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Author Topic: Sony A7RII ISO Invariance  (Read 18093 times)

amolitor

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Re: Sony A7RII ISO Invariance
« Reply #60 on: November 21, 2015, 11:13:36 am »

Photographers do this a lot. They think that if one quantity is bigger than another, then the other one kind of goes away.

We see the same fallacy in sharpness discussions.


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tony field

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Re: Sony A7RII ISO Invariance
« Reply #61 on: November 21, 2015, 02:19:50 pm »

I disagree. ISO invariance is not desirable, having a large dynamic range sensor is.

The problem with Canon sensors is not that they are not ISO invariant, but the fact they have a relatively poor dynamic range at base ISO.

Should Canon sensors begin at ISO100 having the same DR as Sony sensors, anyone would prefer the Canon behaviour (holding nearly the same DR at ISO200 or even ISO400 as at ISO100).

Regards
In a practical sense, "Iso Invariant" means that, after the sensel charge is read, there is no further significant noise contribution to the pixel.  You can alter the effect of ISO amplification and see no additional noise.  This allows the maximum dynamic range to be achieved --- remember that DR is a signal to noise relationship.  ISO Invariance implies maximum dynamic range for that sensor.  Of course, you want a sensor that has the lowest possible sensel read noise and ISO invariance together.

The Canon (like my 5D-III) is "Iso Variant" because, after the pixel is read (which is very low noise on Canon sensors - among the best available), additional downstream noise is created in the sensor processing system.   The implication is that some amplification of the pixel values can be done in such away that it raises the signal above the down-stream added noise.  In a simplistic sense, if the pixel voltage is one volt and the down-stream noise is 2 volts, you can, for example, amplify the pixel above the noise to 3 volts by setting an ISO value.  This means that the actual signal can be detected cleanly since it well above the noise.  Unfortunately overall, this also means that the base ISO DR of the Canon is not very good - but turns out to be very good ISO 1600 and higher.
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BJL

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Photographers do this a lot. They think that if one quantity is bigger than another, then the other one kind of goes away.

We see the same fallacy in sharpness discussions.

Indeed, but one should also avoid the opposite mistake of pursuing perfection in one respect (like minimizing quantization noise) at the cost of ignoring faults in other respects (the greater imperfections of higher levels of analog gain, like amplifier nonlinearity and clipping).  I have perhaps given the wrong impression that I support the dogma that "more that unity gain is pointless", so let me clarify:

At some ISO speed setting (seemingly in the range 400 to 800 with many good current sensors) the analog gain brings the noise level entering the ADC up to the noise level then caused by quantization in the ADC: I believe that this is what some call "unity gain".  Contrary to the above dogma, I see that it can still help to amplify a few stops beyond that, so that the noise floor in the incoming analog signal is at about two or four times the quantization noise, but beyond about that, quantization noise is insignificant, and reducing it further with further analog gain is of no practical value.  Indeed, digital gain in raw conversion becomes a better approach: it is perfectly linear, and one can back-off from clipping – which I suppose is then more accurately called "arithmetic overflow".

To quantify: describing the ADC quantization noise as of level 1 (a 1 bit error?), if the noise in the input signal is amplified to be two stops higher it is at level 4. Then the combined effect in the deepest shadows is the RMS combination sqrt(1^2 + 4^2) = 4.12, so raising the deep shadow noise floor and lowering the total engineering dynamic range by 0.04 stops compared to what one would get by avoiding ADC quantization noise effects entirely.  This I would call measurable but in practice negligible.

Raising the ISO speed by one more stop, to three stops over unity gain, the noise floor in the amplified analog signal is at eight times the quantization noise floor, and the quantization noise raises that total noise from 8 to 8.06, a change of 0.01 stops, so a tiny 0.03 stops better than the previous "two stops over unity gain" case.  If the cost of this 0.03 stop improvement in the deep shadows is the risk of clipping the highlights one stop earlier . . .

