Sony A7RII ISO Invariance

[AlterEgo]

A7R2, tricks with (unswitchable) NR @ 1sec, 4sec, etc = http://blog.lexa.ru/2015/11/20/sony_a7r_ii_temnovoy_shum_v_zavisimosti_ot_vyderzhki_i_chuvstvitelnosti.html

use translate google com (this blog is of rawdigger/fastrawviewer code developer, who works in team with IB).

[Ray]

I think some of you guys have missed the point. (Not naming anyone, because I'm so polite.  )

ISO invariance is mainly useful for those who shoot in manual mode, selecting shutter speed and aperture, visible in the viewfinder, and taking note of the exposure scale, also visible in the OVF.

It is often described, by fanatical purists, that ETTR applies only to base ISO, where full-well capacity can be achieved.
In relation to this point of view, when it is obvious that the combination of chosen shutter speed and aperture will result in a higher-than-base ISO, the ISO-Invariance camera allows one to ignore the time-consuming procedures of determining  the correct ISO for a 'pseudo' ETTR exposure.

If the variance is huge, like 5 stops, then there might be some advantage in raising ISO. If we're talking about one, or two, or even three stops, then the best practice, with a 'so-called' ISO-Invariant camera, is simply to underexpose, in accordance with choice of shutter speed and aperture at base ISO, or even higher than base ISO, if that's what the camera is already set at.

It's quicker and it might enable one to get the shot, rather than miss it as a result of fussing around with regard to ETTR in relation to one's chosen ISO setting. It removes the fastidious and time-consuming concern about ETTR in relation to a chosen ISO

I always shoot in manual mode. I'm in charge. 

[AlterEgo]

It's quicker and it might enable one to get the shot, rather than miss it as a result of fussing around with regard to ETTR in relation to one's chosen ISO setting. It removes the fastidious and time-consuming concern about ETTR in relation to a chosen ISO

when you have a decent drop in noise in deep shadows with ISO variant camera then you simply put your camera in matrix metering and auto ISO and then use your M mode... much better than fixed ISO with M mode with ISO invariant camera in real life when you don't have time to patiently meter the scene... so __IF__ DR @ base ISO is the same, ISO variant camera simply better - period

[Guillermo Luijk]

ISO Invariance is certainly something to be wished for.   However, many cameras are iso-invariant when the ISO amplification overcomes the read/downstream noise of the sensor system.   For example, my Canon 5D-III and 1D-IV are virtually iso-invariant at/over ISO 1600, and "usefully iso-invariant" at/over ISO 800.  Testing this in a practical sense is trivial.  My testing comparison is approximate based on "observing" the point where there is less  that 1/3 stop improvement  in visual S/N.  If anyone is interested, I can show the results of my 5D-III tests.

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

[AlterEgo]

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

exactly as I noted above (if DR @ base ISO...)

[Ray]

when you have a decent drop in noise in deep shadows with ISO variant camera then you simply put your camera in matrix metering and auto ISO and then use your M mode... much better than fixed ISO with M mode with ISO invariant camera in real life when you don't have time to patiently meter the scene... so __IF__ DR @ base ISO is the same, ISO variant camera simply better - period

Wouldn't that procedure result in the risk of blown highlights?

[AlterEgo]

Wouldn't that procedure result in the risk of blown highlights?

where was the last time you saw a camera overexposing with it's automatic metering ? on the contrary - they underexpose all the time ... so matrix metering is the safe bet.

[Stefan12345]

Saying that ISO invariance is better than ISO variance is like saying that a 2.8 lens is better than a 4.0 lens. We all know that some 4.0 lenses are better than some 2.8 lenses.
But everything else equal, ISO invariance is preferable above ISO variance because it offers the opportunity to adjust the ISO after the shot has been taken.

[AlterEgo]

But everything else equal, ISO invariance is preferable above ISO variance because it offers the opportunity to adjust the ISO after the shot has been taken.

you can't change the readout noise after the shot /I mean when you have raw file already/ (which is the only point in gain change for ISO variant sensors before exposure starts)... so if you on purpose (whatever it is) or forced (by the subject - for example it is moving) undersaturate the sensor you are better off with an option to trade the unused sensel capacity for reduced readout noise...

and no you are not "changing ISO" after the shot - you simply multiply the data (with potentially lower S/N, baked in that data forever, than was possible)...

so any ISO variant sensor with equal DR @ base ISO to your ISO invariant sensor is logically always better = the only /and real/ opposite case is when ISO variant sensor (like Canon's) are not offering same DR @ low gains -> so there you beat Canon's there... but it is not because they are ISO variant... their ISO variantness is also a result of the real reason why they have lower DR @ low gains.

