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Author Topic: Can you expose too far to the right even if not clipping?  (Read 14011 times)
FranciscoDisilvestro
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« Reply #20 on: December 10, 2010, 01:27:37 PM »
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I'll try to give a numerical example of why ETTR is not a flawed concept,

For simplicity, let's suppose that the possible values for the RGB channels go from 0 to 100. This are the linear values as output from the A/D converter.

The raw data for any given image will be values from 0 to 100 in any channel.

Suppose that as a result of a given exposure we have the following ranges:

R= from 1 to 35
G= from 2 to 48
B= from 1 to 25

If I input those values to a Raw converter, I will get a result, which I'll call result A

Now, let's just multiply all values by 2, we get:

R= from 2 to 70
G= from 4 to 96
B= from 2 to 50

After input this values to the same Raw converter, I will get result B

Now, if we consider both sets of data coming from a black box, we could say that the difference between both is 1EV or 1 stop

How to get output A from the second set of data? Two choices:

1) Divide by 2 all values (which will result in the exact first set of data)
2) Reduce exposure by 1EV in the Raw converter

In 1) you will get the exact result A, no question about it.

In 2) you may or may not get the same result than A, since it depends on how and when exposure correction takes place in the Raw converter

Now, going back to the black box, if we didn't know that the second set of data is just the first set multiplied by 2, we could suppose:

a) The difference is due to a 1EV exposure (time or aperture)
b) The difference is due to different ISO (double)
c) The difference is due to ETTR exposure (note that now the range of G = 4 to 96, covering almost all possible values)

Up to this point, Is there anything on the data that will compromise color rendition? NO, nothing.

If we knew the exact proportion of the ETTR exposure related to the "correct" exposure and divide all raw values by that factor, then we will have as a result the same raw values as if we used the "correct" exposure, but with less noise.

So let me change the conditions of the example: The second set of data is a the result of ETTR exposure which correspond to twice the correct exposure, then just divide the second set by 2 and you get the first one.
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joofa
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« Reply #21 on: December 10, 2010, 01:40:20 PM »
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The linearity of a sensor is an assumption in ETTR. Please see the following post on this topic:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=37133825

and Iliah Borg's response that includes a link that shows some non-linearity of a sensor:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=37136263

Joofa
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NikoJorj
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« Reply #22 on: December 10, 2010, 02:15:57 PM »
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a link that shows some non-linearity of a sensor:
Let's quote it :
http://www.maxmax.com/nikon_d700_study.htm
The linearity is not perfect in these curves... And it would be very interesting to know from what that comes.

NotNote (sorry for the typo Embarrassed), of course that the big non-linearity in the red channel comes from clipping.
However, the behaviour just before saturation is strongly non-linear : one point made.
« Last Edit: December 10, 2010, 02:37:04 PM by NikoJorj » Logged

Nicolas from Grenoble
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« Reply #23 on: December 10, 2010, 02:18:18 PM »
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But there is one situation where ETTR can help - when you're already at the lowest ISO setting you camera offers.

Sorry but I have to say “duh”. We all know raising ISO produces more noise and often we need that trade off. And by using the base ISO and implementing ETTR, Sandy shows and agrees the result is less noise because at this ISO, using proper exposure for raw results in less noise in the shadows. If that’s something you desire, you should use it.

We analog shooters know from experience we had to test or film and processing to determine the true (optimal) ISO. We also knew that if we pushed our film (because sometimes we had too), the result was kind of the equivalent today of raising the ISO, more “noise”. Sometimes it just had to be done (push processing beat an under exposed chrome by a mile and we lived with the trade off).

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While the G10 has a ISO 100 setting it doesn't have a ISO 50 setting. (It does a ISO 80 setting, but I'm ignoring that as it's too close to 100 to make much of a difference.) So what ETTR is doing here is allowing us to synthesize a lower ISO setting, and hence a better noise performance, than the camera actually has. The disadvantage of course is that the camera's dynamic range is reduced by one stop, but if you have a low contrast image, that might be a price worth paying.

What makes this affect the dynamic range?

And yes, the entire idea is better noise performance (and we could say, the optimal, true (?) ISO).
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Andrew Rodney
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FranciscoDisilvestro
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« Reply #24 on: December 10, 2010, 02:18:49 PM »
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Thanks for those links Joofa, very interesting information.

