are equivalent to the middle being cropped out of a "full frame" sensor image.

I don't really like either term - the 35mm frame size is wholly arbitrary, provided the viewfinder shows you the correct field of view then it doesn't matter that the sensor is smaller than 35mm - "full frame" sensors are in tern smaller than 645 medium format sensors, which are smaller than large format sensors etc.

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I wouldn't call it arbitrary, for it was a doubling of the normal movie image by Barnack. Now, one could argue that the film size was arbitrary at one point, but the film decision was based on existing media, and thus, not arbitrary. –  dpollitt Jan 19 '12 at 3:07

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