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Author Topic: Link between high ISO noise and DR?  (Read 8824 times)
Guillermo Luijk
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« Reply #40 on: October 28, 2008, 09:33:23 AM »
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Quote from: Ray
That's a good point. I'd forgotten that the A900 doesn't have 14 bit encoding like some of its competitors. One might also wonder how a camera with 12 bit encoding can deliver greater DR than another camera, such as the 1Ds3, which has 14 bit encoding.

Nevertheless, the fact that 12.6 stops might be a bit of an exaggeration is not significant in paractical terms, provided the DR of the other cameras with which the A900 is compared, is exaggerated to the same degree.
Hi Ray, the problem is not that they give a 12-bit camera a higher DR than a 14-bit camera. In fact, that's perfectly possible since DR is not bitdepth-limited but noise-limited, and the 14-bit camera could have a worse SNR than the 12-bit.

The problem is that a 12-bit linear encoding can _NEVER_ encode (I am not talking about noise here, just about encoding) more than 12 f-stops of DR. That's why anyone looking at a table where a 12-bit camera is given more than 12 f-stops of DR should not think of any exageration, but simply of wrong information.

Ideally the following levels would be encoded in a 12-bit RAW file where the whole range 0..4095 were used:

    0EV: 2048 levels, 2048..4095
   -1EV: 1024 levels, 1024..2047
   -2EV: 512 levels, 512..1023
   -3EV: 256 levels, 256..511
   -4EV: 128 levels, 128..255
   -5EV: 64 levels, 64..127
   -6EV: 32 levels, 32..63
   -7EV: 16 levels, 16..31
   -8EV: 8 levels, 8..15
   -9EV: 4 levels, 4..7
   -10EV: 2 levels, 2..3
   -11EV: 1 level, 1

There is no room for more than 12 f-stops, and in any case the lowest f-stops would be poorly represented (1 level, 2 levels, 4 levels,...)

BR
« Last Edit: October 28, 2008, 09:35:08 AM by GLuijk » Logged

Tony Beach
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« Reply #41 on: October 28, 2008, 10:38:13 AM »
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Quote from: GLuijk
The problem is that a 12-bit linear encoding can _NEVER_ encode (I am not talking about noise here, just about encoding) more than 12 f-stops of DR. That's why anyone looking at a table where a 12-bit camera is given more than 12 f-stops of DR should not think of any exageration, but simply of wrong information.

What if the data is not encoded linearly?
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Guillermo Luijk
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« Reply #42 on: October 28, 2008, 10:50:01 AM »
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Quote from: Tony Beach
What if the data is not encoded linearly?
Then it is possible (like in the M8), but the A900 uses a linear encoding as far as I know.
« Last Edit: October 28, 2008, 10:50:40 AM by GLuijk » Logged

Ray
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« Reply #43 on: October 28, 2008, 11:02:41 AM »
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Quote from: GLuijk
The problem is that a 12-bit linear encoding can _NEVER_ encode (I am not talking about noise here, just about encoding) more than 12 f-stops of DR. That's why anyone looking at a table where a 12-bit camera is given more than 12 f-stops of DR should not think of any exageration, but simply of wrong information.

Well, there's nothing new there, regarding wrong information. It happens all the time  . I imagine not only the lowest stop would be poorly represented but the highest stop also.

Since useful DR is to some extent a subjective factor, then I suggest the precise figure doesn't matter. Even if you use a program like Rawnalize to ensure that absolutely nothing is clipped in the highlights, you still have to determine what level of image degradation in the shadows is acceptable and constitutes another stop of DR.


What I find significant and interesting is not that the DR of the A900 is 12.6 stop or 11.6 stops or whatever, but that it is claimed to be 0.8 stops better than the 1Ds3 at base ISO. It is the accuracy of this figure which should be questioned. The DR test of 1ds3 RAW images would have been carried out a few months ago with a previous version of ACR, possibly by a different person and possibly with different adjustments in ACR in the attempt to get the best outcome.

