Diffraction Aperture
Diffraction aperture is defined as the interference or bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture.
When light passes through a small hole (such as the aperture iris in your camera's lens), it spreads out. This diffraction visibly reduces image sharpness. The smaller the aperture, the greater the effect. Diffraction occurs to some degree at small apertures with all lenses, but is more visible on photos taken with digital cameras than in film cameras, because the pixels on the digital camera's sensor are more sensitive than film is to the softening effect of diffraction.
The exact aperture at which the softening caused by diffraction becomes visible depends on several variables, including sensor and pixel size. Generally speaking, the smaller the sensor, and the smaller the pixels on the sensor, the sooner you'll notice the effect of diffraction as you stop down.
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