Secondary Effects
When a lens designed for 35 mm format is used on a smaller-format DSLR, besides the obvious reduction in field of view, there may be secondary effects on depth of field, perspective, camera-motion blur, and other photographic parameters.
The depth of field may change, depending on what conditions are compared. Shooting from the same position, with the same lens and same f-number as a non-cropped (full-frame) 35 mm camera, but enlarging the image to a given reference size, will yield a reduced depth of field. On the other hand, using a different lens with the same field of view as the non-cropped camera (matching the 35 mm-equivalent focal length), at same f-number, the smaller camera's depth of field is greater.
Perspective is a property that depends only on viewpoint (camera position). But if moving a lens to a smaller-format camera causes a photographer to move further from the subject, then the perspective will be affected.
The extra amount of enlargement required with smaller-format cameras increases the blur due to defocus, and also increases the blur due to camera motion (shake). As a result, the focal length that can be reliably hand-held at a given shutter speed for a sharp image is reduced by the crop factor. The old rule of thumb that shutter speed should be at least equal to focal length for hand-holding will work equivalently if the actual focal length is multiplied by the FLM first before applying the rule.
Many photographic lenses produce a superior image in the center of the frame than around the edges. When using a lens designed to expose a 35mm film frame with a smaller-format sensor, only the central "sweet spot" of the image is used; a lens that is unacceptably soft or dark around the edges can give better results on a smaller sensor. However, since the image projected onto the sensor must be magnified more to make a print using a smaller sensor, lenses used on smaller formats must deliver a higher-resolution image to the sensor for acceptable quality.
Read more about this topic: Crop Factor
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