Tech info: Dynamic Range of DSLRs

[jbeale]

For the technically inclined, Emil Martinec has a very technical and in-depth, but well-illustrated and readable discussion of sensor dynamic range here:
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/

This is about DSLRs, but I suspect it is quite relevant to RED as well. He has quite a bit of data for various DSLRs, including the current top models. He has an interesting idea about dual ADC readouts of the sensor (eg. simultaneously ISO 100 and ISO 1600, then merged) to increase total delivered dynamic range. I think some Redusers may have previously suggested this also (?) He mentions no production DSLR uses this technique currently, but I'll bet Red is planning something along these lines, if they don't already.

"This windowing of DR is occurring because the camera doesn't deliver the full DR that the sensor is capable of at base ISO; rather it is limited by the rest of the electronics -- the ISO amplifier and ADC. The [Canon 1D mark 3] sensor has a DR of about 14 stops, while the amp/ADC have a bit less than 12 stops DR. There are about two stops of extra dynamic range to be had in DSLR's that is currently being thrown away by limitations of components other than the sensor. This is true of both Canons' and Nikons' CMOS DSLR's."
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3a.html

 

[NateWeaver]

The [Canon 1D mark 3] sensor has a DR of about 14 stops, while the amp/ADC have a bit less than 12 stops DR. There are about two stops of extra dynamic range to be had in DSLR's that is currently being thrown away by limitations of components other than the sensor.

I wonder if this is why modifying the ISO of the Red by amplifier was rejected in favor of doing it later in the chain?

I always wondered about that...

Graeme?

 

[GlennChan]

I recall Graeme saying that there was less noise if you increased gain digitally instead of analog.

 

[Daniel Browning]

For the technically inclined, Emil Martinec has a very technical and in-depth, but well-illustrated and readable discussion of sensor dynamic range here:
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/

Emil's essay contains assertions that run counter to the prevailing wisdom of all the most respected popular photography web sites and forums, such as dpreview.com, luminous landscape, and even reduser. For example, he asserts:

• High pixel densities do not have worse noise, SNR, DR.
• Almost all 14-bit ADC are a total waste.
• Quantization error is insignificant.
• 100% crop is a fundamentally flawed measuring tool.

Fortunately, unlike the prevailing "wisdom", Emil's assertions are based on facts, logic, and repeatable measurements. No surprise; he's a theoretical physicist that helped get string theory started.

I highly recommend his essay and only hope that some day the myths it debunks will fall into obscurity.

 

[Daniel Browning]

He has an interesting idea about dual ADC readouts of the sensor (eg. simultaneously ISO 100 and ISO 1600, then merged) to increase total delivered dynamic range. I think some Redusers may have previously suggested this also (?) He mentions no production DSLR uses this technique currently, but I'll bet Red is planning something along these lines, if they don't already.

Emil discussed his idea with Eric Fossum, inventor of the CMOS APS; Eric said "been there, done that, got the patent":

US Patent 6734905 - Dynamic range extension for CMOS

RED did test the sensor with higher analog gain and did not see a reduction in read noise. Maybe RED has already implemented the improvement, or maybe it's a design trade off.

 

[Pawel Achtel]

One interesting thing in the article is that higher end DSLRs implement base gain settings as hardware amplification. This is different as software multiplication (effectively metadata) approach used in extended ISO and also used in Red One.

This approach shows great benefit of the read noise for ISO 100 right up to some point around ISO 1600 to 3200. This can be seen on read noise vs. ISO graphs.

...Setting the highest analog ISO amplification keeps the headroom, and one can always do as much additional software amplification as is needed afterward during raw conversion....

I think this is the key to improvment of the DR at range of ISO settings that currently Red One does not offer as it uses software multiplication approach irrespective of ISO settings (metadata).

My bet is that both EPIC and DSMC will use the analogue hardware amplification approach as this is clearly the best way to provide high dynamic range at variety of ISO settings as opposed to a single ISO rating at ISO 320.

However, such approach requires hardware changes, so it is unlikely to be offered in Red One as an upgrade. In my (speculative) view this is going to be a major (if not the main and most significant) architectural difference between Red One and Epic.

The analogue hardware amplification method would result in greatly extended ISO range with only minimal DR loss and no additional data overhead. So, it is "no brainer" to me.

