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#41
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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 |
#42
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In message ,
Alan Browne wrote: Yes, I owuld guess that it is so. When below, eg, ISO 1600, the ADC is stepped, and at 1600 and above the sensor readings are left shifted. There may or may not be an overlapped zone where some gain is in the ADC and some gain is in shifting left. Then, there may be some filtering in the ADC as well as (of course) making the RGB image from the Bayer image that will disgusise (filter, badly or otherwise) the quantization effects. The dynamics of the noise are greater than the dynamics of quantization at the highest ISOs. The lower ISOs are the ones that show the most quantization effect. -- John P Sheehy |
#43
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wrote:
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. It's not outrageous at all -- it just bears no resemblance to any way I've ever approached photography. I guess it makes sense in the above situation if you actually believed the camera's light meter, which was wildly incorrect; if the better exposure was at ISO 800, then the meter was two full stops off in its judgement, because you weren't underexposing by one stop at ISO 100, you were underexposing by three stops. So you're still comparing a proper exposure at ISO 800 to a two-stops-under exposure at ISO 200, and the moral of the story is still that it's better to boost ISO than to underexpose. 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. Do the Canons really make you use full-stop increments?? -- Jeremy | |
#44
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In message ,
Alan Browne wrote: Quantization noise is further disguised in character, of course, by the conversion from bayer to RGB. Not if you look directly at the RAW data. Demosaicing and white-balancing certainly hide a multitude of quantization sins. If a camera had no CFA, and was greyscale, then an 8-bit Tiff with a 2.2-gamma-adjusted output from a greyscale RAW file would have values of something like 0, 9, 16, 20, 22, etc, in the deepest shadows, with nothing in-between. The WB and demosaicing create all kinds of intermediate values that fool a histogram, but doesn't have quite the shadows that it could, if the camera's sensor "sensed" in a gamma-adjusted (and already white-balanced) manner. -- John P Sheehy |
#45
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In message ,
Jeremy Nixon wrote: wrote: 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. It's not outrageous at all -- it just bears no resemblance to any way I've ever approached photography. I guess it makes sense in the above situation if you actually believed the camera's light meter, which was wildly incorrect; if the better exposure was at ISO 800, then the meter was two full stops off in its judgement, because you weren't underexposing by one stop at ISO 100, you were underexposing by three stops. So you're still comparing a proper exposure at ISO 800 to a two-stops-under exposure at ISO 200, and the moral of the story is still that it's better to boost ISO than to underexpose. You misunderstood; there is nothing wrong with the metering in this scenario. The scene had the approximate reflectivity of a grey card. 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. Do the Canons really make you use full-stop increments?? For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the non-1-series cameras. -- John P Sheehy |
#46
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wrote:
You misunderstood; there is nothing wrong with the metering in this scenario. The scene had the approximate reflectivity of a grey card. No, there was; if the meter indicates one stop underexposure, but you can increase three stops and end up with a proper exposure, then the meter was actually off by two stops. You're comparing "trust the meter" at ISO 200 with "don't trust the meter" at ISO 800, and that's not a useful comparison, because if you trusted the meter in the first situation you'd trust it in the second and still underexpose by two stops, but if you weren't going to trust the meter in the first place (and thus be willing to go up two more stops from what it said at ISO 800) then you'd have been willing to do so at ISO 200, too, if you could -- leaving the choices being either underexpose at 200 or expose properly at 800. Do the Canons really make you use full-stop increments?? For ISO? Yes, the ISOs are 100, 200, 400, 800, 1600, 3200 On the non-1-series cameras. Ah, so it's one of those "held back" features. Annoying how they use (seemingly) simple features to convince you to spend more money. -- Jeremy | |
#47
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In message ,
