Expose to the Right and High(er) ISO

[KrisCarnmarker]

OK, so I'm sitting there viewing the newly release "From Camera to Print" videos from Luminous Landscape, and more or less in passing, they talk about the "Expose to the Right" technique.

Now, they mention that doing this is essentially like lowering the ISO sensitivity. Assuming we are talking about exposure, this is understood. However, they claim that there is no point in using the technique at higher ISOs, e.g. 200, as you may as well lower the ISO to 100. That is what I fail to understand. The technique is used in order to make use of the most amount of bits. How does that change when using ISO 200, for example?

[Doug_Kerr]

Hi, Chris,

OK, so I'm sitting there viewing the newly release "From Camera to Print" videos from Luminous Landscape, and more or less in passing, they talk about the "Expose to the Right" technique.

Now, they mention that doing this is essentially like lowering the ISO sensitivity. Assuming we are talking about exposure, this is understood. However, they claim that there is no point in using the technique at higher ISOs, e.g. 200, as you may as well lower the ISO to 100. That is what I fail to understand. The technique is used in order to make use of the most amount of bits. How does that change when using ISO 200, for example?

On the typical digital camera with various "ISO speed" modes, assuming the use of automatic exposure control, changing the "ISO" setting will not (ordinarily) move the exposure to the right; that is, shift the range of photometric exposure on the sensor to a higher point on the range of the imaging system (as it is in that "ISO" mode). We often describe places in that range in terms of fractions of the saturation value of photometric exposure (as it is in that "ISO" mode).

The reason of course is that the change in setting changes the actual sensitivity of the system (thus changing its saturation value) and advises the metering system of that, so it will enact an exposure (in the sense of shutter speed and aperture) that will give the same result as before.

For example, if, on a certain camera, with the "ISO" set to "ISO 100", a metered exposure of a frame-filling gray card gives (in the JPEG output) a pixel value across the image of R/G/B 116/116/116, then if we change the setting to "ISO 200", and take the same test shot, we should expect that the image will still have its pixels at R/B/G 116/116/116.

Now in fact it might not work out exactly that way, owing to subtleties in how the camera metering and image processing work. But it will not be far off.

Regarding "making best use of the number of bits", classically, changes in the ISO sensitivity of the camera are implemented by changing the gain of the analog amplifiers between the actual photodetectors and the analog-to-digital converters. If this is wholly true, then changes in the ISO sensitivity (again assuming metered exposure) do not have any effect on the "use of the bits" issue.

(There will of course be noise implications, which I do not treat here.)

In some cases, the highest ISO sensitivity modes are obtained with the amplifiers having the gain for a lower mode and changing the range of the system by digital (numerical) manipulation of the output numbers from the analog to digital converters. In some case, the amplifiers are not able to be set to distinct gains for the full repertoire of available "ISO speed" modes, perhaps only distinct gains for the settings (other than the highest) at full-stop intervals, with intermediate settings done by digital manipulation.

In such a case, the "best use of the bits" occurs at those "ISO speed" settings having distinct amplifier gains (perhaps the "full stop" values except the highest one).

[Asher Kelman]

Doug,

If it were possible, would re-reading the same voltage and A-D converting again numerous times be a method to deal with gain-noise.

Asher

[KrisCarnmarker]

Thanks Doug. What you describe is how I've understood it (if I've understood your post correctly).

What i meant in my OP about effectively lowering the ISO was not exposure, but exposure settings. For instance, assuming ISO 100, if I would use f2.8 and 1/500 s for a "correctly exposed" image, I would need to modify that to (e.g.) f.2.8 and 1/250 in order to expose to the right by one stop. This would be the same modification I would have to make if I were to lower the ISO to 50 but not expose to the right. At least that is how I understood what MR meant in the video.

So, just to clarify: you agree that there is no reason why "exposing to the right" is not equally applicable at other ISO settings? Disregarding noise issues, etc..

[Doug_Kerr]

Hi, Kris,

Thanks Doug. What you describe is how I've understood it (if I've understood your post correctly).

What i meant in my OP about effectively lowering the ISO was not exposure, but exposure settings. For instance, assuming ISO 100, if I would use f2.8 and 1/500 s for a "correctly exposed" image, I would need to modify that to (e.g.) f.2.8 and 1/250 in order to expose to the right by one stop. This would be the same modification I would have to make if I were to lower the ISO to 50 but not expose to the right.