P. S. In all but the deepest shadows, photon shot noise overwhelms these other noise sources anyway.


TL;DR  Pardon all the numbers: my point is that these numbers are negligible, and we should not obsess about pushing up the analog ISO speed gain in pursuit of perfect ETTR histogram placement in low light situations!  Good modern sensors often let us use any ISO speed in a safe range that avoids the problems at either extreme, so we can think more about composition instead of worrying about absolutely perfect light metering and settings choices.
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tony field

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...
At some ISO speed setting (seemingly in the range 400 to 800 with many good current sensors) the analog gain brings the noise level entering the ADC up to the noise level then caused by quantization in the ADC: I believe that this is what some call "unity gain".  Contrary to the above dogma, I see that it can still help to amplify a few stops beyond that, so that the noise floor in the incoming analog signal is at about two or four times the quantization noise, but beyond about that, quantization noise is insignificant, and reducing it further with further analog gain is of no practical value.  Indeed, digital gain in raw conversion becomes a better approach: it is perfectly linear, and one can back-off from clipping – which I suppose is then more accurately called "arithmetic overflow".
...
TL;DR  Pardon all the numbers: my point is that these numbers are negligible, and we should not obsess about pushing up the analog ISO speed gain in pursuit of perfect ETTR histogram placement in low light situations!  Good modern sensors often let us use any ISO speed in a safe range that avoids the problems at either extreme, so we can think more about composition instead of worrying about absolutely perfect light metering and settings choices.
My interpretation of "unity gain" (from simple reading) :

Unity gain happens at some value of the camera ISO setting when 1 electron produced in a pixel results in 1 ADU at the ADC output, that is the conversion factor is now equal to "1".  I suspect that higher than unity gain, in general, does not result in significant improvement ... other than to suppress pattern noise.  Apparently, this unity gain is the preferred operating point for astronomical photographers.

I interpret this to apply to my Canon 5D-III - at the point where the sensor system becomes virtually "iso invariant" at  ISO 1600 when simple digital scaling in ACR is "identical" to in-camera ISO scaling.
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Ray

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Re: Sony A7RII ISO Invariance
« Reply #64 on: November 21, 2015, 10:55:57 pm »

There seems to be a major point which has been overlooked in this thread. The Sony A7R2 is not an ISO-Invariant camera, according to DXOMark tests, especially with regard to DR.

For example, at ISO 100 (actually ISO 74) the DR of the A7R2 is 13.9 EV, as measured by DXO. If one underexposes by 5 stops at ISO 100, instead of increasing ISO to 3200 for a pseudo-ETTR shot, then the DR of the resulting underexposed shot will be 5 EV lower (ie. 13.9 - 5 = 8.9 V).

However, the DXOmark Dynamic Range measurements at ISO 3200 show a DR of 11.01 EV. That means that DR is only 2.89 stops down instead of the expected 5 stops down that would result if the camera were truly ISO-Invariant.

It seems clear from these results that choosing to raise ISO instead of underexposing will always produce a better dynamic range with the Sony A7R2. However, the gain in DR varies according to the starting ISO. For example, underexposing 5 stops at ISO 3200, instead of raising ISO to 102,400, will result in a DR which is worse by only 0.4 EV, approximately.

http://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D810-versus-Sony-A7R-II___963_1035
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dchew

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Re: Sony A7RII ISO Invariance
« Reply #65 on: November 22, 2015, 09:43:25 am »

However, the gain in DR varies according to the starting ISO. For example, underexposing 5 stops at ISO 3200, instead of raising ISO to 102,400, will result in a DR which is worse by only 0.4 EV, approximately.

http://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D810-versus-Sony-A7R-II___963_1035

Thank you Ray. That has been my understanding from DXO and from Jim Kasson's blogs. The a7rII is ISO invariant in steps.

Dave
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Ray

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Re: Sony A7RII ISO Invariance
« Reply #66 on: November 22, 2015, 12:03:51 pm »

Thank you Ray. That has been my understanding from DXO and from Jim Kasson's blogs. The a7rII is ISO invariant in steps.