I'm suggesting that it's this procedure of not carrying out simultaneous tests which represents a flaw in the methodology. Nevertheless, it would be better if we could have an international standard for DR measurement, like the ISO standard for exposure.

I'm thinking of buying a Panasonic 50" plasma display which boasts a contrast ratio of 1,000,000:1. I wonder what method was used to calculate that  .
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Guillermo Luijk
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« Reply #44 on: October 28, 2008, 11:41:05 AM »
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Quote from: Ray
I'm thinking of buying a Panasonic 50" plasma display which boasts a contrast ratio of 1,000,000:1. I wonder what method was used to calculate that  .

That's nearly 20 f-stops and sounds quite exagerated. But you can check it with your camera. Just make 2 shots, one with a completely white display (i.e. maximum luminosity) and second with a black display (i.e. minimum luminosity, which in plasma and LCD is never pure black) in a completely dark room, making sure to obtain a good exposure in both cases. By comparing the resulting RAW values, and after correcting them by the difference in the exposure settings, you can trust your sensor's linearity to get the figure of contrast.

I did something similar on my windows, which are of that kind of glass that is seen as a mirror when looking from outside so I can walk naked without any neighbour noticing me . And I found out they eliminate 2/3 of the incident light!!. I deeply regret now having paid for such kind of windows since my place is not the best lighted one could expect.
« Last Edit: October 28, 2008, 11:41:40 AM by GLuijk » Logged

Panopeeper
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« Reply #45 on: October 28, 2008, 02:12:32 PM »
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My observations are:

1. the A900 raw is nonlinear (due to the lossy compression); perhaps it *can* create lossless raw files as well?

2. The noise at ISO 200 is about 0.4 EV better than the 1DsMkIII. However, it is possible that the A900 applies some smoothing to the raw data. The images I have DO show some degradation of very fine noise compared to the 1DsMkIII despite the higher resolution of the A900, but I would not state firmly, that a noise reduction took place. I guess Emil could make a Furier analysis in order to clarify this.
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Gabor
ejmartin
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« Reply #46 on: October 28, 2008, 02:25:34 PM »
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Quote from: Panopeeper
My observations are:

1. the A900 raw is nonlinear (due to the lossy compression); perhaps it *can* create lossless raw files as well?

2. The noise at ISO 200 is about 0.4 EV better than the 1DsMkIII. However, it is possible that the A900 applies some smoothing to the raw data. The images I have DO show some degradation of very fine noise compared to the 1DsMkIII despite the higher resolution of the A900, but I would not state firmly, that a noise reduction took place. I guess Emil could make a Furier analysis in order to clarify this.


It would depend on the nature of the compression.  I wasn't aware that the A900 used lossy compression; are you saying the raw files don't use lossless compression algorithms?  If so, what are they doing?  That would certainly affect the analysis of raw data.  

I can believe that the A900 is better than the 1Ds3 at low ISO; the D300 is better than the 40D at low ISO.  This may be due to the column-parallel ADC's on the Sony CMOS sensor, which allows slower  data rates and correspondingly lower noise.  At high ISO, though, the amplifier/ADC noise is irrelevant, and the photosite readout noise dominates; there Canon is the leader in low noise designs.
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emil
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« Reply #47 on: October 28, 2008, 04:17:15 PM »
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Quote from: ejmartin
I wasn't aware that the A900 used lossy compression; are you saying the raw files don't use lossless compression algorithms?  If so, what are they doing?
Both the A700 and A900 offer the selection betwen 12bit, lossless data and a lossy, non-linear version. I did not know this from the A900 when I wrote my previous message; I just found one A900 lossless file. I have not implemented the decompression, I don't know the algorythm. I am converting the lossy Sony files in DNG for analysis.

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That would certainly affect the analysis of raw data

I'm afraid it would; see the attached histograms. However, the lossless files (if you find some) should be suitable. The one I found is with ISO 1000, and it looks like manipulated for BOTH smoothing and to achieve this ISO from something else (see the attachment), thus it is probably not suitable for a Furier analysis.

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Gabor
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