“Expose to the Right” & relation to ISO Invariance

[Bart_van_der_Wolf]

@Bart... Ok, I see the confusion, and it's probably my fault.  I've been talking about the concept of exposing to the rightest.  Not necessarily at the very rightmost point.  I went down that path as it more accurately represents the scenario of the OP.  Along the way I've carried that into a more specific discussion about ETTR.  But as you say, ETTR is more specifically about exposing right up to the clipping point.  So if you manage that at base ISO, then there is obviously nothing to be gained (and a lot to be lost) by raising ISO.

Apologies for the mixup.

Hi Bernie, no problem. Glad it got clarified (at least for us two )
And, more importantly, we were both Right in a way. Oops

Unfortunately we need to get specific about the semantics at times, to avoid talking at cross purposes.

Cheers,
Bart

[digitaldog]

Hi Andrew, thanks for taking the time to explain. Assuming these were Raw captures, the brightness of the converted ISO 800 shot should have been much brighter, so I assume that the brightness was made equal in Lightroom with a.o.  the Exposure slider. Is that correct?

To use Michael’s terms, there were, like all ETTR work 'normalized' so yes.

Sure. Since the actual exposure was identical, either the ISO 100 shot needs to be brightened (which will only show the shot-noise with a large percentage of read noise added), or the ISO 800 shot darkened.

The ISO 100 wasn’t touched at all. It is after all, according to the meter, the correct exposure.

By increasing the ISO to 800, in the Canon specifically, there is an amplification added to the captured signal before reading it out. Amplifying the signal and then adding read noise is different from adding read noise and then amplifying that. The order of things matters, in Canons.

That's not pertinent to the overall, fixed comments found here and elsewhere about ISO and ETTR.

So as useful the demonstration is, IMHO it's not about ETTR on ISO invariant cameras.

I disagree.

[bjanes]

Surely with all cameras, whether they are ISO invariant or not, one should attempt to apply ETTR at base ISO for best results, if the required shutter speed and aperture allow it.

Likewise, with all cameras, whether they are ISO invariant or not, one needs to apply the ETTR process regardless of  the ISO setting chosen, in order to get the best results, the best results being the lowest noise in shadows and mid-tones, without blowing wanted highlights.

For example, if I'm using a D7000 or D7200, which I would say are pretty close to being ISO invariant cameras, and I decide to use ISO 800, not only because I need a fast shutter speed, or because the light is not good, but also because I want to clearly see what I've shot on the camera's LCD screen, and/or share the images with others in the field, then I still need to apply the ETTR process at ISO 800 in order to achieve the lowest noise and the best review image.

In other words, any exposure which does not push the histogram to the right, whatever the chosen ISO, will result in more image noise.
Where's the confusion? The fact that one might not need to use a high ISO with an ISOless camera, if one doesn't feel the need to be able to clearly view the shots taken on the camera's LCD screen, does not mean that the ETTR principle does not apply if one does choose to raise ISO, rather than underexpose at base ISO.

However, that is not to deny that the ISOless camera has clear advantages. But that is a separate issue

Well stated Ray, but the critical restraints in this situation are the required f/stop and shutter speed. If one is shooting an event in limited light and requires a shutter speed of 1/320 sec to freeze motion and an aperture of f/8 for depth of field, one sets the camera with those parameters. Once this is done, exposure (in lux seconds) is fixed. If the histogram is to the left at base ISO, one could increase the ISO and move the histogram to the right, but this does not change the number of photons collected, but it would brighten the LCD preview. With a non-isoless camera, such as Andrew's Canon, use of a higher ISO will decrease read noise and improve the image. The decrease in read noise with that camera levels out at around ISO 1600, and a further increase in ISO will not improve the read noise, but will decrease highlight headroom and risk blown highlights. Interested readers should refer to Emil's post.

If the histogram is still to the left at ISO 1600, one could increase the ISO further and move the histogram to the right and get a brighter LCD preview at the risk of blown highlights, but there would otherwise be no improvement in image quality. True ETTR would be to increase the exposure (shutter speed, f/stop), but this is prevented by the chosen restraints on shutter speed and aperture.

With an iso-less camera, one could increase the ISO sufficiently to give a readable LCD preview but not further in order to preserve highlight headroom. I agree with Bart that ETTR is best applied at base ISO. At higher ISO one often can not, because of the above mentioned constraints, move the histogram to the right by increasing exposure. ETTR is a useful concept, but it places undue emphasis on the appearance of the histogram.
Giving maximum exposure would be a better term.