Dave

Yes. Having checked the graph again, I would describe the A7R2 as being ISO-Invariant only for the 3 stops between ISO 3200 and ISO 25,600, and the one stop between ISO 51,200 and 102,400. For all other full-stop steps there is at least 1/3rd of an EV advantage in DR by increasing ISO just one stop instead of underexposing, and sometimes as much as a 0.5 EV advantage.
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Telecaster

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Re: Sony A7RII ISO Invariance
« Reply #67 on: November 22, 2015, 04:59:10 pm »

My takeaway: unless I'm working slowly and/or methodically with a tripod I can ignore all this stuff. In practice I've been able to get more than good results with the ISO cranked up, provided I expose enough for a pic that already looks good SOOC.

-Dave-
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Ray

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Re: Sony A7RII ISO Invariance
« Reply #68 on: November 23, 2015, 02:06:35 am »

My takeaway: unless I'm working slowly and/or methodically with a tripod I can ignore all this stuff. In practice I've been able to get more than good results with the ISO cranked up, provided I expose enough for a pic that already looks good SOOC.

-Dave-

Haven't you got that the wrong way round, Dave? If you are able to work slowly and methodically with a tripod, then  you can ignore all this stuff because you have the time to get ETTR exposures (or pseudo-ETTRs) right.

If your subject is stationary, and/or lighting good, and/or desired aperture wide, you can use base ISO for maximum DR, or bracket exposure for merging to HDR if necessary. In such circumstances, the ISO-Invariance qualities of the camera become irrelevant.
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bjanes

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Re: Sony A7RII ISO Invariance
« Reply #69 on: November 23, 2015, 01:01:17 pm »

Thank you Ray. That has been my understanding from DXO and from Jim Kasson's blogs. The a7rII is ISO invariant in steps.

That is correct. As I understand things, ISO invariance (ISO-Less) occurs when the input referred read noise (expressed in electrons) holds constant as the ISO is raised. As the chart shown below taken from Bill Claff's web site demonstrates, the Sony shows 3 discrete decrements in read noise as the ISO is raised. This is likely due to the Aptina Aptina DR-pix technology that is said to be implemented in this sensor.

The 7RMii demonstrates only minor changes in the input read noise from ISO 640 to 25600 and is nearly ISO-less. This is the range that Michael discussed in his post. I do not think DR changes of less than 0.5 EV are really significant.

The Nikon D7200 is virtually ISO-less from ISO 400 to 5100. The Canon 5DS R does not appear to be ISO-less at any ISO, but it's readnoise is competitive above ISO 1600. The Nikon D800e falls between the two extremes.

Regards,

bill
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Telecaster

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Re: Sony A7RII ISO Invariance
« Reply #70 on: November 23, 2015, 04:20:04 pm »

Haven't you got that the wrong way round, Dave? If you are able to work slowly and methodically with a tripod, then you can ignore all this stuff because you have the time to get ETTR exposures (or pseudo-ETTRs) right.

Unless for some reason I've chosen a specific ISO value I just let it float in camera, caring not a whit whether the gaining up—when it happens—is happening in the analog domain, the digital domain or some combo of the two. That's what I mean by ignoring "all this stuff."

-Dave-
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Ray

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Re: Sony A7RII ISO Invariance
« Reply #71 on: November 23, 2015, 09:48:48 pm »

Unless for some reason I've chosen a specific ISO value I just let it float in camera, caring not a whit whether the gaining up—when it happens—is happening in the analog domain, the digital domain or some combo of the two. That's what I mean by ignoring "all this stuff."

-Dave-

I see! It's been mentioned before that matrix metering with automatic ISO rarely overexposes. I must admit I haven't tried this since I began using the 'effectively' ISO-less Nikon cameras. I've assumed that an automatic ISO setting would tend to overexpose the very bright areas in a scene if those areas cover only a small proportion of the scene, since the automatic metering produces an average reading which is not necessarily ideal for any particular part of the scene.

My usual procedure is to separate focusing from metering, allocating the AF-On button to focusing, using a single focusing square, and allocating the half-pressed shutter to the exposure reading in relation to the position of the focusing square.