[Stefan12345]

Please note that I said "everything else equal", which includes noise at any ISO setting.

[AlterEgo]

Please note that I said "everything else equal", which includes noise at any ISO setting.

you can't have that by defintion... because the point of ISO variant sensor is exactly in the lower readout noise :-) ... the DR drops by less than a stop when you go stop higher in gain just exactly because readout noise drops... and with ISO invariant sensor the noise does not drop - otherwise it is no longer invariant.

[tongelsing]

 I'm a theatre-photographer and for me Iso-invariance is a blessing, but I don't push it further then maximal 3 stops.
Most of the theatre-photography you do in manual because you cannot trust your meters anymore because of the vastly black backgrounds and the changing intensity of the lighting.
So if I shoot manually close to normal ranges there is always the danger of blowing out the highlights.  I measure once at very close distance an actors face and the turn my exposure value two steps down. I push camera monitor almost up to maximal and then I photograph the performance without any risk of overexposure.
Later in LR I will make the necessary corrections.

I works very well for me and I use this method for over a year now.

Working with a nikon D4s
Ton

[AlterEgo]

I'm a theater photographer and for me Iso-invariance is a blessing, but I don't push it further then maximal 3 stops.
Most of the theatre photography you do in manual because you cannot trust your meters anymore because of the vastly black backgrounds  and the changing intensity of the lighting.
So if I shoot manually close to normal ranges there is always the danger of blowing out the highlights.  I measure once at very close distance an actors face and the turn my exposure value two steps down. I push camera monitor almost up to maximal and then I photograph the performance without any risk of overexposure.
Later in LR I will make the necessary corrections.

I works very wel for me and I use this method for over a year now.
Ton

you simply miss the point that if you think that matrix metering will overexpose (they don't unless you have ancient camera with like a dozen segments - or you are saving specular highlights) then you can simply lock your gain (ISO) on ISO variant camera with the same effect as you have now  ... just think a little bit - you are not losing anything with ISO variant camera because at any fixed ISO it is has either equal (at base) or less (further up from base) noise (readout) vs ISO invariant one and you can use ISO variant camera as ISO invariant but not vice versa... try to move your mind from Canon's sensors for a while.

[Stefan12345]

No, you can't. If you use an ISO variant camera at ISO 400 and later on, you discover that you should have used ISO 800 (with the same exposure), you're out of luck: you'll never get the quality that you would have gotten if you used ISO 800 in the first place.
With an ISO invariant camera, you are able to develop the photo as ISO 800 and get the same quality.

[AlterEgo]

With an ISO invariant camera, you are able to develop the photo as ISO 800 and get the same quality.

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 ... 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

[BJL]

By all means, preserve the highlights you're interested in preserving, but you're better off using the analog amplifiers to whatever extent you can than you are amplifying in the digital domain, because, quantization noise.

I see that you have qualified this comment later, but let me describe what I see as the trade-offs, and a sweet spot between "constant ISO speed; fix the levels in post" and "pseudo-ETTR by increasing the ISO speed". (*)

At low levels of analogue gain, noise from the ADC ("quantization noise") might be a significant part of all shadow noise, so that increased analogue gain helps.  But beyond some point (maybe around ISO 800 with many recent non-Canon sensors?), the noise in the analogue amplified signal is well above the ADC's noise contribution so that further analogue gain does not really help, and to much of it can hurt, for example by causing clipping of some highlights that one's metering overlooked.  I refer to highlights that did not overfill the photosites, but whose signal is then amplified into clipping.

The practical conclusion for me is that I will normally set the ISO speed to get the desired levels in the standard JPEG conversion, but in some low light situations where "proper exposure" requires an elevated ISO speed, but there is also a risk of amplifier clipping from highlights, the safe strategy with an "ISO invariant" sensor is to err on the side of a bit of underexpose.  Thus, I no longer see much point to the "high ISO speed pseudo-ETTR" strategy of raising the ISO speed with careful metering with the goal of placing the brightest highlights at raw levels just a hair below maximum.  (There is still a place for choosing exposure time and sensor exposure levels to maximize photon counts without overfilling too many photosites.)


(*) I put that "pseudo" in "pseudo-ETTR" because increasing the ISO speed with the same exposure time etc. does not increase exposure, as in the amount of light gathered by the sensor.  Some might call this "purist" or "extremist"; I just call it using words correctly.