Bill

[Bart_van_der_Wolf]

Now we're getting into semantics, Bart. The word 'right' in the phrase ETTR refers to the 'right' of the histogram. The histogram, as seen on the camera's LCD screen, does not distinguish between the different states of 'full-well highlights' (ETTR at base ISO) and the amplification of 'less-than-full-well highlights' resulting from ETTR at a higher-than-base ISO.

That's right correct.

If one feels the need to distinguish between these two states with regard to optimum exposure, then one needs two different acronyms. ETTR at base ISO could become EFFWH (Expose For Full Well Highlights), and ETTR at higher-than-base ISO could become ATTR (Amplify To The Right).  

Well, assuming we want to optimize image quality (why else shoot Raw instead of OOC JPEG), it might help to distinguish between the two. Not sure if Expose to the right (ETTR) and Amplify (or adjust) to the right (ATTR) are going to help much, because they still need the clarification about the dilemma between shooting constraints an technical image quality.

Cheers,
Bart

[Bart_van_der_Wolf]

ETTR is a useful concept, but it places undue emphasis on the appearance of the histogram.
Giving maximum exposure would be a better term.

Bill, I fully agree with that (with your entire post in fact).

Cheers,
Bart

[digitaldog]

Well, assuming we want to optimize image quality (why else shoot Raw instead of OOC JPEG), it might help to distinguish between the two. Not sure if Expose to the right (ETTR) and Amplify (or adjust) to the right (ATTR) are going to help much, because they still need the clarification about the dilemma between shooting constraints an technical image quality.

Ah, I think we're reaching critical agreement!
When I simply wrote to you in my last post that I disagree, it was based on what you just wrote.

What are the benefits one expects from using an ETTR approach? I think we all agree less noise due to better S/N. The adjustment of the ISO on a Canon system provided that goal and result. So now it's semantics but that's important and I've found that such back and forth discussions here on LuLa, thanks to the level of the audience often accomplishes the goal of having well defined terms as well an explanation of the whys. Then we can attempt to perhaps persuade others to do the same.

Exposure. Is it simply the interaction of shutter and aperture? Is ETTR solely achieved by controlling those two items in an attempt to produce the best quality data? Where does ISO fit? If we agree on that terminology, what do we call the process whereby altering the ISO as I did with a Canon camera produces a similar effect? ATTR? Perhaps although I'm not certain anything is happening to the right of something else.

There are a lot of Canon cameras out there which presumably can produce less noise by tweaking the ISO as I did. So I think we should address this audience. Maybe we don't want to call it ETTR per se. But we need to come up with something to illustrate the effect.

One item my test provided that I'd hope we can all agree upon. Stating that 'raising ISO always produces more noise' isn't correct. Now what do we call this and how do we explain the results in as simple a sentence or two as possible?

[spidermike]

what do we call the process whereby altering the ISO as I did with a Canon camera produces a similar effect?

The problem arises because people are introducing a variable that was never there in the original articles? 

Why not simply define ETTR as pushing the histogram* to the right (without blowing highlights) as a method of reducing noise and maximising information. How you do that (be it pushing ISO or longer shutter speed etc) is dependent on the constraints of that particular shoot.
The unmentioned corollary is that its effect is maximised at base ISO, but then again any photographer venturing into ETTR already knows that and applies whether using ETTR or not.

* yes, I now that the aim is not to push the histogram but the histogram is the best tool we have to know we are achieving what we are setting out to do.

[digitaldog]

The problem arises because people are introducing a variable that was never there in the original articles? 

Yes (if original you mean Michael's way back when).

Why not simply define ETTR as pushing the histogram* to the right (without blowing highlights) as a method of reducing noise and maximising information. How you do that (be it pushing ISO or longer shutter speed etc) is dependent on the constraints of that particular shoot.

I have no issues with that as long as others are OK with the E for Exposure lumping ISO for certain camera systems into that mix.

The unmentioned corollary is that its effect is maximised at base ISO, but then again any photographer venturing into ETTR already knows that and applies whether using ETTR or not.

Thus far, I'm OK with that. But someone else might have a reason to sway my opinion which is the reason these discussions are useful.

[Bart_van_der_Wolf]

Ah, I think we're reaching critical agreement!
When I simply wrote to you in my last post that I disagree, it was based on what you just wrote.

What are the benefits one expects from using an ETTR approach? I think we all agree less noise due to better S/N. The adjustment of the ISO on a Canon system provided that goal and result. So now it's semantics but that's important and I've found that such back and forth discussions here on LuLa, thanks to the level of the audience often accomplishes the goal of having well defined terms as well an explanation of the whys. Then we can attempt to perhaps persuade others to do the same.

Yes, fully agree.

