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The Low Light Photography Field Guide by Michael Freeman

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Focus at Full Aperture

If you are used to the forgiving focusing range of a normal lens (and these days, full aperture on many zooms is ƒ/3.5–ƒ/5.6) then working to precision with a fast lens wide open can come as a shock.

For example, on my Nikon DSLR, using a Zeiss 85mm Planar at ƒ/1.4 and focusing on a subject at 16.4 ft (5 m), the depth of field is around 7.4 in (19 cm). Compare this with the 30 in (77 cm) when the lens is stopped down to ƒ/5.6. Depth of field is at best a fuzzy concept, and the calculations are not to be taken literally as it is ultimately based on the concept of “sharp enough,” involving both judgment and the definition of the circle of confusion. The circle of confusion is the smallest diameter that an imaged point can ...

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