Crop Factor
When used with a Canon APS-C (1.6x crop) DSLR camera or APS-H (1.3x crop), the field of view of this lens is similar to a 320mm or 260mm on full frame camera. There will be an apparent magnification of approximately 1.6x in the final image, since the "cropped" image will fill up the sensor. This is due to the crop factor inherent with APS-C or APS-H (crop) sensor digital SLR cameras.
An example would be taking an image of a rock using two cameras with the same lens. The first camera a 18mp full frame and the second a 18mp APS-C, both shooting the same composition in a stationary position. The first image will be more "wide" while the second image will be more "magnified". After bringing the results into an image editing program and enlarging the first image so that the rock is the same size in both images, one will see that the enlarged image is approximately 160% (1.6x) of the original.
The major advantage to this extra "reach" would be the utilizing of the full sensor space for a cropped image rather than having to crop afterwards, thus utilizing parts of the sensor that would have otherwise been wasted. The major disadvantage would be the lack of change in perspective, since the focal length has not actually changed it will be like shooting with the field of view of a 320mm lens on a full frame sensor while having the perspective of 200mm lens. The resulting image will appear to have a less pleasing background blur and unlike using an actual 320mm lens on a full frame sensor.
Read more about this topic: Canon EF 200mm Lens
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