This set-up allows me to move the focusing square around the scene and watch how the exposure reading changes on the metering scale visible in the viewfinder. If I consider a small portion of bright, cloudy sky within the scene might be overexposed, and I want to capture the maximum detail in that small portion of the scene, I simply increase the shutter speed with my thumb on the wheel until the meter in the viewfinder looks right when the focusing square is positioned over that brightest part of the sky. I then recompose the scene and take the shot.
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tony field

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Re: Sony A7RII ISO Invariance
« Reply #72 on: November 24, 2015, 12:10:29 am »

That is correct. As I understand things, ISO invariance (ISO-Less) occurs when the input referred read noise (expressed in electrons) holds constant as the ISO is raised. As the chart shown below taken from Bill Claff's web site demonstrates, the Sony shows 3 discrete decrements in read noise as the ISO is raised. This is likely due to the Aptina Aptina DR-pix technology that is said to be implemented in this sensor.   .....

One thing to consider how "constant" is the input-referred noise.   One way is to think it is "truly constant".  The other is to put bounds of the noise change.   Bill Claff personally chooses to infer "ISO invariance" is when the noise varies by 1/3 or less EV for higher  "measured ISO".
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bjanes

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Re: Sony A7RII ISO Invariance
« Reply #73 on: November 24, 2015, 08:25:58 am »

One thing to consider how "constant" is the input-referred noise.   One way is to think it is "truly constant".  The other is to put bounds of the noise change.   Bill Claff personally chooses to infer "ISO invariance" is when the noise varies by 1/3 or less EV for higher  "measured ISO".

I haven't seen Bill's post on that subject, but it is in essential agreement with what I said. There is not much difference between <0.5 and <=0.3.

Bill Janes
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Ray

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Re: Sony A7RII ISO Invariance
« Reply #74 on: November 25, 2015, 12:35:22 am »

I haven't seen Bill's post on that subject, but it is in essential agreement with what I said. There is not much difference between <0.5 and <=0.3.

Bill Janes

Bill,
As I understand, and have observed, a difference of 0.5 EV in dynamic range is noticeable and therefore significant at a pixel-peeping level, although generally not particularly significant.

If one underexposes by 2 stops at ISO 100, with the A7R2, instead of raising ISO to 400, then according to the DXOMark tests, one will have sacrificed a full stop (or EV) of DR, which is definitely significant.

However, adopting the same procedure with the Nikon D810, underexposing by 2 stops at ISO 100 instead of raising ISO to 400, one sacrifices approximately 1/4th of an EV worth of DR, which is definitely insignificant.
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Stefan12345

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Re: Sony A7RII ISO Invariance
« Reply #75 on: November 25, 2015, 07:11:19 am »

you totally miss the point that with ISO variant camera I already have better readout noise @ ISO400 than you with ISO invariant camera by definition (of variantness) so if I need to push by a stop in raw converter I am in a better situation than you ;D... you start with the worse readout noise, I start with better - we both push by a stop... and if I am paranoid about blowing something then I can just start @ base ISO where we by definition have the same situation... so I have a choice - you don't
I stand corrected. I did not know that an ISO variant camera by definition has the same readout noise at base ISO as an ISO invariant camera.
I thought ISO invariance just meant that shooting at increased ISO delivers the same results as shooting at base ISO and then pushing the image an equivalent number of stops. (using the same exposure settings ofcourse)
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AlterEgo

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Re: Sony A7RII ISO Invariance
« Reply #76 on: November 25, 2015, 09:18:37 am »

I thought ISO invariance just meant that shooting at increased ISO delivers the same results as shooting at base ISO and then pushing the image an equivalent number of stops. (using the same exposure settings ofcourse)
you can have that either with ISO-by-tag (the true ISO-less camera, no analog anb/or digital gain - just firmware writes a tag telling raw converters to push behind the scene) or with constant readout related noise when gain is applied (of any kind - analog or whatever)... now because we consider theoretical sensors where the only difference is what happens with increase in ISO (gain) we shall assume that readout noise is the same at "base" ISO...

if we will start to consider real cameras then things get tainted because Canon sensors are not using the same technology as Sony sensors for example and that makes readout related noise not equal.
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