depth of focus

Definition: Measure of zone or distance over which an image projected by the lens will appear acceptably sharp: it lies in front of and behind the focal plane. * Varies proportionally with aperture for given circle of confusion and inversely proportional with focal length. * Depth of focus is the reason why, in manual focusing, it is sometimes tricky to determine the exact point of focus.
Eyes wide focused
Depth of focus is the reason why lenses with small maximum apertures are so hard to focus and why we focus at maximum aperture with single-lens reflex cameras and enlarging lenses. This practice assumes that the focus does not shift with stopping down - not guaranteed with cheaper enlarger lenses. And, it has to be said: do not confuse with depth of field; a common and egregious error.

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