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Old Feb 10, 2012, 9:31 AM   #11
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Thanks corkypix. So could you achieve the same result by using say 1000 1/200s exposures instead of one 5s exposure and then combining them in-camera?
Yes. Stacking exposures will increase signal to noise ratio and therefore DR.
This technique is often used in astrophotography. As you know, you can
greatly increase the DR by using a different exposure for each frame. I know
very little about the subject, but I know you have considerable expertise in
this area. I look forward to seeing your results.

The basic principle is that noise is more or less random in nature
so that it tends to average out over successive frames. The wanted
signal which in this case is the image data. Is not random or at least
it is less random so that it tends to be constant from frame to frame.
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Old Feb 10, 2012, 11:04 AM   #12
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OK, here goes. I set my camera to take 100 individual 1/200s exposures each of which *just* preserved the highlights. If I'd done one 0.5s exposure it would have been totally ruined. So I have 100 shots like this:



I used jpeg, not raw, so noise in the deep shadows is horrendous as you'd expect. Next step is to add them up in such a way that i don't introduce clipping. I was going to write something to do it but then realised that my AutoHDR would do almost the same thing as it converts 8-bit jpegs into 16-bit and then averages them so I just used that to align and merge all 100 exposures (after resizing them because this is a lot of data). That gives a 16-bit image that looks just like a single exposure but has great shadow detail even though the display can't handle it. THIS is equivalent to what I want from the camera. Using say 14-bit raw, 2 exposures would give me 15-bit, 4 exposures would give me 16-bit and so on. By combining 100 jpegs I've got a 16-bit image from a lot of 8-bit ones. Using RAW and a few tweaks to the code I could have turned 14-bit RAW into a 20-bit image.

Finally I tonemapped it which is what we already do to get the extended range to display. It's a bit grungy because I was working on reduced resolution and pushed it hard to make a point but you could get something much more natural. This is basically creating an HDR with improved shadow detail but using multiple exposures at the same setting instead of using different settings.



Here's a 300% crop of the handle of the cabinet in the corner showing the difference in detail using 1 vs 100 frames. The bit that amazes me is that the colour information is recovered as well.




So the process would be:

1/ Set a 0.5s exposure because you want motion blur say
2/ The camera determines that at its native ISO it can only manage 1/200s before the highlights are blown so it takes 100 1/200s exposures instead which add up to 0.5s as requested
3/ The 100 14-bit images are then added together to create say a 20-bit image which contains a far greater dynamic range.
4/ Now, you can either:
a/ just divide every pixel in the 20-bit image by whatever it takes so that the maximum value is scaled to white. This is like deciding the ISO after the shot.
b/ You tonemap the full 20-bits to give an HDR image.
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Old Feb 10, 2012, 11:22 AM   #13
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I'm not sure about the sensor 'photon buckets' getting full. If I take a 1600ISO shot they might get full (blown) but the same shot at 100ISO would look fine. That suggests that the sensor can handle the number of photons coming in because it's the same each time but that something somewhere else in the processing is struggling. Isn't it just that at 1600ISO the photon count in the highlihgts is multiplied up beyond the 12- or 14-bit limit? If that's the case though, it really just needs the camera to take the basic 100ISO shot and then pick a multiplier that doesn't clip the highlights.
The difference between the shots is the amount of amplification applied. There are many more photons actually captured in the ISO 100 shot, because it will have been taken at a slower shutter speed. The ISO 1600 shot is very underexposed, but the result is amplified to a higher level. The small variations are amplified as well, which is why the noise is higher.
You can do what you are talking about by taking the shot at ISO 100 and then using a curves adjustment to raise the levels of shadows and highlights. This is done in some cameras and called in-camera HDR or D-lighting or something else.
The sensor tests I have seen show that the actual dynamic range of the best sensors available now is an the order of 14 bits (for aps type sensors), so you really aren't going to get more than that out of them. Increasing the number of bits from the ADC will give you finer resolution of levels within that range, though.

brian

If you convert from Raw to a 16 bit format, and maintain 16 bits throughout the processing, you should be able to obtain results similar to what you show with the multiple exposures. As you mention though, most monitors won't display 16 bits (or printers, for that matter).

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Old Feb 10, 2012, 11:33 AM   #14
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The sensor tests I have seen show that the actual dynamic range of the best sensors available now is an the order of 14 bits (for aps type sensors), so you really aren't going to get more than that out of them.
But it does appear that you can get a good 15-bit image by combining 2 14-bit images and so on. I guess that's just what multi-frame noise reduction on the Sony does - it takes 6 shots I think and averages them. I'd just like it to take 6 shots and add them up instead.
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Old Feb 10, 2012, 1:30 PM   #15
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That's an encouraging result Martin. I can certainly see detail in the stacked image which I can't get out of the single shot, even if I boost the brightness and contrast.
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