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Photo Forum / Digital Photography / DSLR Cameras / November 2006Digital camera sensor performance
I have a new page up describing digital camera sensor performance for a fair range in digital cameras and commercial sensors. Enough data has now been measured to show trends in full-well capacity, dynamic range, signal-to-noise ratios, unity gain ISO (high ISO performance), and how read noise does not follow the same trends. Included are some newly derived parameters showing low light performance. I've plotted results as functions of pixel size and there are clear trends. Some trends can be used to predict basic performance of new cameras. See: http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary Roger >I have a new page up describing digital camera sensor performance > for a fair range in digital cameras and commercial sensors. [quoted text clipped - 7 lines] > trends. > Some trends can be used to predict basic performance of new cameras. Thanks Roger ... that is very useful and well-done! >I have a new page up describing digital camera sensor performance >for a fair range in digital cameras and commercial sensors. [quoted text clipped - 11 lines] > >Roger Looks like I made the right choice when I ordered my 5D <g>  SignatureScott in Florida > I have a new page up describing digital camera sensor performance > for a fair range in digital cameras and commercial sensors. [quoted text clipped - 9 lines] > See: > http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary What are the KAI-11002... (kodak sensors) used for, old digital backs? In figure 7 they have awful low light sensitivity in spite of large pixels. Pixel pitch is the spacing, not the actual size, right? Wouldn't Full Well Capacity be the same as the actual pixel size? So in figure 1, the D50 has a small well for it's pitch, same for SNR but the unity gain in fig 6 is good for it's pitch, almost as good as the 5D. Are the Canon S60/70 much better than typical modern P&S or sort of average? Is it fair to say that pixel count effects SNR? As the grain gets smaller, it's certainly less noticeable but I don't know maybe that doesn't really count. SignaturePaul Furman http://www.edgehill.net/1 Bay Natives http://www.baynatives.com >> I have a new page up describing digital camera sensor performance >> for a fair range in digital cameras and commercial sensors. [quoted text clipped - 13 lines] > What are the KAI-11002... (kodak sensors) used for, old digital backs? > In figure 7 they have awful low light sensitivity in spite of large pixels. Paul, I am not sure what the Kodak sensors are used in. They have good sensitivity for signals above a few tens of photons. Their low value in Figure 7 is due to their high read noise, double the Nikon CCDs and about 4 time the Canon CMOS sensors. > Pixel pitch is the spacing, not the actual size, right? Correct. Pixels have an active area that is somewhat smaller than the pixel pitch squared. The active area divided by the pitch squared is the fill factor. The fill factor, or effective fill factor with the micro lenses over each pixel is usually in the 80-90% range. So pixel pitch is a pretty good measure, and usually the only parameter us consumers can get for pixel size > Wouldn't Full > Well Capacity be the same as the actual pixel size? So in figure 1, the > D50 has a small well for it's pitch, same for SNR but the unity gain in > fig 6 is good for it's pitch, almost as good as the 5D. Yes, it must have a high Quantum Efficiency. > Are the Canon S60/70 much better than typical modern P&S or sort of > average? The S60 is a couple of years old and probably pretty average. The S70 is probably also pretty average except that the read noise is about half that of other CCDs. > Is it fair to say that pixel count effects SNR? As the grain gets > smaller, it's certainly less noticeable but I don't know maybe that > doesn't really count. Yes, that is a good point, and I think it does count, at least to some degree. After all, film has very low SNR, but with many grain clumps, like pixels, the finer they are, the less noticeable they become. But as you try and make bigger enlargements, the warts show. Roger > I have a new page up describing digital camera sensor performance > for a fair range in digital cameras and commercial sensors. [quoted text clipped - 11 lines] > > Roger Roger, thanks for all the hard work you've put into this. Do you have any idea why the 5D and 20D have such high read noise in low ISOs? I imagine it will not be a pleasant experience to significantly lift the shadows in an ISO 