The truth about high ISO dynamic range - does this apply to other cameras?

erotavlas

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Hi I stumbled upon this other forum where one user explains the following

The truth about high-ISO dynamic range. Yes, really!

Does anyone know if this is how all digital cameras behave? Or just this particular Fuji camera/sensor?

A particular annoying false "fact" that keeps being repeated in such reading matter is the claim that the dynamic range of your camera and images decreases with rising ISO numbers. This is wrong, as every reader of my book already knows. Looking at my book's JPEG ISO series at Index of /XPro1/Abbildungen/ISO-Reihe - ISO Test Series (yep, you can look at this stuff for free, no need to buy anything), it's easy to compare the sample images at ISO 200 and ISO 6400. You can see that the ISO 6400 sample covers pretty much the same dynamic range as the ISO 200 samples. In other words: The ISO 6400 sample (almost) doesn't show more blown highlights or blocked shadows than the ISO 200 sample.
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Yes, I know: It's true that the dynamic range of your camera's sensor decreases with rising ISO numbers, but you don't render images with a sensor, you do that with a camera (or an external RAW conversion software like Lightroom). Your camera isn't just a sensor, most of it is image processing. This image processing takes care of keeping the practical dynamic range intact over the full ISO bandwidth. Yep, those engineers are smart, they know their stuff.
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How does the camera (or the RAW conversion software) do it? Basically, it uses the same method that is used for the DR expansion function: underexposure combined with tone-mapping. You can read all about it in my book and most of it here for free.

Here comes the really fun part: Your Fuji APS-C camera offers MORE dynamic highlight headroom at ISO 3200 and yet even MORE again at ISO 6400 than between ISO 200-1600. Really! How can that be? Isn't that....totally crazy?

It is in theory, but not practically. Here's why: Your dear Fuji camera only processes and stores real RAW data between ISOs 200 and 1600. All the other ISOs (like ISO 100, or ISOs 3200-25600) are simply mathematical divisions of ISO 200 (to get ISO 100) or multiplications of ISO 1600 (to get ISOs 3200, 6400, 12800 and 25600).

So if you happen to overexpose an image at ISO 1600, you are in a tough spot, because the camera is recording the actual overexposure in the RAW file. Highlights will be blown and be quite hard or impossible to recover. However, at ISO 6400, it's all peachy, because your "overexposure" is actually an underexposure of 2 stops – the difference between ISO 1600 (the actual RAW data) and ISO 6400 (the RAW data the camera pretends to record). This means that you can easily recover blown highlights by simply moving the exposure slider in your RAW conversion software to the left. Shooting at ISO 6400, two full stops of highlights will magically reappear this way, w/o any visible quality loss. At the -2 EV setting of Lightroom's exposure slider, your ISO 6400 image will look exactly as it would have looked if the shot was taken at ISO 1600 in the first place (using the same aperture and shutter speed). No difference at all.
 

Garbz

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This guy is on crack.

There's a reason why dynamic range numbers exist, because they are measurable and they are nearly always measured on the RAW data. That doesn't mean the sensor minus all processing, since a lot of the processing is actually pre-processing (bias adjustments, changing adc values, changing amplification etc). Dynamic range goes down as ISO goes up. End of story. Anyone claiming otherwise is overthinking it and has lost their grasp on how things actually work.

For one he is right about the base ISO bit, all cameras do that. The Nikon D800 does it as well, and ISOs above 6400 are derived from that ISO. However that's where his little reality ends. RAW images are created by the camera. They aren't really a RAW dump of the sensor data, but rather the camera's interpretation of that dump. End result you ISO12800 image looks brighter than the ISO6400 image because it is, not because of some post processing, but rather because of pre-processing. You can't darken this and recovery detail from magic data that isn't there.

The reason his samples work is because at high ISOs the camera is approximating the brightness change and doing it poorly. Have a look and you'll see that the high ISO picture is no where near as bright as the lower ISO picture. Colour and tonal accuracy has gone out of the window. It worked in his favour on his camera in this picture, but it's a silly way to go about it. If you really wanted a non-linear image, just take one at a lower ISO setting and play with some curves.
 

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