[David Horn]

Sony a7RII ISO Invariance, should you underexpose, or use high ISO?
My answer: Best option is neither. Use as low an ISO as possible coupled with the use of a tripod, a wide aperture, and a long exposure.
When that is not possible, then use a higher ISO and “expose to the right”.
Never underexpose, unless to accommodate highlights.
Why?
Camera pixels have a characteristic called the “Full Well Capacity”, i.e. the maximum number of electrons the pixel can hold without saturating or “blooming” (spilling electrons out to adjacent pixels). See
http://www.clarkvision.com/articles/digital.sensor.performance.summary/
for an excellent explanation.
I do not know exactly how the analog-to-digital conversion (ADC) is implemented in the Sony A7RII, but I have designed sensor interfaces in CCD cameras and I suspect the general principles are the same. That is, the analog voltage from each pixel, that is proportional to the number of electrons in that pixel after exposure, is fed to an “Analog Front End (AFE)”. The first function in the AFE is a “Sample and Hold”. That is followed by a Variable Gain Amplifier, which is followed by an Analog-to-Digital-Converter. The gain (amount of amplification) of the variable Gain Amplifier corresponds to the ISO setting. For a weak signal (low light) it amplifies the signal a lot to match the voltage range of the ADC. For a strong signal (strong light), the gain is set to a low value (low ISO) so that the ADC is not overloaded.
What will happen if you deliberately underexpose?
The gain of the Variable Gain Amplifier will be set to a low value, and the voltage going to the ADC will be very low.
For a 5-stop underexposure, you will only be using 1/32 of the range of the ADC.
In other words, you will only be using 9 bits of a 14-bit A to D converter, and you will be multiplying the Quantizing Noise by a factor of 32.
Compare Fig #2 withe Fig #4 in Michael’s article. To my eyes this extra noise in Fig #4 is very obvious.

[earlybird]

For a 5-stop underexposure, you will only be using 1/32 of the range of the ADC.
In other words, you will only be using 9 bits of a 14-bit A to D converter,

I can't figure out why so many people have adopted the phrase "dynamic range" to describe "low noise shadow recovery". It seems to me that more often than not, one desires to recover shadows after making a relatively low dynamic range exposure.

When ever I encounter the phrase dynamic range I think of dynamic range and so I find the recent discussions about "dynamic range" confusing.

It seems simpler to call shadow recovery, "shadow recovery" so that people can focus on and discuss the differences in the ADC systems.

[BJL]

When that is not possible, then use a higher ISO and “expose to the right”.
Never underexpose, unless to accommodate highlights.
Why?
. . . Camera pixels have a characteristic called the “Full Well Capacity”,
. . .
For a 5-stop underexposure, you will only be using 1/32 of the range of the ADC.
In other words, you will only be using 9 bits of a 14-bit A to D converter, and you will be multiplying the Quantizing Noise by a factor of 32.
Compare Fig #2 withe Fig #4 in Michael’s article. To my eyes this extra noise in Fig #4 is very obvious.

I disagree about "ETTR at high ISO" or the reasons stated above: once the ISO speed is much beyond the "unity gain" level where noise from the sensor is amplified above the nose level forth ADC, further amplification just increases the risk of blowing highlights by amplified clipping, while adding nothing to the image quality in the rest of the image, because shot noise and sensor noise are dominant over "ADC noise".

Yes fives stop of underexposure leaves a 14-bit ADC using only the bottom 9 bits of its range – but those are enough to cover all the actual information from the sensor anyway in situations like Michael's example where you would have to push the ISO more than 5 stops above minimum, and specifically more than fives stops above "unity gain" level.  Because then all the photosites are underexposed by more than 5 stops (getting less than 1/32 of their full well capacity) so the signal from the sensor has less than 9 stop of DR: any extra amplification just adds less significant bits of pure noise at the bottom of the useful signal, with only at most the 9 most significant bits containing any useful information: this does not improve signal quality, and if taken too far, increases the risk of clipping.

As has been pointed out, Michael's comparison photos are off by one stop: ISO 640 pushed by 5 stops should be compared to ISO 204800 (not 102400) to be comparing equal exposure level, meaning equal amount of light reaching the photosites, as with equal exposure time and aperture.

[earlybird]

ISO 640 pushed by 5 stops should be compared to...

It was my impression that the example of ISO 640 pushed by 5 stops was meant to be compared to the "properly" exposed ISO 640 example. The recently popularized notion of "invariance" seems related to a practice of leaving the ISO alone while shooting with a priority for both shutter and aperture regardless of resulting exposure.

I inferred that the higher ISO shot was included as an after thought, or as a casual comparison.