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Messages - sarangiman

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196
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 12, 2012, 02:02:34 PM »
jrista:
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One of the things I personally really want to know is whether analog gain, either at the pixel or via ADC or something else, is done for every ISO setting. The whole push/pull third-stop approach is rather annoying, as it always chops off/shifts at lest 1/3rd stop DR at those ISO settings.

It's kind of stupid that on the 5D2 you can set ISO steps at 1-stop increments, thereby never selecting any of those intermediate ISOs, but Auto ISO will still use the intermediate ISOs. Seems to me like it'd be helpful if Auto ISO also considered your setting of ISO increments. Along this train of thought, I wish Canon/Nikon/etc. would allow us full access to the hardware to modify the firmware ourselves... kind of like writing apps on iOS... would open up a world of possibilities for photographers with special interests. But I doubt we'll ever see that happen.

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given that there appears to be practically no fixed patter noise in 5D III sample images, and at worst minor vertical banding noise...I'd venture a guess that it eliminates fixed pattern noise.

I don't know about that... please see my previous post. That particular model of the 5D III has as bad or worse vertical banding than my 5D II...

197
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 11, 2012, 06:53:30 PM »
I forget who posted this .CR2 file (I think it was either here or on dpreview), so, sorry I'm missing credit for it, but, vertical banding is pretty poor. This is +100 Fill Light, ISO 100 (it's a crop, not full image):



That just looks like FPN that shoulda been removed in internal camera processing...

That's really going to limit usability of shadows in high dynamic range landscapes (& yes, I do shoot w/ Singh-Ray filters... but sometimes it's either not practical b/c of you're, say, shooting a cityscape without a simple horizon, or when even those filters aren't enough... e.g. when shooting long telephoto where effects of grad ND filters are minimal or, say, when shooting w/ the moon in the frame).

198
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 11:36:15 PM »
jrista:
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Do you mean "charge reading" or "conversion of charge to a digital unit"?

Not sure what happened to your post, but I'm answering anyway... I mean during 'charge reading'. If it's just during 'conversion of charge to digital unit', then LTRLI's statement "part of the read out noise stage that doesn't come into play much at high ISO is not so hot so it shows up a lot at low ISO and you get diminishing returns compared to what you'd expect looking at the high iso numbers as you go to near base ISO since you get worse read performance at lower ISOs" wouldn't make much sense (if I understood what he was trying to say correctly).

I may be totally wrong.

But I'm basing my notion from this comment in the chapter on CMOS Image Sensors in "Image Sensors & Signal Processing for Digital Still Cameras" (assuming these are active, not passive, pixel sensors): "The active pixel concept in which a photogenerated charge is amplified in a pixel and the amplified signal is read out has its roots in the phototransistor array image sensor. One advantage of the active pixel is its suppression of noise generated and/or injected in the signal readout path..." (pg. 145).

Then again, no idea if this 'amplification' is the one you can set by ISO... someone more knowledgeable will have to chime in.

199
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 08:47:23 PM »
neuroanatomist, or anyone: Though I asked this previously, I think it got lost in all the back-and-forth -- is 'gain' at the pixel-level applied during charge reading? I.e. does the ISO setting determine the 'conversion gain' (µV/e-)? Meaning, if you have a very low signal, is increasing this gain advantageous b/c it will help offset the read noise to be injected & b/c of the limited precision of reading?

200
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 07:25:21 PM »
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It means that SNR would be better up in the realm where photon noise dominated read noise. But it does relatively little, in comparison, for the performance in the deep shadows compared to the read noise. So it can be that they have the same SNR in the deepest shadows near black but that one has better SNR say at semi-dark gray and brighter.


Ah, all you're saying is that the absolute difference in SNR will be less for low signals vs. high, as is clear in this simple linear plot here of the SNRs for QE 1 vs QE of 0.5, assuming a read noise of 6e-:



This always begged the question in my mind: if you increase QE dramatically for a sensor without increasing full well capacity, don't you run the risk of blowing out the sensor for any given f-ratio & exposure combination compared to an older sensor with lower QE & same full well capacity? I realize that the increased QE sensor would have a lower gain for any f-ratio/exposure combination, but blown out is blown out... this always confused me.

Thanks.

201
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 05:45:36 PM »
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I think they say records and captures because the read electronics can mess up what the sensor itself captured. I believe it has been shown that the 1D4 sensor, for instance, actually captures close to 14 stops but then the readout loses a few stops and that is what gets recorded in the RAW file and it's only 11.5 stops instead of almost 14.

