Taking The Guesswork Out Of Aperture Selection

Depth of field an be defined as the range of distances from the lens in which objects appear sharply focused. When focusing a lens, the distance shown on the distance scale on the lens (if it has one) will be the point of sharpest focus; objects at the selected distance from the lens will be the most clearly focused elements in the image. The clarity of focus will gradually deteriorate as one moves closer to or farther away from the lens. If a minimum acceptable criteria is defined for focus clarity, there will be a range of distances in front of and behind the selected focus distance that are acceptably "in focus". This range of distances is depth of field, or DOF. The way focus clarity is defined for DOF calculation is by specifying a Circle Of Confusion (COC), which is the size a point light source will be projected on the imaging surface (either film or a digital sensor) by the lens. In digital imaging, the COC value chosen should generally match the pixel pitch of the sensor (the center-to-center distance between pixels on the sensor surface), since the sensor cannot resolve details smaller than 1 pixel. Calculating DOF in this way will tell you what settings to use to obtain pixel-sharp images with your camera.