The double readout approach (effectively HDR) would additionally (and substantially) increase DR at any ISO settings but, at this stage, it is less obvious whether such approach will be implemented by Red Team. Such approach would require much higher quantization or/and introduction of log (or other non linear) conversion before writing to RAW. Going from 12 bits to 16+bits would certainly add data overhead and pose challanges in compression, bandwidth and storage. Introduction of gamma curves pre-RAW quantization would be appealing as lower bit depth can be used. However, this would add complications in providing transfer curves to the camera.

Certainly interesting reading and food for thought. Thanks for posting the links!

 

[Daniel Browning]

One interesting thing in the article is that higher end DSLRs implement base gain settings as hardware amplification. This is different as software multiplication (effectively metadata) approach used in extended ISO and also used in Red One.

Extended ISO on all Canon and Nikon DSLR is not "effectively metadata" because it's applied to the RAW before storage. The one exception is the Canon 10D ISO 1600, which was hardware 800 with a +1 push in metadata. That means all Canon DSLR cameras throw away a stop of dynamic range for every ISO setting over 1600 (ISO 25,600 loses *four* stops of dynamic range!) with no benefit in reduced read noise.

Of course, it would be very simple for Canon to implement software amplification and metadata. In fact, they could release a firmware, right now, for dozens of their cameras, boosting dynamic range by one stop per ISO setting over 1600. Heck, I would write it for them. But that's one more "feature" they can sell some day in the future.

 

[Pawel Achtel]

He has an interesting idea about dual ADC readouts of the sensor (eg. simultaneously ISO 100 and ISO 1600, then merged) to increase total delivered dynamic range. I think some Redusers may have previously suggested this also (?) He mentions no production DSLR uses this technique currently, but I'll bet Red is planning something along these lines, if they don't already.

What he proposes is slightly different than what was proposed here. He suggests of applying two different analogue hardware gains and merging them. What was suggested here was to use different exposure times and merge the readouts.

The first approach is easier and "artefact free" and I quite like it. But, it is also somewhat less effective than the second approach. The second approach gives better result and higher DR, but timing issues can cause temporal artefacts, like uneven motion blur for shadows and highlights.

If I was Red I would be taking to the guy about that patent. I wish I thought of it earlier :clown2:

 

[Pawel Achtel]

Extended ISO on all Canon and Nikon DSLR is not "effectively metadata" because it's applied to the RAW before storage.

Yes, but the effect is the same: you do not get any extra information out of it as opposed to analogue hardware gain.

Whether you multiply before RAW or after makes no difference as long as you do not clip values. In fact multiplying after RAW can result in better precision of the multiplication.

 

[J. Bernard Vallon]

• High pixel densities do not have worse noise, SNR, DR.
  
• Almost all 14-bit ADC are a total waste.

• 
  
  I believe graeme has said both of these things before. 14 bits is only worth it if you genuinely have greater than 12 stops of DR, few chips can handle that.

 

[Graeme Nattress]

Yeah, there's not much point pushing the AtoD bit depth much beyond what the sensor is capable of in terms of DR. That's when I have to laugh when Wikipedia tells me the Dalsa records 16bits

Graeme

 

[Daniel Browning]

Extended ISO on all Canon and Nikon DSLR is not "effectively metadata" because it's applied to the RAW before storage.

Yes, but the effect is the same: you do not get any extra information out of it as opposed to analogue hardware gain.

I disagree.

Applying a digital gain to the RAW always throws away highlights, so it should never be done. Applying analog gain to the RAW also throws away highlights, but if it has less read noise, then it makes sense.

Most cameras (most CCD DSLR, digicam, and MFDB) do not reduce read noise with analog gain, so they should never use it. Some of them, such as most MFDB, some digicams, and RED are smart about this and just use metadata. Other manufacturers are not so smart and just throw away dynamic range for no reason.

Some cameras, such as Canon DSLR, are different: they have less read noise at 400, 800, and 1600 ISO gain. So it makes sense to use analog gain for those ISO settings. However, 3200, 6400, 12,800, and 25,600 should *not* be applied to RAW before recording, because they have the same exact read noise as ISO 1600, but 1, 2, 3, or 4 stops less dynamic range.

Whether you multiply before RAW or after makes no difference as long as you do not clip values.

