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Forum: General Photography
08-29-2018, 01:09 PM
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Norm, nobody said that ISO invariance is something useful for photographers or even that they should care about it.
ISO invariance is only relevant to this thread to show that ISO, as it is implemented in modern cameras, isn't a physical property of the sensor itself but mostly the result of some electronic treatment and amplification of the signal. So, to answer the original question of this thread, manufacturers could launch cameras with much lower ISO than 100, if they wanted to, with the sensors they actually have on hand. To do this, they only have to modify the treatment of the data generated by the sensor. But to do so, they would also have to sacrify the highest available ISO, because sensor can't capture an infinite amount light. Thus, they don't do because people overwhelmingly prefer good performance at high ISO than being able to use extra long shutter speed in good light (which they can already achieve with a ND filter)... Or, said otherwise, not much people would buy a K-1 with an usable ISO range of 2 to 100, even if Pentax could tweak its sensor to do so.
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Forum: General Photography
08-29-2018, 08:43 AM
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Believe it or not, that's the case, if the sensor is ISO invariant or close enough to it. You can find plenty of articles on this or just try it yourself.
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Forum: General Photography
08-29-2018, 08:28 AM
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On paper, it works both ways: you could just as well overexpose a picture and pulling it back to "normal" exposure level. However, in the real world, sensor has a limit on how much light they can capture. And, as we all know (or should know!), overexposing a picture increase the risk of clipping the highlights. Thus, yes,in practice we usually talk of ISO invariance for pushing up the ISO from an underexpose picture and not the other way.
---------- Post added 08-29-18 at 11:40 AM ----------
No Norm, it's not an hoax. Sure, the same picture taken at different ISO at the same exposure will have different DR. But ISO invariance rather talk about two pictures, at different ISO, but over/underexposed by the same amount of stops. Or, said otherwise, for a same amount of light the DR and noise will be the same (although exposure will be wrong).
In practice, ISO invariance just means that if the aperture and shutter speed stay the same, the resulting image will have the same DR and noise level when brought back to the same exposure in PP.
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Forum: General Photography
08-29-2018, 07:24 AM
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I think that "base ISO" means the sensitivity giving the best signal/noise ratio. That said, with ISO-invariant modern sensors, it's more or less irrelevant. I guess that now the lowest selectable ISO is more a decision of the manufacturer than a technical limitation of the sensor. Although there's certainly is a point under which the sensor isn't ISO invariant anymore, it's almost certainly much lower than ISO100...
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