On Jul 5, 2006, at 11:41 PM, Toralf Lund wrote: > I was talking about the number of different voltage levels that may be > output from the sensor itself. That number is not necessarily 4096; it > is the analogue-to-digital converter that has 4096 different values. > This distinction is far from irrelevant. Unless the actual sensor can > produce more than 4096 different levels, your whole exercise is quite > pointless at ISO settings much higher than the native rating of the > sensor. Heck, at low-light conditions, there is not even point in > metering at all, if you are going gamma correct and/or "scale" the > data > as part of our raw conversion anyway. > > I think you'll understand why if you consider the following > "experiment" > for a while: > Get a digital camera with a native ISO of 100. > Turn the ISO dial to 100, and set shutter/aperture so that you get > correct exposure, and start firing away. Check the maximum pixel > value. > Reduce the amount of light by 50%. Don't update the camera > settings. Try > again. What is the pixel value range like now? > Now switch to ISO 200, and repeat. > > The big question is whether the last step actually changed the > number of > different pixel values, or just replaced the range 0,1,2,3...2047 with > 0,2,4,6...4094. Like I said, think about it for a while...
I've thought about it for long enough ... a year or two ago. There's no point in considering the A/D conversion as separate from the sensor *because you can't do anything about it*. The RAW data output is what you can manipulate and represents conceptually a linear gamma space. If I look at the output with a proper exposure at ISO 200, I get values from 0-4095. If I look at the output at ISO 1600 with a proper exposure, I also get values from 0-4095. Using a standard target and developing a curve fit will demonstrate that the dynamic range and gamma change between the ISO settings, but do not change the notions of how to evaluate the best exposure for a given scene. With regard to the data you can manipulate, using the strategy of providing enough exposure to place Zone IX properly just under the saturation threshold will *always* produce the best data to work with regardless of ISO because the reason it works is founded in numerical operations. If you plot difference in the response curve of the sensor at different ISOs, you could also use that information to expand or constrain the dynamic range you want to apply to scene capture, but that says nothing about proper exposure evaluation ... which was the point of my explanation. I take the tactic that I use the lowest ISO that I can all the time, consistent with achieving the focus zone and exposure time desired, since I can compress or expand the dynamic range in software with little constraint in post processing. The lower ISO setting will allow more data and thus greater flexibility in rendering. Godfrey -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