Jeremy Nixon wrote: wrote: You misunderstood; there is nothing wrong with the metering in this scenario. The scene had the approximate reflectivity of a grey card. No, there was; if the meter indicates one stop underexposure, but you can increase three stops and end up with a proper exposure, What is "proper exposure"? then the meter was actually off by two stops. No, the meter is dumb, and it puts the mid-key scene at middle grey, which is not *optimum*, unless your goal is to use a JPEG as-is, or print directly from the camera. In terms of RAW capture, it is a relatively poor digitization and/or exposure. You're comparing "trust the meter" at ISO 200 with "don't trust the meter" at ISO 800, and that's not a useful comparison, because if you trusted the meter in the first situation you'd trust it in the second and still underexpose by two stops, but if you weren't going to trust the meter in the first place (and thus be willing to go up two more stops from what it said at ISO 800) then you'd have been willing to do so at ISO 200, too, if you could -- leaving the choices being either underexpose at 200 or expose properly at 800. I clearly stated that we were a stop under because of *NEEDED* f-stop and shutter speed! You're going to blur the image now? Let me try another way. Let's pretend the camera is greyscale (no CFA), to simplify matters. The camera meters a grey card (or the ~18% grey scene) as if to wind up with an average RAW value of 400 (fairly typical) out of about 4000 possible levels. At ISO 100, you fall a stop short of that with your needed f-stop and shutter speed, so the RAW output would be 200. Using the same aperture and shutter speed (as needed), with ISO 800, the average now is a RAW value of 1600. If you used ISO 1600, the average RAW value would be 3200, and you might have a few brighter spots clipping past 4095. -- John P Sheehy |
#48
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wrote:
Jeremy Nixon wrote: No, there was; if the meter indicates one stop underexposure, but you can increase three stops and end up with a proper exposure, What is "proper exposure"? The exposure you want is the proper exposure. In this case, +2 from the meter reading. then the meter was actually off by two stops. No, the meter is dumb, and it puts the mid-key scene at middle grey, which is not *optimum*, unless your goal is to use a JPEG as-is, or print directly from the camera. In terms of RAW capture, it is a relatively poor digitization and/or exposure. Right. The meter is off by two stops from the exposure you want. You're comparing "trust the meter" at ISO 200 with "don't trust the meter" at ISO 800, and that's not a useful comparison, because if you trusted the meter in the first situation you'd trust it in the second and still underexpose by two stops, but if you weren't going to trust the meter in the first place (and thus be willing to go up two more stops from what it said at ISO 800) then you'd have been willing to do so at ISO 200, too, if you could -- leaving the choices being either underexpose at 200 or expose properly at 800. I clearly stated that we were a stop under because of *NEEDED* f-stop and shutter speed! You're going to blur the image now? No. Using the needed f-stop and shutter speed, at ISO 200 you are underexposing, and at 800 you are not. You're suggesting that at ISO 200 you are blindly trusting the meter, but once you go to 800 you suddenly aren't and are willing to use +2 EC from what it's telling you to get a good exposure. In real life, if someone is willing to use +2 EC to get a good exposure at ISO 800, then they were willing to do so at ISO 200, and the only reason they didn't is because the shutter speed would end up too low. So, the conclusion is that there isn't enough light for ISO 200, and the shot would be underexposed. Since it's better to boost the ISO than to underexpose, you boost the ISO. The properly exposed shot at ISO 800 will be of better technical quality than the underexposed one at 200. However, the properly exposed shot at 800 will have *more* noise than the properly exposed shot at ISO 200; it's just that you couldn't get the shot at 200. The usual thought process here, from the photographer's perspective, would be: good exposure is at +2 EC. I can't get there with shutter speed because it'll be too slow to hand-hold; I can't get there with aperture because I need the depth of field; so I get there with ISO. Let me try another way. Let's pretend the camera is greyscale (no CFA), to simplify matters. The camera meters a grey card (or the ~18% grey scene) as if to wind up with an average RAW value of 400 (fairly typical) out of about 4000 possible levels. At ISO 100, you fall a stop short of that with your needed f-stop and shutter speed, so the RAW output would be 200. Using the same aperture and shutter speed (as needed), with ISO 800, the average now is a RAW value of 1600. If you used ISO 1600, the average RAW value would be 3200, and you might have a few brighter spots clipping past 4095. Right. At ISO 100, you're drastically underexposing. At 200, you're still underexposing. At 800, you're not. At 1600, you're overexposing, unless you intended for those bright spots to clip (spectral reflections, point light sources, etc). -- Jeremy | |
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