Gotcha.

So, just to clarify: you agree that there is no reason why "exposing to the right" is not equally applicable at other ISO settings? Disregarding noise issues, etc..

If bumping the exposure by one stop makes sense at one ISO speed setting, it would generally also make the same sense at another.

However, that's not my concept of "expose to the right". To me the term means:

Pick an exposure (shutter speed and aperture, what I call "exposure1") such that the highest luminance part of the scene receives a photometric exposure (what I call "expsure2) on the sensor at almost saturation (whatever that is for the ISO speed setting in use).

That is not necessarily always an exposure1 that is one stop hotter than the metering system would choose (which I assume is what you mean would be for "correct exposure").

Best regards,

Doug

[Bart_van_der_Wolf]

That is not necessarily always an exposure1 that is one stop hotter than the metering system would choose (which I assume is what you mean would be for "correct exposure").

In general, expose-to-the-right (ETTR) is done with an override to the exposure metering, as an +/- EV adjustment. I often use a +1/3rd EV as a basis for my shooting style.

Bart

[Doug_Kerr]

Hi, Asher,

Doug,

If it were possible, would re-reading the same voltage and A-D converting again numerous times be a method to deal with gain-noise.

Let's first talk about "possible".

In a CCD sensor array, we do not really read out the voltage. We dump the charge from the photodetector (and eventually read the voltage that charge causes when it eventually reaches the end of the "bucket brigade"). So you can't read it more than once for one exposure.

In a CMOS sensor array, we really do read out the voltage from the photodetector, and could perhaps do so again (although the way it is read out may disturb it a little).

But most of the noise (not all) manifests itself in randomness of the voltage on the photodetector at the end of the exposure time, so reading it out several times and averaging it wouldn't really do anything. (There is some noise introduced in the amplifiers, and that might be mitigated by the approach you propose.)

You can get a general idea of how the CMOS sensor works from my article, "The CMOS APS Digital Camera Sensor", available on The Pumpkin. I'm operating in peculiar circumstances right now, so I can't easily give you a link. By the way, "APS" there has nothing to do with the accursed convention for describing the sizes of digital sensors; it means "active pixel sensor".

Best regards,

Doug

[Doug_Kerr]

In general, expose-to-the-right (ETTR) is done with an override to the exposure metering, as an +/- EV adjustment.

Sure.

I often use a +1/3rd EV as a basis for my shooting style.

If I want to truly ETTR, I will chimp a test shot and see how much headroom I have, then burn most of it with +EC.

Best regards,

Doug

[Bart_van_der_Wolf]

If I want to truly ETTR, I will chimp a test shot and see how much headroom I have, then burn most of it with +EC.

Same with me, although it may be difficult to judge the LCD in bright outdoor conditions at times.

Bart

[Doug_Kerr]

Hi, Bart,

Same with me, although it may be difficult to judge the LCD in bright outdoor conditions at times.

That's what the histogram is for.

Best regards,

Doug

[Asher Kelman]

Unfortunately, not!

The LCD histogram is based on the jpg and misses out much of the space to the right of interest in the last stop!

Asher

[KrisCarnmarker]

That is not necessarily always an exposure1 that is one stop hotter than the metering system would choose (which I assume is what you mean would be for "correct exposure").


Right, that's understood. It's hard to know what Michale Reichman really means with his "equivalent to ISO 50 statement", but I was just assuming what I wrote above.

I also use exposure compensation when exposing to the right, and certainly I need to look at the histogram.

So it seems that we are all in agreement. I wonder what he means then...

[Doug_Kerr]

Hi, Asher,

Unfortunately, not!

The LCD histogram is based on the jpg and misses out much of the space to the right of interest in the last stop!

I should have mentioned that I shoot to the JPEG output.

Best regards,

Doug

[Doug_Kerr]

Hi, Asher,
Unfortunately, not!

The LCD histogram is based on the jpg and misses out much of the space to the right of interest in the last stop!

As you know, I have not so far operated in the raw regime, so there is much common wisdom that I do not have at hand.

Typically, how can we quantify the "headroom" that remains (considering the availablilty of the raw data) in a shot that is "exposed fully to the right" with respect to the JPEG output (that is, a shot in which highest value of either R, G, or B is essentially 255)?