100 image from them! Still, it dramatically decreases with increasing ISO. I would be extremely interested to find out how Canon do this (not that they'll tell us any time soon). Very impressive. >> I have a new page up describing digital camera sensor performance >> for a fair range in digital cameras and commercial sensors. [quoted text clipped - 21 lines] > extremely interested to find out how Canon do this (not that they'll > tell us any time soon). Very impressive. While cameras like the 5D, 20D, and 1D Mark II have apparently higher low ISO read noise in electrons, the image noise is low. The read noise is high because of A/D quantization and a small amount of noise in the amplifier. For example, the 20D has a read noise of only 3.6 at ISO 1600 but 25 electrons at ISO 100. But in terms of data numbers (DN) in the image, that 25.3 electrons becomes a noise of 2.0 DN. Thus, there is amazing detail in the shadows of an image. One may also have some fixed pattern noise, but that can be calibrated out. (Take the read noise, Table 4, and divide by the gain in Table 3 to get the data numbers in an image.) Roger > While cameras like the 5D, 20D, and 1D Mark II have apparently higher > low ISO read noise in electrons, the image noise is low. The read noise [quoted text clipped - 8 lines] > > Roger Yes, that's what I did. The D200, for instance, would appear from these numbers to have better shadow detail at ISO 100 (its read noise corresponds to 1.25 DN as opposed to around 2DN for both the 5D and the 20D at the same ISO; things change a lot at higher ISOs!). At any rate, read noise of this magnitude (DN 1 or 2) is not going to affect the image too much. My question, though, is if you have any idea how they manage to decrease read noise so much when ISO is increased (as unity gain is approached, judging from the compacts that you have also tested). I am not an expert in imaging systems, but this seems almost like magic to me. > Yes, that's what I did. The D200, for instance, would appear from these > numbers to have better shadow detail at ISO 100 (its read noise > corresponds to 1.25 DN as opposed to around 2DN for both the 5D and the > 20D at the same ISO; things change a lot at higher ISOs!). But fixed pattern noise can easily be larger than a DN or so. So to say which is better needs images from both cameras to be evaluated. Then note that the larger pixels of the 5D versus D200 would result in more photons per pixel for the same f/ratio lens so the 5D image would have a signal/noise advantage boosted by about 2x. > At any rate, read noise of this magnitude (DN 1 or 2) is not going to > affect the image too much. [quoted text clipped - 3 lines] > judging from the compacts that you have also tested). I am not an expert > in imaging systems, but this seems almost like magic to me. I have heard something about double correlated reads, but have not been able to confirm that is what Canon actually does. Another speculation is something about the reset cycle reducing fixed noise and minimizing electrons in the capacitor on each pixel to reduce the read noise. But without a technical paper from Canon, one can't be sure. I have not found a Canon technical paper, and perhaps it is because Canon recognizes they have an advantage and want to keep it to themselves. Roger >At any rate, read noise of this magnitude (DN 1 or 2) is not going to >affect the image too much. [quoted text clipped - 3 lines] >judging from the compacts that you have also tested). I am not an expert >in imaging systems, but this seems almost like magic to me. One trick might be to improve readout accuracy by pre-biasing the photosites with a charge proportional to the ISO setting, then subtracting the bias value in software after conversion. Now that I think about it, that might also reduce the effect of thermal noise. >>At any rate, read noise of this magnitude (DN 1 or 2) is not going to >>affect the image too much. [quoted text clipped - 9 lines] > Now that I think about it, that might also reduce the effect of > thermal noise. I see. That would decrease the max number of electrons that can be held before dielectric breakdown, no? So decrease max well capacity. It's like throwing away your shadow range, isn't it? Except you just push it up, throwing away highlight range instead. Do I have this right? But can this be done for CCDs? (I have no clue, I am not an engineer; it's a question). >>> At any rate, read noise of this magnitude (DN 1 or 2) is not going to >>> affect the image too much. [quoted text clipped - 18 lines] > But can this be done for CCDs? (I have no clue, I am not an