This page is provided both for general background information on the need for accurate focusing, as well as to provide actionable tips on how to reliably achieve sharp focus. It is a general discussion not aimed at any particular camera brand or lens.
For examples of focus error issues, please see Focus Accuracy at diglloyd.com.
Some photographers still believe that focusing by the distance scale on the lens is sufficient to extract top results if one simply stops down to f/8 or f/11. Experienced digital shooters know that such an approach is inadequate with today’s 10+ megapixel cameras (or even 4-megapixel cameras, to a reduced degree). Stopping down to f/8 from f/2 delivers 4X the depth of field (in theory). If the depth of field is 1 foot at f/2, then it’s only 4 feet at f/8, and with real lenses theoretical and actual depth of field are two different animals. Depth of field does not mean that contrast remains uniform throughout that zone, or that aberrations are all controlled (though stopping down reduces some optical aberrations).
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Since 2006, Diglloyd Advanced Photography is the authoritative review and reference for DSLRs and their lenses: Canon and Nikon primarily, but also Pentax and some medium format. Reviews of key DSLRs are included, but the primary focus is on lens performance. Also included is a wealth of technique and workflow approaches.
DAP includes thousands of pages of exclusive coverage and high resolution Retina-grade examples detailing Canon and Nikon DSLR bodies and their lenses, with in-depth analysis of sharpness and contrast, color, bokeh, vignetting, distortion, MTF, and flare, behavioral and practical usage.