I'm not sure I'm explaining myself properly. My concern is that by just looking at the RAW file, are you really measuring the number of stops recorded in the actual scene that was being shot? Seems to me that to be able to do that you'd have to have a wide range of subjects with different luminosities (i.e. the multiple ND filter set up that DXO says it uses), measure those luminosities using a reliable spot meter, then look at the RAW data and see the range of luminosities for which the camera is able to:

  • Give a reasonable SNR for the darkest luminosity recorded
  • Differentiate one luminosity vs. the next on the higher (brightest) end

By just looking at the RAW file, with no actual scene information, the only way the test would be valid would be if there was a directly linear relationship between the scene luminosity & the luminosity recorded in the RAW. Which your statement about the 14 stops going to 11.5 itself seems to show isn't the case.

202
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 05:18:45 PM »
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part of the read out noise stage that doesn't come into play much at high ISO is not so hot so it shows up a lot at low ISO and you get diminishing returns compared to what you'd expect looking at the high iso numbers as you go to near base ISO since you get worse read performance at lower ISOs

LTRLI: Does the amplification based on the ISO setting occur before charge read out (before noise injection due to read)?

203
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 05:54:32 AM »
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High ISO measurements often miss out on quantum efficiency and can be misleading due to that.

Why is that (mechanistically)?

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A test that I would like to see is mk2 and mk3 shooting the exact same test scene perfectly ETTR, and then push a shadow area (preferably containing a color checker) 3 stops and show the crops side by side

Why can't you do that with the RAW files posted for both cameras on Imaging Resource right now yourself? There's a Color Checker in those shots.

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If this chart is true and not a result of some miscalculation, that would indicate that the D800 applies some sort of artificial curve to balance the shadows and highlights. An aggressive HTP sort of thing.

HTP purposefully underexposes the shots (physically), & then applies a non-linear curve. If the methodology used in generating that graph for the D800 is similar to the method LTRLI used, then any sort of non-linear manipulation done after the exposure would not change the resulting calculated DR (b/c the end points of any non-linear curve would likely still be the same as for the linear curve, since there's no reason to change the absolute values of saturation or black). So I don't think that explains it.

204
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 02:16:21 AM »
I went back to the book 'Image Sensors and Signal Processing for Digital Still Cameras' and it speaks of the 'optical black' (OB) pixels around the active pixels that are necessary to determine a proper black level. I'm assuming that, by whatever mechanism, they have no light gathering capability (b/c, as neuroanatomist mentioned, light from the lens can fall on those pixels).

So, if those OB pixels are used to determine the black level, they may also reflect the noise generated by the read event & subsequent electronics. It is, however, curious that they're around 1024 for the 5D II & 2048 for the 5D III. That's highly suspicious... I'd love to know what Canon is doing there. Isn't that # roughly what the black level is set to? If so, I don't think you'd want to be setting the black level to ~2048... seems to me that'd lower DR... unless the blackest pixel in the Active Pixel area really has a lowest level of 2048 (seems highly unlikely).

I have to go back to dcraw & see what it sets as default black level for my 5D II.

Still, my other problem with this methodology is that recorded signal in the RAW file does not necessarily reflect input signal until someone shows this relationship is completely linear... at least that's what I think.

205
EOS Bodies / Re: 5D3 same max dynamic range as the 5D2???
« on: March 10, 2012, 12:05:34 AM »
Heck I gave you all +1's! I think all the arguing back & forth is great, as there is a lot to learn/understand! As long as we're civil about it :)

I have a # of questions, especially for LTRLI, neuroanatomist, & jrista:

(1) If Canon claims quantum efficiency is up due to better microlens design, and if initial tests show better ISO performance for the MkIII vs the MkII with RAW files (i.e. no influence of noise reduction, hopefully), then doesn't all that mean that SNR is necessarily higher? And doesn't higher SNR necessarily mean greater DR, for the same general sensor design (assuming nothing actually got *worse* btwn the MkIII & MkII)? Am I missing something here?

(2) Let's say those masked pixels really are sealed/blocked off from light OR are somehow turned off. However, let's say they're still being read by the electronics & so they can still be an indication of dark noise. Then, you use:

Equation 1:     DR = log(base 2) [highest signal/stdev]

But for that calculated DR to be indicative of the max possible DR of the scene being recorded -- doesn't that presuppose a linear relationship between incoming light & recorded signal in the RAW file? Do we know this to be true?

(3) Related to question (2)... Has anyone actually established that Equation 1 is a standard, accepted, measure of DR? DXO says "Dynamic range is defined as the ratio between the highest and lowest gray luminance a sensor can capture"... note it doesn't say "... a sensor records". Again, assuming a linear relationship between incoming light & signal recorded (which is not true, for example, for film) in the RAW file, I guess this'd be the same. But but from DXO's testing methodology description (http://www.dxomark.com/index.php/About/In-depth-measurements/DxOMark-testing-protocols/Noise-dynamic-range), I thought they were using actual light source/ND filter combinations to measure the SNR & DR.

