Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 55,644
Re: Understanding ISO

Great Bustard wrote:

I agree that the theory must correspond to reality. However, as I said, banding is a separate issue from read noise. So, taking banding out of the equation, what is bad about read noise that is not also bad about photon noise?

That much is quite straight forward. Photon noise is less as the signal is less, while read noise is there whatever the signal level. So, with zero read noise a sensor has an infinite dynamic range, whatever the signal level. That dynamic range applies even if the signal level is very low (very low exposures). However, the result of very low exposures is that the brighter parts will still appear noisy, even if the darker parts have no noise (with in between parts having in between noise). Essentially it's like printing the image through a random texture mask, which is not unattractive. Read noise adds a layer of random grey (or worse, mixed colours) over that, which messes up the shadows and leads to a very low SNR.

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