Absolutely right, if the response is not linear, ETTR just don't make any sense
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Chris_Brown
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« Reply #25 on: December 10, 2010, 02:27:05 PM »
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I think this brief article explains it well. The recovery of detail in the Niagara Falls shot is noteworthy.
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« Reply #26 on: December 11, 2010, 12:19:15 AM »
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Sorry but I have to say “duh”. We all know raising ISO produces more noise and often we need that trade off. And by using the base ISO and implementing ETTR, Sandy shows and agrees the result is less noise because at this ISO, using proper exposure for raw results in less noise in the shadows. If that’s something you desire, you should use it.

Andrew, yes, agree, it is "duh" - to you, me, and probably (maybe?) many of the readers of this particular forum. But it sure isn't at all clear to a huge number of ETTR proponents.....

Sandy
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« Reply #27 on: December 11, 2010, 06:42:59 AM »
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Yes, Francisco, that's what Sandy's article shows but it also shows that there's really no difference in noise by using a normal exposure at a lower ISO setting.  Leaving us with the idea, as he outlined, that the only situation where ETTR may be useful is when you're already at base ISO and can't go any lower.

Joofa, yes and I corrected myself by saying the communication of ETTR in so far as there being more data in highlights in non-nominal terms was flawed.  Sorry for the confusion.

Anti-blooming circuitry on 'modern' sensors will cause a slight non-linearity in the response at the top of the curve (line).
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FranciscoDisilvestro
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« Reply #28 on: December 11, 2010, 07:41:18 AM »
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Non-linearity is a very strong point against ETTR, so strong as to relegate it to a theoretical exercise

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it also shows that there's really no difference in noise by using a normal exposure at a lower ISO setting

The difference in noise would be more apparent in the deep shadows or in one color channel if the light source is very far from white (like a low power incandescent light, where a normal exposure will result in a very underexposed blue channel)
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ejmartin
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« Reply #29 on: December 11, 2010, 07:43:44 AM »
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Sorry but I have to say “duh”. We all know raising ISO produces more noise and often we need that trade off. And by using the base ISO and implementing ETTR, Sandy shows and agrees the result is less noise because at this ISO, using proper exposure for raw results in less noise in the shadows. If that’s something you desire, you should use it.

I have to quibble with the wording here.  Raising the ISO does not 'produce more noise' (ie lower the SNR); rather, lowering the exposure lowers the SNR ('produces more noise').  Only in an indirect sense does raising the ISO result in more noise -- that clipping in the raw data occurs at a lower absolute exposure, so if the ISO is raised one may have to lower the absolute exposure to prevent clipping.

It is a fact that in most modern CMOS sensored cameras, raising the ISO actually lowers the noise relative to absolute exposure -- any noise introduced after the ISO amplification is smaller relative to signal when the amplification is raised.  This can be quite significant in Canon DSLRs and the Nikon D3/D3s/D700 at low ISO, and is the major reason why there is a benefit to raising the ISO in low light in raw shooting.
« Last Edit: December 11, 2010, 09:13:44 AM by ejmartin » Logged

emil
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« Reply #30 on: December 11, 2010, 03:31:10 PM »
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I think the point Andrew was making was that nobody (that I know of) recommends using ETTR at higher ISOs. The whole point of raising ISO is to get a faster shutter speed, so why would you then go an increase your exposure time just to ETTR?
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Luis Argerich
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« Reply #31 on: December 11, 2010, 04:33:14 PM »
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I think the point Andrew was making was that nobody (that I know of) recommends using ETTR at higher ISOs. The whole point of raising ISO is to get a faster shutter speed, so why would you then go an increase your exposure time just to ETTR?

Because you need a certain exposure time?
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Jim Pascoe
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« Reply #32 on: December 13, 2010, 04:06:51 AM »
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Looking at the OP and assuming that the object of the exercise is to get an image that resembles the subject matter, the author says that his subject was mainly dark browns.  Now if I am photographing a black cat in coal bunker, personally I would under-expose by one to two stops.  In my humble experience with digital (and film), this would give me the final result desired. Using ETTR would just not be logical - surely?  I can understand the concept for more typical scenes where very light tones are present however.

Jim
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FranciscoDisilvestro
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« Reply #33 on: December 13, 2010, 10:06:01 AM »
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Now if I am photographing a black cat in coal bunker, personally I would under-expose by one to two stops.  In my humble experience with digital (and film), this would give me the final result desired. Using ETTR would just not be logical - surely? 

Jim

I guess you are refering to a reflective metering of the scene. If you use a graycard or incident metering, then you would not underexpose. This would give you the desired result if you want a final image straight out of the camera. ETTR capture will require postprocessing to get the desired results.

Suppose you don't underexpose from a reflective metering, the result will be a light gray cat on a light gray background instead of the dark grays you are expecting. To be able to obtain the desired result, you will have to compensate exposure in the Raw converter by -2 EV.

