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 p.4 #16 · Utility of dynamic range beyond a certain point | |
racetratr wrote:
Thanks for your suggestion.
I kind of understand the theory. I cap ISO in some situations for that reason, in better light. But would it be practical in the case I mentioned?
I often have to shoot at, say, 1/1000 sec and f1.8 on the street at night. I manually set those parameters--they are kind of inflexible. I set auto ISO for up to 6400 as the third part of the exposure triangle. I often have to push in post a bit anyhow, since 6400 may not be enough.
Looking at the A7RV's sensor measurements, changing from ISO 6400 to 800 would seem to add slightly more than two stops of dynamic range (not 3).
But meanwhile, my biggest problem is holding the shadows, not DR in the abstract. Doesn't increasing the ISO past 800 increase gain or sensitivity in the shadows?
If it is it true that I would lose nothing in the deep shadows if I set auto ISO to 800 instead of 6400 in the dark, I would reset my viewfinder, do it, and gain DR. But that doesn't match my experience.
Always ready to learn, tho!
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The "exposure triangle" is an incorrect construct. Only two settings directly decide the exposure: the shutter speed and the aperture. The ISO has no direct effect on the exposure; neither the sensitivity of the sensor changes when changing the ISO.
In practical terms (that is ignoring the question on what the ISO actually does in camera), the ISO is an indicator. In Auto-ISO, seeing one stop change in ISO, e.g., from 100 to 200, indicates that the exposure decreased by 1 stop.
A detailed good article debunking the "the fetishistic “exposure triangle,” which, as makes sense with fetishes, is not just unnecessary and not just incorrect – it is, in point of fact, extremely harmful" is
The Unbearable Lightness of Mystic "Exposure" Triangle https://share.google/tQcbXDELqRTsw2POb
In your case, when the aperture is already at f/1.8, the only way to improve the dynamic range, that is the signal-to-noise ratio, is by increasing the signal by using a slower than 1/1000 shutter speed. I don't know your reasons for using this fast shutter speed, but it is this setting that you may want to decrease, to 1/500, then more, to get as close as possible to the shutter speed that should give acceptable results most times.
(Edit: perhaps this is obvious, nevertheless, while the photographic dynamic range is presented as a function of ISO, this dynamic range cannot be changed by changing the ISO directly, only by changing the exposure by changing the shutter speed and aperture. In practical terms, the only reason to manually change the ISO is when one suspects that the gain was excessive and resulted in ADC overflow - but this is rare, I believe. For sensors with "dual gain", of course, it helps to know the ISO when it kicks in, but this is a special point.)
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