That is, for that same scene, how much hotter could we have exposed it before any of the raw CFA "channels" reached their saturation digital output value of 4095?

I realize that of course the actual numerical answer would depend on the chromaticity of the highest luminance part of the scene. But perhaps the typical answer would be known for the chromaticity of a neutral scene object illuminated by light of some handy standard "white" flavor (perhaps the white point of the sRGB color space).

I plan to do some testing on this myself, but owing to various equipment problems here at the moment doing so would be cumbersome, so I am putting it off. So I hoped there was a "well-known" broad answer. There must be, since people speak so often of the existence of this headroom.

Thanks for helping me get into this important new compartment of my understanding.

Best regards,

Doug

[Andrew Rodney]

Based on tests I just complete for an article in DPP, the histogram is all but useless to me with my Canon 5D.

Using controlled lighting, shooting a number of test targets, at least one with an amazingly white tile and neutral reading from a Spectrophotometer, I was able to shoot at 1.5 stops OVER the meter recommendation (Sekonic flash meter), get full non-clipped highlights by simply altering the Exposure slider in Lightroom. 2 stops over did blow out the data (full sensor saturation). When I viewed the bracketed images (half stop) on the LCD, ½ over normal showed highlight clipping (based on the JPEG I'm not asking for).

In all cases, the ETTR images produced superior data, most notable in the last stop of shadows. The higher the ISO, the more apparent.

From this set of tests, even after playing around with JPEG picture styles to get a flatter image, the Histogram and clipping were far from ideal or based on the actual Raw data I eventually got.

[Doug_Kerr]

Hi, Andrew,

. . .I was able to shoot at 1.5 stops OVER the meter recommendation (Sekonic flash meter), get full non-clipped highlights . . .

Interesting.

How much hotter would this exposure (1.5 stops over metered) have been compared to the exposure that gave full ETTR as seen on the JPEG output (which would of course have been hotter than the metered exposure)?

Thanks for the input.

Best regards,

Doug

[Andrew Rodney]

How much hotter would this exposure (1.5 stops over metered) have been compared to the exposure that gave full ETTR as seen on the JPEG output (which would of course have been hotter than the metered exposure)?


The 'normal' exposure looks normal on the camera LCD and using the default rendering in CR/LR. I came up with 1.5 over as the ideal ETTR as I was bracketing (via flash power) in ½ stop increments and the Plus 2 was indeed truly over exposed. The white tiles clipped. The plus 1 stop could be adjusted but so could the plus 1 ½ to produce the same appearance as the normal image but without white clipping.

[Michael Fontana]

No proof, but feelings out of experience, it's not only about quantity of ETTR but depends on the light's contrast, as well. Might that be correct?

[Don Lashier]

No proof, but feelings out of experience, it's not only about quantity of ETTR but depends on the light's contrast, as well. Might that be correct?
Indeed. In fact with my typical shooting conditions (outdoors on a sunny day), ETTR doesn't make any sense at all as you're already flirting with highlight blowout at "normal" exposure due to the high DR of the scene.

- DL

[Doug_Kerr]

Just as a matter of curiosity, I did some very simplistic tests to look a little into the matter of the "headroom" of the raw output compared to the headroom of the JPEG output in my EOS 20D.

I used an admittedly special case (one that makes analysis simple, of course): a frame-filling uniformly-illuminated "uniform" reflectance neutral target (gray card). (We'll see later why I put "uniform" in quotes!)

The illumination was from a constellation of three flash units, at fixed output. I did a custom white balance with the setup and used CWB in the camera (pertains only to the JPEG output, of course).

I took exposures at different apertures at 1/3 stop spacing and for each, looked at the RGB values of the JPEG output (in my editor) and the values of the r, g1, and b channels of the raw output (with IRIS).

An interesting "point on the curve" was where the RGB values, averaged over a central part of the image, were 253/253/253. The histogram peaks were fairly broad (owing to the fact that the test target was not quite of uniform reflectance, having a noticeably mottled surface). The histogram showed that 17.6% of all pixels were above RGB 254/254/254 (and thus could be considered clipped) In terms of the JPEG output, this is really pretty "exposed to the right".

For the raw output, I was not able to conveniently get an average value but only a histogram per channel (r, g1, and b). Again, the peaks were of significant width.

