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Author Topic: DxO Sensor Mark  (Read 17944 times)
BartvanderWolf
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« Reply #60 on: February 13, 2011, 07:53:13 AM »
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If that means taking an arbitrary photo and measuring the noise, you will get stuck unless the subject is well defined (patches, test chart): as you seem to suspect you cannot distinguish between signal and noise if you can't accuractly predict the signal. So it sounds like a dead end for automated/objective testing. And taking a photo of a well defined subject (patches, test chart) is what DxO is doing.

One can go a long way by subtracting two images, which is of course easier with stationary subjects. Simple subtraction of images with equal exposure times will result in random noise (Standard Deviation needs to be divided by Sqrt(2) when 2 images are used) and elimination of signal and pattern noise. This is the better way of determining random noise levels in test patches. I don't know if DxO uses that to reduce the fixed pattern noise e.g. caused by amplifier variations.

Cheers,
Bart
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Peter van den Hamer
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« Reply #61 on: February 13, 2011, 01:07:36 PM »
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This is the better way of determining random noise levels in test patches. I don't know if DxO uses that to reduce the fixed pattern noise e.g. caused by amplifier variations.

DxO said that their noise figures include fixed pattern noise. Your approach would, as you indicate, not see the fixed pattern noise. Fixed pattern noise should be reported as it can be substantial part of the noise at high ISO (at least in the 5D2 generation cameras - not sure about the K-5 generation).

What I expect DxO does is generate a smoothed polynomial describing the spatial intensity of the light reflected off the patch.
An then subsract this background level from each pixel to determine noise. This compensates for low-frequency gradients (non-uniformity due to lighting, filters, vignetting) which are not perceived as noise by observers. In fact, humans hardly see such low-frequency gradients at all. And this technique would measure high-frequency (temporal and fixed) variations which are perceived as noise.

The statement by DxO that FPN is included in their noise figures is consistent with their protocol which mentions that it is important that the test targets are cleaned (dusted?) regularly. Dust on the test target would be measured as noise. It is also consistent with their avoidance of printed targets which tend to have some degree of noise.

Peter
« Last Edit: February 18, 2011, 06:12:29 AM by Peter van den Hamer » Logged
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