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D > Diffraction Effect - Pg. 89

D Diffraction Effect 89 Closing the lens aperture increases depth of field. To get maximum depth of field ­ and therefore the sharpest image from front to back ­ you would expect to stop the lens down all the way to its minimum aperture. Lenses have a `sweet spot' aperture; anything smaller than this causes the diffraction effect, which begins to reduce resolution by increasing the size of the circles of confusion. The best analogy is to think of a hose being pinched tighter and tighter, which will spray wider and wider; instead of creating a neat, focused circle it creates a rippled diffraction pattern called an Airy disc. Sometimes the extra depth of field is worth the slightly lower resolution.