The centers of the peaks of the histograms for the three raw channels (on a 4095-unit maximum scale, presumably with black at about 128 units) were at approximately:

r: 1900
g1: 3800 [but see below]
b: 2600

But a noticeable part of the upper skirt of the g1 channel peak would have been above 4095, and thus those pixel values were clipped to that level. So, for this particular uninteresting object, that exposure resulted in a little clipping of (the green component of) its highlight detail (such as it was).

Overall, for this particular contrived setup, "when the JPEG output was just out of headroom, the raw output was just out of headroom.

Now, in another test shot, 1/3 stop lower in exposure, the RGB averages were 244/244/244,with only 0.3% of the pixels above 253 (essentially no clipping). Then, for the raw output, the peaks of the channel histograms were at approximately:


r: 1500
g1: 2800
b: 2000

again, with essentially no clipping.

(Note that these are consistent, between the two tests, with the raw values being essentially linear with photometric exposure.)

Of course, all the relationships in the camera are so complicated that it is difficult to reliably extrapolate these findings, taken for an unrealistic, idealized situation, into cases involving real scenes.

Therefore, I emphasize that although these "idealized situation" tests give some useful insights into this matter, we cannot (necessarily) from these results develop any policies governing exposure planning for real scenes. It does, however, raise some questions about the notion that "in the raw context, there is significantly more headroom than in the JPEG context."

Best regards,

Doug

[Andrew Rodney]

Indeed. In fact with my typical shooting conditions (outdoors on a sunny day), ETTR doesn't make any sense at all as you're already flirting with highlight blowout at "normal" exposure due to the high DR of the scene.


Actually you're still using ETTR, as you're ensuring you're not blowing out the highlight detail in a scene with an enormous dynamic range. ETTR would suggest, if there's bright highlights you want to capture, you absolutely do not clip them but exposure just below that value. ETTR isn't about over exposing, its about exposing for the highlights and developing for the highlights. How we decide to expose for this is the tricky stuff as well as how we use the LCD to provide anything useful based on these behaviors.

[John Sheehy]

It does, however, raise some questions about the notion that "in the raw context, there is significantly more headroom than in the JPEG context."

What is the contrast setting in the 20D?

What is the average green RAW value for the grey card exposed with the metering at 0 EC, and what histogram peak does that result in on the camera?

These factors all affect the difference between RAW clipping and JPEG clipping.

[Doug_Kerr]

Hi, John,

What is the contrast setting in the 20D?

I forgot to mention that I set that to minimum for this test.

What is the average green RAW value for the grey card exposed with the metering at 0 EC, and what histogram peak does that result in on the camera?

I will determine those.

These factors all affect the difference between RAW clipping and JPEG clipping.

Thanks.

Best regards,

Doug

[John Sheehy]

Right, that's understood. It's hard to know what Michale Reichman really means with his "equivalent to ISO 50 statement", but I was just assuming what I wrote above.

You can use the camera at any ISO exposure index you want; the only issues are the noise floor, relative to middle grey, and the clipping point, relative to middle grey. Pushing and pulling push and pull these levels.

If your camera's lowest ISO setting is 100, and you meter for ISO 12, and the image does not clip anything matte and reflective in the scene, you truly are shooting at ISO 12, probably with higher IQ than the camera would probably have if it really did have ISO 12 and you wasted the headroom. Canon 1Dmk* and 5D DSLRs, for example, have "ISO 50", which really should be ISO 70 or so. ISO 50 and 100 have almost exactly the same read noise, relative to metering on these cameras. This means that ISO 100 with +1 EC has less read noise, relative to signal, than ISO 50 has. IOW, ISO 100 pulled to 50 is a better ISO 50 than the one the camera offers explicitly (in the RAW data; specific converters may not take proper advantage). The only difference is that ISO 50 offers almost 1/3 stop more headroom (but which may possibly be non-linear).

[John Sheehy]

Hi, John,

I forgot to mention that I set that to minimum for this test.


There you have it, then. This generally pulls 2/3 to 1 stop more RAW highlights into the JPEG's highlights. I set all my cameras to minimum contrast, for a more RAW-relevant histogram, and I find that using a little positive EC along with low contrast results in less noisy JPEGs, which can easily be brought back to reasonable tone curve if they are pale, with a gamma setting of around 0.8 in PP.

