To improve results it would required to do calibration shot just before shooting session - similar what is done with Gray/wite/black card before session for later WB adjustement
It is not quite constant. I did some experiments merging hundreds of images (don't ask), and the areas where banding seems to occur are clustered but not constant (over periods of a few hours).
However, if your exposure is more than 1s, turn on long exposure noise reduction. On my 5DIII at least, this removes most of the banding. The downside is that the general noise level rises (which also helps hide the banding), so you will need to average several such frames to get the noise back down again.
Another thing that seems to work well with multi-exposures is for images that need contrast stretching. For example, if you have an image with very low contrast (fog), combining multiple frames (eg in PS) can give you more latitude for contrast adjustment.
In general though, if shadow noise is a problem, you are better off shooting multiple exposures, as for HDR. With a three-shot bracket, you can extend the usable dynamic range by six stops. To do the same by averaging frames you would need an awful lot more - and if the pattern noise is correlated the result will be worse.
Thanks, interesting info about banding.
Also I agree, one could use HDR to increase DR.
And I was using that occasionally (in Photomatix or PS) but did not like that much for reasons listed below.
Of course if 1Dx could do in-camera HDR similar to 5DMIII than this would be very nice.
One of the problems with HDR is that this requires careful post-processing which is time consuming.
And “time is money” – very valuable thing.
Also known problem with HDR is that though it allows to increase image DR significantly it also increases image noise which becomes more visible. So one of the practices for HDR is to do NR on each image before merging them into final HDR. This also additional time in post processing
My goal was to try to get out of 1Dx cleanest image that practically does not require any post processing . Almost 2 stops SNR improvement for shadow areas (actually 4.7 dB) is sufficient for many if not most of high DR evening/night shots. And all that with just one button press ( or two if you want to have mirror lock to reduce camera vibrations to get best possible resolution).
And if a little bit of NR applied then you can get very clean image with good details and low noise level in deep shadows.
As for long exposure noise reduction – sure - this is very useful feature.
I do not know how Canon implementing this but in my first digital camera (Sony DSC-828) it was done by subtracting pure noise shot ( sensor read with shutter closed ) from image done after that.
And that was working very well.