(4) I must say I find it strange that in DXO's methodology in the link above, they state that they use a setup "in order to test across a dynamic range of 4 density steps (= 13.3 f-stops — a dynamic range much greater than today’s digital cameras)"... but then report that the Nikon D7000 has 13.9 stops of DR. Is this just an outdated description?

Thanks in advance!

206
epsiloneri,

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[With 16-bit ADC] On the other hand, you could stop worrying about ISO altogether (essentially set it in post if you wished - same thing as "changing brightness levels" in post that we already do now).

Right, b/c with higher bit-rate ADCs & higher full-well capacities, you're more likely that for a full well, you can still get 1e- = 1DU.

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Ideally, you don't gain anything by going beyond unity gain, because if you have sufficient numerical precision to count every single electron, then that's enough. The reason you can still gain somewhat by using higher ISO is that A/D converters aren't ideal and cannot count electrons exactly. E.g. 5 electrons might be measured as 5.613756732 electrons. If you only have precision for integer number of electrons, that would measure to the equivalent of 6 electrons, but with some additional precision you might determine that there are 5.6 electrons worth of charge, which is closer to the true 5 electrons.

Ah, I see. So in this case, treating 1/2e- as 1DU would mean the 5e- would be treated as 10DU, and any error in that measurement (noise?) would be better masked by the larger signal (5.6e- x 2 = 11.2e- = 11DU vs. 5.6e- = 6DU --> in post if you double the exposure you then get 12DU). And 11DU, in this case, is more accurate... so there's some advantage to 2x unity gain ISO. Am I thinking about that right?

I wonder where along this pathway noise is added as well... if noise is added before multiplication (gain), then the benefit of higher ISO is less (i.e. in the 11DU vs 12DU example I posted above, the 0.6e- noise was added prior to A/D conversion... presumably during the read event) than if there are still sources of noise after gain & A/D conversion (where a larger signal would be less affected by noise, relatively).

207
Also, then, if the 5D2 had a 16-bit ADC, then with the 60,000 well capacity, unity gain would correspond to ISO 100?

And just out of curiosity -- if you had a full well capacity of ~120,000, and the same 14-bit ADC, and therefore used 8e- per DU (ISO 100), you'd still get a cleaner image than the 5D2 with 60,000 well capacity, just b/c of increased DNR, yes? Though, of course ideally you'd want a higher ADC.

Which makes me wonder why Canon doesn't use 16-bit ADCs with sensors that have full well capacity of ~60,000?

208
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. I.e., if the full well is 60000 and you have 14 bits, using a gain of 4 electrons per DU gives you optimal S/N (and DR) for well-exposed images (since 4*2^14 is about 60000).

epsiloneri: First of all, very nice, intuitive, explanation!

I have a question: I know there are different methods to determine ISO of a sensor, but in your example above, for that system where a gain of 4 e- per DU gives you optimal SNR, is that defined as the 'base ISO'? Since you don't want to fill the wells anymore, you shouldn't really be allowed to go to a lower ISO (which'd cause the metering algorithm to increase the exposure).

You mention that for the 5D2, ISO800 is still useful for quantization, even though ISO400 is unity gain. But aren't higher ISOs still useful IF you're really really photon starved? For example, if after an exposure your fullest well only has 2000 e- on the 5D2, wouldn't you benefit from 1/8 e- being a DU (ISO 3200), thereby making that 'fullest pixel' translate to a DU of 16,000? Because then you have more bits to represent the data (the whole philosophy behind 'expose to the right')?

Thanks!

209
EOS Bodies / Re: The (un)official I'm switching to Nikon thread
« on: March 01, 2012, 04:50:07 PM »
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Breaking news - 5DII AF forces tog to move to Nikon...

Not really... I could've just as well upgraded to a 1D series Canon body. But chose to wait for the revamp of the lines first. Which is why today is so exciting :)

210
EOS Bodies / Re: The (un)official I'm switching to Nikon thread
« on: March 01, 2012, 03:29:34 PM »
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I actually only took one single photo above 1.4, for more DOF I think the 70-200 is the way to go indeed. Sometimes a certain love cannot be explained....

Sometimes I like to go smaller than f/1.4 just to ensure I nail focus on off-center compositions, b/c the 5D Mark II AF severely underperforms. And in those cases, I'm stuck with non-circular OOF highlights.

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As the OP said, CA at 1.2 is a huge issue. I'd stop down to 1.4 or 1.8 to try and avoid it. but that's my 2cents.

Actually, I've found CA to persist & soften a subject on the side of the frame so much that I have to go to f/2 to equivalent sharpness compared to the Sigma 85/1.4 at f/1.4 (which, to be fair, is more like f/1.6 on the Canon lens).

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