a sharp image from corner to corner, which is 3 f/ stops smaller than the maximum aperture of the lens. For an enlarging lens with a maximum aperture of f/2.8, the optimal aperture would be f/8. The lens is normally set to this aperture and any color filtration dialed in, if making a color print or one on variable-contrast black-and-white paper. A series of test strips, and/or a stepped series of exposures made on the one sheet of paper, are undertaken to determine exposure, and then contrast or colour filtration. Alternatively a custom incident light meter (densitometer or 'colour-' or 'darkroom analyser') may be used in setting exposure once the degree of enlargement has been decided, and in colour printing may also be used to establish a base neutral filtration from the negative rebate. The enlarger's lamp or shutter mechanism is controlled either by an electronic timer, or by the operator - who marks time with a clock, metronome or simply by counting seconds - shuttering or turning off the lamp when the exposure is complete. The exposed paper can be processed immediately or placed in a light-tight container for later processing. Digitally controlled commercial enlargers typically adjust exposure in steps known as printer points; twelve printer points makes a factor of two change in exposure. If a greater or lesser enlargement from the same negative is then required, a calculator – analogue, digital or in app format – may be used to quickly extrapolate from the original settings the exposure without the need for labour-intensive re-testing. Paper processing After exposure, photographic paper is developed, fixed, washed and dried using the gelatin silver or C-print process. Automated print machines Automated photo print machines have the same basic elements and integrate each of the steps outlined above in a single complex machine under operator and computer control. Rather than project directly from the film negative to the print paper, a digital image may first be captured from the negative. This allows the operator or computer to quickly determine adjustments to brightness, contrast, clipping, and other characteristics. The image is then rendered by passing light through the negative and a built-in computer-controlled enlarger optically projects this image to the paper for final exposure. As a byproduct of the process a compact disc recording may be made of the digital images, although a subsequent print made from these may be quite inferior to