engineer; > it's a question). It wouldn't reduce thermal noise, though (I think). >>At any rate, read noise of this magnitude (DN 1 or 2) is not going to >>affect the image too much. [quoted text clipped - 9 lines] > Now that I think about it, that might also reduce the effect of > thermal noise. That would add noise. Any signal has an uncertainty equal to the square root of the number of electrons. Roger > > One trick might be to improve readout accuracy by pre-biasing the > > photosites with a charge proportional to the ISO setting, then [quoted text clipped - 4 lines] > That would add noise. Any signal has an uncertainty equal > to the square root of the number of electrons. I think his idea is that low electron counts are not counted accurately, so let's avoid them. If that's the case, it would work by avoiding low electron counts being read. I have no clue if it is so. I don't know if what he says works, but I am pretty sure he did not mean it as a way to avoid shot noise (indeed, it will add its own noise, as you say). >> > One trick might be to improve readout accuracy by pre-biasing the >> > photosites with a charge proportional to the ISO setting, then [quoted text clipped - 8 lines] >accurately, so let's avoid them. If that's the case, it would work by >avoiding low electron counts being read. Exactly. > I have no clue if it is so. I have no hard evidence that it would either, I'm merely making an educated guess - albeit one based on my technical experience with data-aquisition design (which I'll cheerfully admit is dated, but still broadly valid), & my subjective experience with visible noise in images captured with Canon CMOS image sensors. >I don't know if what he says works, but I am pretty sure he did not >mean it as a way to avoid shot noise (indeed, it will add its own >noise, as you say). Correct. I'm talking about a tradeoff off (virtual) well-depth for amp noise at very low signal levels. I believe the concept is reasonably sound in theory, but its usefulness in practice would depend a lot on the noise & thermal characteristics of the particular sensor & read amps. That said, don't forget that DLSR image sensors (& their amps!) have to work at temperatures much higher than optimum, so thermal noise is very likely one of the mjor design issues, & anything that helps with that would be valuable. >>>>One trick might be to improve readout accuracy by pre-biasing the >>>>photosites with a charge proportional to the ISO setting, then [quoted text clipped - 10 lines] > > Exactly. I just saw this post! OK, this makes sense then. Ignore my other post. >>I have no clue if it is so. > [quoted text clipped - 16 lines] > noise is very likely one of the mjor design issues, & anything that > helps with that would be valuable. >>>At any rate, read noise of this magnitude (DN 1 or 2) is not going to >>>affect the image too much. [quoted text clipped - 12 lines] >That would add noise. Any signal has an uncertainty equal >to the square root of the number of electrons. Of course, but if it adds *less* noise than the extra noise you get from the extra amplification required to bump up the effective ISO of the sensor, then it's a net gain. I don't have the data required for a rigourous analysis, (or any realistic way of obtaining it, short of being hired by Canon for an R&D role), but my gut instinct is that it would help a lot - particularly in reducing the influence of thermal noise (eg; coloured hot pixels on Bayer sensors) on low-light images. > Of course, but if it adds *less* noise than the extra noise you get > from the extra amplification required to bump up the effective ISO of [quoted text clipped - 3 lines] > would help a lot - particularly in reducing the influence of thermal > noise (eg; coloured hot pixels on Bayer sensors) on low-light images. But increasing the ISO does not have the same effect as adding a constant to the measured value. Also, even if you could add this value and subtract it without any noise, the only advantage you will get will be to avoid any problems related to quantising small charges. This is what I thought you meant. You will not avoid problems such as a small actual signal being drowned out by noise (since the signal is still small). If you meant something else, I can't think what it can be (and still have a chance of being right). Can you explain? >See: >http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary Fascinating analysis, Roger, as usual. :) Thanks again for all the time & effort you put into these reports, I for one greatly appreciate them. |
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