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In message ,
"Bart van der Wolf" wrote: wrote in message .. . SNIP The rule of "using the lowest ISO" for maximum capture quality only works when DOF and shutter speed are not issues (static scene with tripod and MLU). True, but for me that's automatically solved when I "expose to the right". Yes, but do you expose to the right *and* raise the ISO to get the motion/DOF parameters you need? I think only a minority of people truly understand that going to a higher ISO is not only sometimes necessary, but it is also *NOT* necessarily a quality compromise; not unless you clip the data or lower the absolute sensor exposure witha faster shutter speed and/or a smaller aperture. Let's say that you're shooting a black subject against a middle-grey background with a 100mm lens on a full-frame DSLR, at a distance, and you want the DOF that is had at f8. The camera's metering tells you that at ISO 100 and 1/100 and f/8, you are "under-exposed" (or, in my suggested terms, "under-digitized") by one stop. Common wisdom would dictate to most people that they need to move to ISO 200. The fact is, you could move to ISO 800, maybe even 1600, with cleaner, more detailed results, with the same aperture and shutter speed (for 1600 you might need to decrease the absolute exposure just a tad to avoid blowing out the green channel). Anyone who thinks that what I just wrote is outrageous is clearly operating in an inefficent exposure/digitization paradigm. I like to look at things with efficient models, and nothing is clearer than looking at ISO choice in terms of getting the most output range for a given absolute sensor exposure. It isn't ISO per se that causes noise; it is the S/N ratio in the absolute analog sensor exposure that determines the starting noise, and ISO has absolutely no effect on that exposure except in how it affects the camera's metering. It has an effect on how that exposure is digitized, and at this stage, the higher the ISO is, the less noise there is in the image, because there is less quantization. Of course, this assumes analog gains proportional to the ISO numbers. If the shutterspeed and aperture are cast in concrete (which they often aren't), then all that's left is to correct with the ISO setting (not to influence the exposure meter, I'd probably use Manual in such a situation) in order to change the amplification on the analog signal before ADC. Too bad this couldn't be done in finer increments than one stop, on most cameras. You have to vary f-stop and shutter speed by 1/2 or 1/3 stop to get the in-between levels of digitization. However, if capturing the full scene Dynamic Range is important, I'd probably choose ISO 100 if I need to get the best sensor DR output, and adjust the Depth-of-Field / camera shake / subject motion trade-off. I don't know what camera you have, but I really can't see much of a difference in image quality between 100 and 200 with my 20D. It seems to me that 200 has just enough noise to dither the lower bits into softening the quantization that happens at ISO 100. So, you get slightly more random noise with 200, and slightly more posterization effect with 100, and they come up about equal. That one stop can often go a long way towards a better shutter speed, or get you away from the poor "wide-open" optics that many lenses have. IS0 400 is noticeably poorer, and from there up the difference becomes more dramatic with each doubling of the ISO setting. This, of course, is in reference to shadow quality. High-key images do not vary as much at the various ISOs. -- John P Sheehy |
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