[Michael Fontana]

Actually you're still using ETTR, as you're ensuring you're not blowing out the highlight detail in a scene with an enormous dynamic range. ETTR would suggest, if there's bright highlights you want to capture, you absolutely do not clip them but exposure just below that value. ETTR isn't about over exposing, its about exposing for the highlights and developing for the highlights. How we decide to expose for this is the tricky stuff as well as how we use the LCD to provide anything useful based on these behaviors.


Here's what I do: with blue sky, I rather tend to underexpose about 1/3 to 1/2, to avoid blown highlights, if no bracketing for HDR is required.

Meanwhile in cloudy scenes, or in the studio; it's often plus 1/3 to 1/2.

Not a big deal, when shooting to 99% from tripod, in manual mode.

[John Sheehy]

How does that change when using ISO 200, for example?

It applies at all ISOs, but it doesn't make any sense to use a high ISO and be limited to its lower maximum absolute signal clipping level, if you don't need the low absolute signal level.

The best strategy, IMO, is to focus on what Av and Tv values you want, then choose the highest ISO that doesn't clip away desired highlight details, with those Av and Tv values. Unless, of course, your camera is one that adds more noise at the higher ISOs (most don't; they just seem to because the high ISO dictates a low absolute exposure to the metering system).

One doesn't always have the time and luxury of accurately gauging which ISO will cause unwanted clipping, so knowing your camera helps estimate risk. Generally speaking, Canon DSLRs, when used from this perspective, will have the greatest IQ gains from the higher ISOs, and may be worth risking a little highlight detail for the cleaner shadows at higher ISOs. Most other DSLRs don't have any optimized readout for high ISOs, and gain less from using higher ISOs as opposed to under-exposing at lower ISOs, so you might lean more towards the lower ISOs and under-exposure, with less to gain at the higher ISOs. This analysis is based upon the actual RAW data, however, and the limitations of certain converters (or their default setting), and the limitations of shooting in JPEG might make the higher ISO advantageous even on these cameras not optimized for high ISO, as the shadows of conversions tend to be poorly rendered and posterized, and in-camera JPEGs add even more junk to the shadows.

[Doug_Kerr]

Hi, John,

I set all my cameras to minimum contrast, for a more RAW-relevant histogram

Do you feel that in general, one can successfully choose exposure for raw ETTR (and I don't mean "more to the right than the metered exposure" - I mean "as far to the right as will fit") by observing test shots on the in-camera histogram?

If not, what is the best way to attain that result?

Thanks.

Best regards,

Doug

[Don Lashier]

Actually you're still using ETTR, as you're ensuring you're not blowing out the highlight detail in a scene with an enormous dynamic range. ETTR would suggest, if there's bright highlights you want to capture, you absolutely do not clip them but exposure just below that value. ETTR isn't about over exposing, its about exposing for the highlights and developing for the highlights. How we decide to expose for this is the tricky stuff as well as how we use the LCD to provide anything useful based on these behaviors.

I suppose it's a matter of semantics, but to me ETTR means purposely over-exposing ("to the right") to gain shadow quality. Underexposing in high DR would for me be ETTL :).

Like Uwe, I've never bought into ETTR as a normal technique primarily because it's too time consuming for my fast shooting style with dubious gains for the typical shots I do where noise free shadows aren't worth the risk. Yes in flat lighting I'll tend to overexpose a bit (up to a stop) because it's safe and easy, but in high DR I'll spot the entire scene and expose accordingly. This obviously includes checking the highlights for blowout but I don't consider this ETTR as a method but rather simply exposing for maximum capture based on conditions and the relative importance of various portions of the scene and this may involve exposing to the "right", to the "left", or down the "middle".

PS: I think if the technique had been labeled "expose FOR the right" (EFTR) it would be less controversial as this is pretty standard advice/procedure for digital. ETTR leads to such blanket practices as setting +EV in auto mode which is dangerous without chimping each shot as AE is notoriously sensitive to scene in some situations.

PPS: I'm not against ETTR - it's obviously a valuable concept applicable to a certain class of photos, but it should be used as an element of a larger exposure methodology taking in the dynamics of the scene and the goals of the capture and not just blindly dialing in +EC.