Impossible. It will always clip values and prevent you from using the full dynamic range that the camera is capable of.

 

[Pawel Achtel]

Yeah, there's not much point pushing the AtoD bit depth much beyond what the sensor is capable of in terms of DR. That's when I have to laugh when Wikipedia tells me the Dalsa records 16bits

Graeme

Totally agree, Graeme. And they store uncompressed - so wasteful. No wonder they need petrabytes of storage for an average feature film. And, for wildlife DALSA is completely useless.

However, if the next implementation of Red/Epic sensor can deliver 14 stops DR, then 16-bit ADC and RedCode would be fully justified, in fact needed.

Either that or you may need to implement transfer curves (knee) to squeeze full sensor's DR into less bits.

 

[Joseph Ward]

Wish there can be a Red test with uncompressed Raw vs RedCode Raw?

 

[Radoslav Karapetkov]

Isn't something like the future Fuji SR sensor the way to go?

Pixel coupling and stuff...

I wonder if the current RED sensors can do something similar, at the expense of resolution.

 

[Graeme Nattress]

I don't know. Fuji trade resolution for DR. I'd prefer to have both....

Graeme

 

[Pawel Achtel]

I disagree.

Applying a digital gain to the RAW always throws away highlights, so it should never be done. Applying analog gain to the RAW also throws away highlights, but if it has less read noise, then it makes sense.

What I was referring to was that from certain ISO, like 1600 the read noise starts affecting the effectivnes of applying analogue gain and at some point there is no benefit of doing so. Compression artefacts aside, at that point there is also no benefit of applying digital gain as this can be done just as effectively in post processing. Hence introduction of "extended" gain settings in Canon DSLRs. Where they screwed up is that there should not have been 12-bit quantization applied for a sensor that can deliver 14 stops. That's suboptimal to say the least. They should have used at least 15+ bit quantization and they could have dispensed with ISO settings, analogue (and digital) gain altogether.

This is because, if quantization step is sufficiently small in comparison to read noise, then analogue gain does not need to be applied at all and all DR of the sensor can be transferred without loss. This is my understanting how Red implemented the ADC transfer. Thus, the ISO is simply metadata and all is good and simple. :biggrin:

What we need next is 14 stop capable sensor and 15-16 bit ADC to read it and that would be the nail in the coffin for film medium :gun:

 

[Pawel Achtel]

Wish there can be a Red test with uncompressed Raw vs RedCode Raw?

Why? RedCode does not seem to have visible artefacts like MPEG does and it is basically visually lossles to my eye :blink:

It also does not seem to affect resolution or dynamic range, so a comparision of uncompressed and compressed would be purely academic.

 

[Dj Joofa]

For the technically inclined, Emil Martinec has a very technical and in-depth, but well-illustrated and readable discussion of sensor dynamic range here:
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/

Emil Martinec has good information, however, unfortunately like many others he also does not appreciate that there is a difference between noise reduction when a number of images are averaged together and the noise reduction resulting when image is resized.

Perhaps the easiest to way to see that is the mean square error in such estimation is given as:

error = (bias).^2 + (variance of noise)

When you average images of the same scene to get a cleaner image, then though image intensity is varying at each pixel, however, the average is in the time domain and each pixel can be considered to have a true value being estimated, though it is different for each pixel, of course. In this case, since average is an unbiased estimator, the bias is zero, and the variance of the noise decreases with an increase in the number of images being averaged, therefore SNR increases as a measure of the square root of the number of images.

However, when image is resized by averaging neighboring pixels, the average is in the spatial domain. Even in the absence of noise, the signal is not constant in neighboring pixels (unfortunately Emil chose a degenerate example of adding noise to constant image on his website), therefore, the bias term is not zero. Though noise variance goes down by averaging a larger number of pixels, on the other hand the bias increases as a square.

There is an optimal number of pixels to be averaged to get best SNR, however, that varies at each pixel position, implying at each pixel position a different number of pixels must be averaged together to get the same max SNR at each location!. The reason that happens is the differential rate of change of signal and noise determine the optimal number of pixels to be averaged together and these parameters change at each location.

 

[jbeale]

My reading of paragraph (a) at the top of this page
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p4.html
is that Emil draws a clear distinction between temporal and spatial averaging. I wasn't able to find anywhere he confused them.