Exposure. Is it simply the interaction of shutter and aperture? Is ETTR solely achieved by controlling those two items in an attempt to produce the best quality data? Where does ISO fit? If we agree on that terminology, what do we call the process whereby altering the ISO as I did with a Canon camera produces a similar effect? ATTR? Perhaps although I'm not certain anything is happening to the right of something else.

This seems to be the major reason for most of the discussions. With the exception for Canons, changing the ISO doesn't change much besides the brightness of the LCD/Thumbnail/preview/histogram after in camera Raw conversion, White balance, and tonemapping.

What does change the Raw image quality is caused by the actual exposure level (the number of photons that achieve a certain level of exposure, due to the combination Aperture and shutterspeed). More photons is better, as long as we can achieve that within the creative (aperture/DOF) and unshaken (shutterspeed/subject motion) constraints.

There are a lot of Canon cameras out there which presumably can produce less noise by tweaking the ISO as I did. So I think we should address this audience. Maybe we don't want to call it ETTR per se. But we need to come up with something to illustrate the effect.

It's not easy without getting very technical, but it is ISO variant read-noise.

One item my test provided that I'd hope we can all agree upon. Stating that 'raising ISO always produces more noise' isn't correct. Now what do we call this and how do we explain the results in as simple a sentence or two as possible?

The leading principle is that more photons will improve the shot noise statistics more than the read noise, hence an improved S/N ratio is the result. If the actual photon exposure cannot be increased, due to constraints e.g. for reduction of camera shake or subject motion, then some cameras (e.g. Canons) can benefit from boosting the ISO to 800 - 1600 ISO which improves only the read noise characteristics.

Cheers,
Bart

[bernie west]

In my (and others, going by what Andrew said) opinion, the principle ethos of ETTR is getting the best SNR.  If that includes using ISO on non-ISOless sensors, then that seems like a reasonable addition to the general concept.  But thinking more about Andrew's example shots earlier I can't really work out how that happened.  Wouldn't boosting the ISO just boost both the signal and the noise at the 100ISO image by the same factor?  Which would leave the SNR the same.  In fact you'd expect the SNR to fall, as more noise would be introduced by the amplifier electronics.  What's going on there?

Edit:  A possible answer to my own question... Does this mean that the majority of electronic noise is introduced into the image data in the ADC?  That would mean that boosting the ISO in non-ISOless cameras would boost the signal in isolation from the majority of the electronic noise.

[Dave Ellis]

Edit:  A possible answer to my own question... Does this mean that the majority of electronic noise is introduced into the image data in the ADC?  That would mean that boosting the ISO in non-ISOless cameras would boost the signal in isolation from the majority of the electronic noise.

Hi Bernie, could I suggest you have a look at my comments in Reply 3 of this thread. That's the sort of thing I was talking about and my thoughts are based on what I have read in Emil Martinec's article referenced by Bill Janes above (reply 62).

Dave

[bernie west]

Oh yeah, cheers for that.  That makes sense.

[bjanes]

I provided two captures.
Capture 1 was shot at ISO 100 with a f-stop and shutter recommendation from an Incident meter (Minolta Flash Meter III). It is what many would suggest is 'correct' exposure.
Capture 2 was shot at ISO 800 with the same settings as capture 1. Identical in every respect expect I changed the ISO setting from 100 to 800!
Capture 2 shows less noise than capture 1. This is a very magnified view of the image but it is clear that the capture with the higher ISO has less noise, better S/N.

This is an interesting exercise, but it would be informative to see the entire images, not crops. A danger of ETTR is blown highlights. The ISO 800 image received 3 stops more exposure than indicated by the meter. This would lift the mid-tones by 3 stops. Since pure white (zone 9) is 2.5 stops over mid-gray, this means that tones above mid-gray would be clipped. The mid-tones have a better SNR, but clipped highlights would have a SNR of zero.

Bill

[Bart_van_der_Wolf]

This is an interesting exercise, but it would be informative to see the entire images, not crops. A danger of ETTR is blown highlights. The ISO 800 image received 3 stops more exposure than indicated by the meter. This would lift the mid-tones by 3 stops. Since pure white (zone 9) is 2.5 stops over mid-gray, this means that tones above mid-gray would be clipped. The mid-tones have a better SNR, but clipped highlights would have a SNR of zero.

Hi Bill,

That was one of my concerns as well, since Andrew said that the ISO 100 shot was (according to the exposure meter) correctly exposed. An ISO 800 setting with the same exposure would then place the mid tones at the clipping level, and leave brighter tones totally clipped.