- DL

[Michael G. Spille]

Bart-Thank you.
I've watched the LL video 3X,and i've read at least a dozen articles on the various forums that address this issue
Yours is the best and simplest answer I've gotten;and the one I will follow.
Mike

[KrisCarnmarker]

John, thanks for that explanation. I may be incorrect, but you are basing your arguments on noise levels, are you not?

Noise levels are obviously not irrelevant, but to me at least, the purpose of ETTR is about tonal gradations more than about noise levels. These two are practically related of course.

If we use the LL article (http://www.luminous-landscape.com/tutorials/expose-right.shtml) examples, a camera capable of 5 stops DR, with a 12-bit ADC provides 4096 distinct values. 2048 of those are used in the first (brightest) f-stop, 1024 for the next, etc.. Practically, this means I get better tonal gradations at the higher f-stops, which is what we want to exploit.

So, if I go to e.g. ISO 400, do I no longer have those 2048 levels available in the first f-stop any more? This is what I don't understand. Of course, I'm simplifying things, and it may be that although I do still have those 2048 levels, other parameters have worsened and whatever gains I have achieved by ETTR at ISO 400 have been made negligible by the other factors.

[Don Lashier]

> Noise levels are obviously not irrelevant, but to me at least, the purpose of ETTR is about tonal gradations more than about noise levels.

Kris, while this is true if you're going to do some rather severe tonal adjustments like pulling hugh amounts of shadow detail or doing double conversions for HDR layering, imo for normal shots where you might just pull a bit of contrast it's largely irrelevant as the existing (normally exposed) data is more than adequate.

IMO even for noise issues, unless you're going to pull shadows ETTR gains you little if anything other than pixel-peeping quality.

- DL

[KrisCarnmarker]

I agree Don. Whether or not I employ the ETTR technique is another matter. I just want to understand it, and until I understand why its not applicable for higher ISOs, I don't feel I understand it.

Having said that, I almost always have to bump the exposure in LR by 0.5-1.5. So the ETTR technique caught my attention.

[Doug_Kerr]

Hi, Kris,
[QUOTE=KrisCarnmarker;31823]So, if I go to e.g. ISO 400, do I no longer have those 2048 levels available in the first f-stop any more?[quote]

Assuming you use a corresponding exposure (one stop less exposure) when you go to from ISO 200 to ISO 400, then that matter is unchanged.

There will always be about 2048 discernible steps in the top half of the range of photometric exposure (H) to which the sensor can respond at a particular "ISO speed" setting. (I say "about" because I don't really know exactly how to come to grips with the black point being at about 128 units).

That top half will also be "the top one stop of the range of scene luminances" if we have fully exposed to the right (the highest scene luminance just barely receiving the saturation value of H and thus a digital output level of 4095).

If we expose one stop less than that, then our highest scene luminance will be at (about) 2047 units, and the top half of the range we are actually using contains (about) 1024 steps.

Note that if we are speaking of ISO speed settings that don't have bona fide gains of the analog amplifiers, but are constructed by digital scaling of the digitized outputs, then all bets are (somewhat) off.

Note also that these "step count" analyses really only works for a subject whose chromaticity is such that the r, g, and b sensor groups give the same output (not true, for example, for anything we could reasonably call "white" or "gray).

Excuse me for saying all this in such a tedious way, but if we don't, then there can be misunderstandings about what is meant.

Best regards,

Doug

[Doug_Kerr]

Hi, John,
What is the contrast setting in the 20D?

What is the average green RAW value for the grey card exposed with the metering at 0 EC, and what histogram peak does that result in on the camera?

To give a more complete response:

1. The contrast setting was "minimum" ("-2").

2. For a metered exposure of a uniformly-illuminated neutral test card, using partial metering, with EC=0:

a. The average RGB values in the sRGB JPEG image were 118/118/118.

b. The camera histogram peak was at about 2.3 scale units (considering the scale to run from 0 at the far left to 5 at the far right, with each line representing an integer value).

c. In the raw data, the peak of the g1 or g2 histogram was at about 440 units (4095 scale).

Note with regard to the matter of the metered exposure, although ideally we would expect the metering equations to produce the same value of H (and thus of RGB) for a given part of a test scene between tests, in fact this is not quite so.

For example, the RGB value will vary somewhat at different ISO speed settings, or with different balances of shutter speed and aperture that make up the Ev the meter adopts. For example, if I use the Tv mode for metering, and set the shutter speed to 1/100 and take a metered shot of the test card, and then set the shutter speed to 1/200 sec and take another metered shot, the RGB values will differ.

