![]() The A2 reliably saturates at about 3 1/2 stops above middle gray. This creates a "blocked-out" effect, for which digital images are famous. All image luminances that map to intensity values above 255 are displayed as maximum white, 255. Photographic film saturates gradually, causing high values to roll off smoothly (the "shoulder" of the characteristic curve). I did not have time to find the optimal "Shadow" value for low values of gamma. This did indeed preserve image detail in the shadows, at the cost of noise. ![]() Falsely assuming that a low gamma would flatten the whole curve, I adjusted "Shadow" so that the target showed very faintly in the darkest values, at 0.2 (default is 0.5). A test at the lower limit, 0.01, is of no interest because the resulting values are flat, light, and noisy so I arbitrarily picked 0.10 after viewing the effect of setting various values of gamma. Arbitrarily, one was the upper limit gamma of dcraw, 1.5. Processing.The resulting series of images were processed at the default gamma of 0.6, and at two gamma values above and below this. (It excludes the "toe" and "shoulder" of the curve.) In sensitometry, "gamma" is the slope of the straight-line portion of the "characteristic curve" - the S-shaped graph that relates image luminance to film or print density. Raw files were recorded and processed using the gimp and the dcraw plugin. The target was placed on Exposure Zones minus-one (-I) through eleven (XI) in order to ensure that the entire dynamic range was captured. A standard test target was exposed under bright cloudy skies at an ambient temperature of 40 degrees F., using an assumed ISO of 64 to calculate exposures. The effect of using the in-camera image processing to adjust gamma ("contrast") is discussed on the next page. This analysis uses raw images processed with dcraw in the gimp. It is useful in understanding both camera function and image processing. The following chart shows the imaging characteristics of the Konica-Minolta DiMAGE A2 ZLR at five different gamma settings. DiMAGE A2 Characteristic Curves for Gamma 0.10 to 1.50 Gamma management using dcraw and the gimp
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