About in-camera long. exp. noise reduction (1DMKIV)

C.Fn II - 1 states for the following for the "Auto" setting:

"For 1 sec. or longer exposures, noise reduction is performed
automatically if noise typical of long exposures is detected. This [Auto]
setting is effective in most cases."

I made a bulb 35/40 seconds long exposure twice, with the cap on, first with C.Fn II - 1 set to "Auto" and then to "On".

In the first exposure, the camera "decided" not to make the second "dark" exposure to subtract the noise. In the second one, surely enough, it spent another 35/40 seconds doing the subtracting operation before displaying the image.

Why aren't the stuck/dead pixels present in the "Auto" enabled exposure (2nd one) enough to trigger the noise reduction function?

What is "noise typical of long exposures" to Canon?



PK

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“Loose praise may feed my ego but constructive criticism advances my skills”
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