The range of the discrepancy is not great - perhaps 1/2 stop at worst - and might in fact result from rounding phenomena in the whole exposure control chain, or perhaps from subtleties in the metering algorithm not embraced by our classical assumptions.

Note also that the Canon "handy check" of AE behavior (applicable to earlier EOS dSLR's, maybe not to the latest ones) would lead us to expect about RGB 116/116/116 for metered exposure of a uniform neutral test target.

Thanks for your input and insight.

Best regards,

Doug

[Bart_van_der_Wolf]

For example, the RGB value will vary somewhat at different ISO speed settings, or with different balances of shutter speed and aperture that make up the Ev the meter adopts. For example, if I use the Tv mode for metering, and set the shutter speed to 1/100 and take a metered shot of the test card, and then set the shutter speed to 1/200 sec and take another metered shot, the RGB values will differ.

A likely candidate is light entering the viewfinder from behind the camera, and throwing off the brightness of the focus screen. Otherwise I've not seen large fluctuations such as you describe.

Bart

[Doug_Kerr]

Hi, Bart,

A likely candidate is light entering the viewfinder from behind the camera, and throwing off the brightness of the focus screen. Otherwise I've not seen large fluctuations such as you describe.

Well, Duh, I knew that, but I have not at all been diligent about preventing that. I will certainly take the necessary precautions in the future (in fact, I have planned to redo some of my tests in just a few minutes!).

Thanks so much.

Best regards,

Doug

[Don Lashier]

Having said that, I almost always have to bump the exposure in LR by 0.5-1.5. So the ETTR technique caught my attention.

Then perhaps you're just under-exposing and should adjust your technique. I have a mental feedback loop between C1 and my exposure technique and try to keep my RC conversion to within +/- 0.5 stops. IMO regularly adjusting +1 or more is not a good idea.

- DL

[Michael Fontana]

Then perhaps you're just under-exposing and should adjust your technique. I have a mental feedback loop between C1 and my exposure technique and try to keep my RC conversion to within +/- 0.5 stops. IMO regularly adjusting +1 or more is not a good idea.
- DL

I agree with Don: >adjusting +1 or more is not a good idea.< as a general rule. The converters have different headroom, though.... some are better in the shadows and weaker at the highlights, and vice versa.

[John Sheehy]

John, thanks for that explanation. I may be incorrect, but you are basing your arguments on noise levels, are you not?

Yes, I am.

Noise levels are obviously not irrelevant, but to me at least, the purpose of ETTR is about tonal gradations more than about noise levels. These two are practically related of course.

Yes, they are, but they are related in such a way that the noise renders the "number of levels" to a level of irrelevancy in actual practice. Noise and the spectral responses of the three color channels, along with clipping, are the only things that really affect image quality at the RAW level, in practical reality. The noise levels would have to be significantly lower than they currently are for the number of tonal levels to make a significant difference in image quality. In a DSLR at ISO 100, even an out-of-focus highlight area of flat illumination will have neighboring pixels in the same color channel differing by scores of RAW levels, due mainly to photon noise statistics. In the deep shadows, they are varying by several levels from both photon noise and read noise at ISO 100, and by scores for some cameras, at ISO 1600. Mild quantization is a relatively minor issue in this context.

If we use the LL article (http://www.luminous-landscape.com/tutorials/expose-right.shtml) examples, a camera capable of 5 stops DR, with a 12-bit ADC provides 4096 distinct values. 2048 of those are used in the first (brightest) f-stop, 1024 for the next, etc.. Practically, this means I get better tonal gradations at the higher f-stops, which is what we want to exploit.

I would call those "stops", not "f-stops", as they have nothing to do with ratios of focal length to aperture.

There is nothing there to exploit with most cameras. Exposing to the right works, but not for the main reason Michael Reichmann gives; it's just a coincidence that using more levels in a given ISO results in a higher signal-to-noise ratio, which *is* significant.

So, if I go to e.g. ISO 400, do I no longer have those 2048 levels available in the first f-stop any more?

Most cameras use 12 bits of RAW data up to at least ISO 800. Not all cameras use all 4096 levels though. My Canon 30D only uses 3040 levels in some of its ISOs (160/320/640/1250), and those give better IQ than their neighboring lower ISOs (125/250/500/1000), which use 3967 levels of RAW data.