But it also shows (a bit OT) that for ISO variant Canons in this case, ISO can be used for "amplification" or ISO bracketing. However, nothing beats a real (Exposure) bracketing sequence if HDR tonemapping is part of the workflow, because collecting more Photons are the quickest route to S/N ratio improvement (at all tone levels). Without that, single exposures benefit from optimizing (maximizing, without clipping of important highlights) the exposure level, by using ETTR if the other constraints allow.

In general that means that low contrast scenes will be exposed 'hotter' than an incident exposure meter would suggest, and darker in a high contrast scene (to protect highlights). The easiest way to quickly achieve that is to (spot) measure the important highlights (e.g. spotmeter the white clouds), and use a +EV of approx 2.5-3 stops. That would take care of scene contrast automatically.

Cheers,
Bart

[digitaldog]

In my (and others, going by what Andrew said) opinion, the principle ethos of ETTR is getting the best SNR.  If that includes using ISO on non-ISOless sensors, then that seems like a reasonable addition to the general concept.

I think so. This raises an interesting concept in my mind which may need correction or explanation based on what others write.
Bart provided a good analogy, that raising ISO is akin to raising the volume on an amplifier rather than the musicians themselves playing louder. So consider the ISO 800 image I provided. Had I used the meters recommendation, I'd have stopped down from the ISO 100 image and with less photon's as a result, more noise. Now what if I'm shooting and I'm at the limit of either the shutter speed of aperture I want using the ISO 100 recommendation so ETTR isn't going to work. I just raise the ISO on the camera leaving the 'exposure' the same, I end up with less noise. That could be a useful technique no?

Also interesting is that when I tested ETTR on this camera, I found I could not go more than 1.5 stops to the right without clipping delicate highlights. 2 stops was blown out. Yet using the amplifier as Bart describes it, I was able to go up three stops. Interesting.

[digitaldog]

The ISO 800 image received 3 stops more exposure than indicated by the meter.

Did it? I'm only asking if this is a fair use of the term exposure as discussed previously.

I'll look at the full image and provide a sample, it's possible I was close to or just at highlight clipping but what about my previous text where 1.5 stops over the meter using exposure (aperture/shutter) was the limit? Is it possible using ISO this way provide more 'headroom' to ETTR if that's even a fair term to use in this example? Or that ISO used this way is not fully correlated with f-stops? Don't know.

Initially the example was made to dismiss the blatant statement that higher ISO always results in more noise. Unless I'm missing something, that statement isn't true.

[bjanes]

Did it? I'm only asking if this is a fair use of the term exposure as discussed previously.

I'll look at the full image and provide a sample, it's possible I was close to or just at highlight clipping but what about my previous text where 1.5 stops over the meter using exposure (aperture/shutter) was the limit? Is it possible using ISO this way provide more 'headroom' to ETTR if that's even a fair term to use in this example? Or that ISO used this way is not fully correlated with f-stops? Don't know.

According to Doug Kerr, who quotes Canon sources, if one photographs a scene of constant luminance (i.e. a gray or white card) with a Canon camera, the rendered gray scale image should have a K value of 55%, which corresponds to a relative luminance of 0.173. Mid-gray is 0.18. A K of 55% corresponds to about 113, 113, 113 in sRGB.

A relative luminance of 0.173 is 2.53 stops below 100% [log2(1/0.173)]. Accordingly, 3 stops over the camera light meter reading would clip scene luminances from slightly below mid-gray and above. According to Doug, Canon does not allow the "famous 1/2 stop cushion" for highlight headroom. You could test your setup by photographing a white card and checking rendered values in Photoshop. The sRGB value should be 113, 113, 113.

Bill 

[AlterEgo]

You could test your setup by photographing a white card and checking rendered values in Photoshop. The sRGB value should be 113, 113, 113.

need to spell raw converter parameters too...

[digitaldog]

This is an interesting exercise, but it would be informative to see the entire images, not crops.

Here's a screen capture of the entire image also showing the somewhat radical settings used in Develop. Clipping indicators are on in Histogram, no clipping. RGB value over brightest area of dogs head reads 98%.
What's interesting (to me) is that while the 'exposure' was boosted 3 stops via ISO, note that the Exposure slider is set to -1.55 stops which as I pointed out earlier is just about the limit of the ETTR testing on this camera done in the past. I can't explain why there is this disconnect between the +3 ISO and -1.5 Exposure slider among the other sliders as set.

[AlterEgo]

I can't explain why there is this disconnect

when dealing with Adobe converters do use gutted .dcp profiles (linear curve, hidden expocorrections compensated, post exposure corrections LUTs removed) + Process 2010 + brightness 50, contrast 25, curve : Medium Contrast, output to the baddest gamut-wise colorspace... something like this... at least in ACR.