Yes, of course you don't want to go beyond the sensor's saturation point in areas where you want detail. But "protecting" an exposure by underexposure is the wrong methodology. It's difficult to explain without some graphs to make it more intelligible, that's why I recommend reading Bruce's first two chapters. He did a great job. But I'll try.
--- A digital sensor is a photon counter. It simply counts up the amount of light falling on a photosite's unit area in the time of exposure and reports that number. As such, it is a linear gamma device ... unlike the human eye or film, intensity values in a scene are simply represented by that linear count of photons falling on the photosite array. The saturation limit happens when the sensor runs out of numbers ... in the case of the sensor used in the Pentax DSLRs, it can count from zero to 4095, 12bits of quantization ... which depends upon how much light energy is falling on the sensor, how much loss is embedded in the photosite's design, and how much time the photosite is exposed. This all conspires together to place saturation on a hard edge ... the number 4090 is not saturated, the number 4095 is because you cannot record the number 4096. By that we've established what "saturation" means. The next thing to understand is how RAW data relates to a tonal scale that makes sense to our eyes. One of the basic operations performed by RAW Conversion is to do a gamma correction on the captured data ... By this is meant that the linear capture of the digital sensor is transformed to re-place the light values in the characteristic curve of human vision's sensitivity and perception. The human eye perceives fewer differences between bright values and greater differences between dark values than the sensor by comparison. So the high values recorded by the sensor are compressed together ... values that are insignificantly different in perception are thrown out ... where the low values are expanded ... values that are crowded too close together are interpolated/stretched to fit the range required. Now, if you consider a linear scale of numbers recorded by the sensor as a binary representation, you'll see that the half the total amount of exposure falling on the sensor is stored in the last bit of representable numbers. Half again is stored in the next bit down. Half again is stored in the next bit down after that. What this means is that, in the range of the sensor's linear number scale, the midpoint of exposure to our eye (call it Zone V) is NOT in the middle of the scale, it's actually down around the 1/8 point in the linear scale. So all the tonal values that make up the important range from Zone II to Zone V are smashed together in the bottom end of the sensor curve, and most of the data values that take up more than 3/4 of the scale are insignificant to human perception. If you underexpose the scene, more interpolation and expansion of that small part of the scale must be performed to fit the data to the proper perceivable range, which has as its byproduct noise and ambiguity in the Zone II to V range through round-off error. What this means to a photographer making an exposure evaluation is that the photographer should consider the linear capture qualities of the sensor. If you pick the brightest points in a scene, the points of specular reflection for instance, you want to place your maximum exposure so that they are just AT the 4095 data value threshold ... this is hard because you can't see when you go over with them. So you look at the Zone IX values, the brightest parts of the exposure where you want to retain detail, and try to keep the values in a capture of those points to somewhere in the range of value 3686 (around 210-220 in 8bit data), and leave the other values to fall where they may. If you look at a scene after capture with the histogram display on the camera, this kind of exposure will "crowd the right" ... The goal in doing it is to capture as much distinct data as possible without saturating the important detail areas. In processing, you place the gamma correction curve to handle a given scene's exposure by adjusting the white clippint point (exposure), the brightness and contrast (essentially, it moves the nodal point of the gamma correction and the angular relationship of the resulting curve inflection) and then the black clipping point (the point at which you decide where the differences between low values are purely ambiguous and insignificant). Exposing as much as possible without saturation means you have more data at the low end to expand through interpolation with the least round-off error and noise. Regarding the current Pentax *ist D series cameras currently available, I'll use the *ist DS as my specific example but I believe the same is true for all of them: The *ist DS is set by default for Auto Picture exposure automation with JPEG *** fine quality, using a Bright color tone and intended to produce a snappy, pleasing image for a 4x6 inch print. What this means is that the in-camera RAW conversion algorithm is tuned to that output, and the meter is calibrated to produce results compatible with that algorithm. The *ist DS does NOT change the meter calibration curve when the user takes control of the rendering engine and requests a RAW file as output. The difference in output requirements is critical: the default JPEG *** rendering and Bright color tone means that meter calibration has to be set optimistic to suppress highlight saturation with the embedded RAW conversion. Switching to RAW capture and using the meter's default calibration results underexposure because RAW format data has more stops of overhead before saturation values are reached at the sensor. With a customized RAW calibration curve, you can obtain better data with more exposure on the 12bit capture. So I find that my average exposure compensation when capturing RAW format runs +0.3 to +0.7 EV, without saturating highlights, and allows much cleaner, lower noise data in the critical Zone II to Zone V range. ---- I hope that helps. I'm not as good at explaining this stuff as Bruce. Godfrey On Jul 4, 2006, at 8:23 AM, Bob Sullivan wrote: > Godfrey, > You've got to explain this. > Digital sensors can't give any detail in overexposed highlights. > You can recover details in underexposed areas with post processing. > So don't you want to avoid blown highlights at all costs? > Regards, Bob S. > > On 7/4/06, Godfrey DiGiorgi <[EMAIL PROTECTED]> wrote: >> Sensors respond to light differently compared to film. Chapters one >> and two of Bruce Fraser's "Real World Camera Raw with Photoshop CS2" >> explains why there is a difference. As a result, exposure evaluation >> requires a different mindset and different settings. JPEG and slide >> film, although they are different, generally end up taking about the >> same exposure. >> >> However, underexposing in RAW by 0.3-0.5 EV is exactly the wrong way >> to go. In general, with the *ist DS, I find my average exposure for >> RAW capture requires +0.3-0.7 EV additional exposure compared to JPEG >> or slide film. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