I'd include a demonstration comparing RAW data that uses various amounts of levels, and various S/N ratios, to show how levels mean almost nothing in practice, but that will take some work, so I will do that if I get time. Nikon realizes they mean almost nothing, and that is why they use lossy compression in their RAWs. You would literally have to start adding many images together, to reduce the noise to very low levels, before the number of tonal levels starts to come into play in the RAW data. Now conversions may be another story. Converters may posterize images more than necessary.

This is what I don't understand. Of course, I'm simplifying things, and it may be that although I do still have those 2048 levels, other parameters have worsened and whatever gains I have achieved by ETTR at ISO 400 have been made negligible by the other factors.

For most cameras, ETTR at a higher ISO will give equal or better RAW data, so long as you don't reduce the absolute exposure to do so. IOW, if you you put a camera in manual mode, meter for ISO 100 and set the Av and Tv values accordingly, and then shot the same scene at every ISO, the one that had the highest ISO which didn't cause any unwanted clipping of highlights will give anywhere from slightly to significantly better results than the lower ISOs.

[Doug_Kerr]

IOW, if you you put a camera in manual mode, meter for ISO 100 and set the Av and Tv values accordingly, and then shot the same scene at every ISO, the one that had the highest ISO which didn't cause any unwanted clipping of highlights will give anywhere from slightly to significantly better results than the lower ISOs.

That makes a lot of sense for me. With the fixed Ev (I mean real Ev) (I always mean real Ev) (and of course a certain scene, with a certain distribution of luminance), the higher ISO settings move the saturation photometric exposure (H) down, until it approaches the highest H in the image. (This actually works separately for each of the three CFA groups, of course, but that doesn't spoil the concept - I suppose that in most cases, the green collision happens first.) At this point, the best use is made of the range of code values.

So, not surprisingly, more generally, to attain this situation ("exposed fully to the right"), for a given scene, we can play with shutter speed, aperture, and/or sensitivity (ISO speed) setting, as best suits all our objectives, and it doesn't really matter which (until we're in a situation in which noise looms into importance in the shadow areas).

As one of our friends, a character actor who's busy now as a corny cowboy in an auto dealership commercial, says, "We don't care how yew git here - just git here!"

Thanks for helping me get this picture.

Regarding my own test data, now that Bart has reminded me not to forget to cork my eyepiece for metered exposures, I have started over. Results presently!

Best regards,

Doug

[KrisCarnmarker]

YIOW, if you you put a camera in manual mode, meter for ISO 100 and set the Av and Tv values accordingly, and then shot the same scene at every ISO, the one that had the highest ISO which didn't cause any unwanted clipping of highlights will give anywhere from slightly to significantly better results than the lower ISOs.

Interesting. I think most people would be surprised to find this out. Most people, me included, will avoid higher ISOs if at all possible.



So, not surprisingly, more generally, to attain this situation ("exposed fully to the right"), for a given scene, we can play with shutter speed, aperture, and/or sensitivity (ISO speed) setting, as best suits all our objectives, and it doesn't really matter which (until we're in a situation in which noise looms into importance in the shadow areas).


Yes, this is what I would have thought "intuitively". That the sensitivity was excluded from my choices was what was bothering me. Now that it is settled (in my mind) that this is not so, I can breathe again :)

Thank you all for taking the time to explain all this to me!

/Kris

[Don Lashier]

Interesting. I think most people would be surprised to find this out. Most people, me included, will avoid higher ISOs if at all possible.


Note the qualification: "so long as you don't reduce the absolute exposure to do so.".

It's not too surprising but also not immediately obvious.

- DL

[KrisCarnmarker]

Note the qualification: "so long as you don't reduce the absolute exposure to do so.".

Yes, that is a given.



It's not too surprising but also not immediately obvious.


Well, tell that to MR, the author of the article I was referring to :) It contradicts his statement.

[John Sheehy]

Interesting. I think most people would be surprised to find this out. Most people, me included, will avoid higher ISOs if at all possible.

Note that I am talking about a fixed absolute exposure (dictated by subject luminance and Av and Tv values); not about dialing the ISO and letting the metering do its ISO-related absolute exposure adjustments.