On May 2, 2008, at 1:37 AM, Davis, Lee wrote: > Are you saying that, regardless of whether you have the camera set to > JPEG or RAW, the meter readout will always be the same for the given > scene BUT that using RAW really needs extra exposure,
Yes. > as if RAW and > JPEGS were two different 'films', each having slightly different ISO > ratings? Not quite ... With RAW, you have 12bit quantization, rather than 8bit, and control of the gamma correction curve so you effectively have more dynamic range to work with. Proper exposure technique for digital capture is to acquire as much data as possible without saturating the sensor: the upper limit of exposure has a hard edge at saturation, where the lower limit is ultimately an arbitrary call to how much noise you find tolerable for given scene dynamic. This is different from film exposure in that film has a "soft" boundary at both ends of the spectrum. Since the capture curve is a linear ramp in powers of 2, you want to get as much data as close to saturation as possible: the top bit is half the data, the next bit is 1/4, etc. Bruce Fraser explains this fundamental eloquently in the first chapter or two of his book "Real World Camera Raw...". The *ist DS metering calibration for RAW capture was significantly less accurate than the K10D. I have not experimented with a K20D to know how it does. The R1 was shockingly accurate. > At first, I was a little disappointed, thinking that many of the shots > were up to a stop underexposed ... Even though we had even (though > dull) > lighting. The histograms are shifted to the left. In Lightroom the > shots > looked way too contrasty and muddy by default. > > I am also trialing Silkypix (too many variables here perhaps) and > really > like it. It seems to render the images better, less contrasty and > perhaps brighter, by default. I rarely worry about the default settings in the RAW conversion applications. Silkypix has perhaps better defaults but I absolutely detest trying to work with it. Lightroom's defaults seem ok for a good deal of stuff, but I hardly ever use default settings. > I got my Sony R1 out and started comparing its meter readings with the > K20D for the same focal length shots, same ISO settings etc, set to > RAW. > (My neighbours must think I'm some sort of freak or nutter). Most > of the > time they were in agreement. What I did see though was that the > K20D was > *much* more sensitive ... Only a slight shift of the camera left or > right would make it fluctuate. I wonder if the Pentaxes 'panic' if > anything in the frame is considered a highlight, even on evaluative > metering? For example, I only need to shift the K20D upwards > slightly to > include a small amount of extra sky and the meter reading changes. The > Sony would need a little more of a shift. Sounds like the K20D has a more sharply defined metering pattern in its evaluative mode, if that's the metering pattern you were using. > So, I am slightly confused but if it is generally true that cameras > need > or ought to meter differently for JPEG or RAW, then that is > interesting > stuff! A JPEG image is a rendered RGB image in a smaller quantization space, post gamma correction. The gamma correction process is lossy: what is lost in largest measure is dynamic range. So it should be no surprise that the exposure requirements are different ... Kind of like processing Plus-X in Microdol-X vs Acufine developer, you need to see exposure on a different curve. > Why can't Pentax just make the firmware change this automtically? A simple bias adjustment is possible, but like so many ahoer aspects of exposure evaluation, there are always mitigating circumstances that might lead you to push the exposure in different directions. Proper exposure is quite a serious study in finesse... Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