An ETTR approach in your example could mean overexposing the reflected measure by 1 or 2 stops (as long as you don't blow any channel) and then compensate exposure by -3 or -4 EV in the Raw processing.

As it was mentioned before, this would work only if the response of the sensor + A/D converter were perfectly linear, so in practice this cannot be guaranteed.

If you think about film, something analog to ETTR was actually performed regularly between negative film users. It was common to overexpose color negative film by 1/3 or 2/3 EV and then compensate in the darkroom. This was due to the characteristics of color negative film: difficult to blow highlights and quality degrades quickly in the shadows.

This of course could not be applied to positive film. you had to use correct exposure to get the desired results.
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Jim Pascoe
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« Reply #34 on: December 13, 2010, 10:43:32 AM »
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Francisco

Yes, of course I was referring to a reflected reading, which is how I now work most of the time, using exposure compensation and experience to get a decent result.  In the past with film I did use an incident meter for almost everything and I can see that if I was to use the same method now it might well make sense to play safe and ETTR to some extent.  I guess with the sort of shooting I now do it is quicker (for me) to rely on the DSLR meter and just use compensation as needed.

Jim
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ejmartin
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« Reply #35 on: December 13, 2010, 10:51:15 AM »
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I think the point Andrew was making was that nobody (that I know of) recommends using ETTR at higher ISOs. The whole point of raising ISO is to get a faster shutter speed, so why would you then go an increase your exposure time just to ETTR?

No, as far as the raw data quality is concerned, the point of raising ISO is to get a lower read noise (better SNR) for a given shutter speed and aperture, provided one is not clipping any highlights one doesn't want clipped.  For those cameras whose DxO DR curve is not a straight line but flattens at low ISO, if the exposure fits within the higher ISO DR then you are better off using it.  

One way to think of it is as follows -- here is the SNR plot of my 1D3 for various ISO, but instead of always placing raw saturation at the same point, let's align them according to absolute exposure:



When the ISO is doubled, one more stop or EV of input is pushed past the saturation point of the ADC and is lost; thus each subsequent one stop increase of ISO removes one stop of highlight exposure. At the low end is the noise "floor", really the point at which one chooses to quit using the sensor data because the signal/noise ratio becomes too poor. This floor varies with ISO, and thus how much room on the noise floor is available depends on ISO. From the figure we see that, in normalized exposure terms, the floor is lowered with increasing ISO.  

So if one has chosen a particular exposure, set by one's needs for DOF and subject motion, one then wants to select an ISO that fits the scene's brightness range so that not too much is clipped on the right, and as much SNR as possible is available on the left.  The camera's meter suggests an exposure for a particular ISO, but if one can get away with raising the ISO, esp at low ISO, then more shadow SNR is available and if the scene brightness range is narrow enough that the ISO chosen is such that the meter is indicating too high an exposure, that can be considered ETTR.  

Of course, better capture SNR always involves increasing the exposure as much as possible to get more photons, and lowering the ISO if needed to preserve highlights.  Finally, there are cameras that don't have this flattening of the DR at low ISO (in particular, those using Sony EXMOR sensors) for which the benefit to raising the in-camera ISO is much, much less; for them, there is little or no point to ETTR, or for that matter raising the ISO at all much over base ISO.
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emil
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« Reply #36 on: December 14, 2010, 08:11:33 AM »
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if sensors were nonlinear, hdr would be one application where it could easily be detected?

-h
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Guillermo Luijk
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« Reply #37 on: December 15, 2010, 01:28:48 PM »
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So it depends on the Raw converter if it scales the ETTR exposure down to the proper/required level before mapping it to a colorspace. Only then, with a colorspace assigned, will the change of exposure potentially impact the saturation due to the remapping model in the given colorspace coordinate system.

Bart, I see no reason for this as long as the colourspace mapping is linearly done (as in DCRAW and I guess most RAW developers) with a typical 3x3 matrix operation. So there is no reason to expect any saturation/hue change because of exposure adjustments done afterwards.

The effects that can lead to this are different from the colourspace conversion: white balance if not applied linearly (in many RAW developers WB is again a linear operation), and specially any contrast/bright curve applied that delinearized the image, or any other non-linear processing done to the image.

But if you just perform a basic RAW development: linear white balance, demosaicing, and output colourspace linear conversion, there should be no difference in adjusting exposure before of after this RAW development. After all is all about scaling the RGB values by the same linear multiplying factor (the non-standard gamma curve in sRGB would need specific adjustments to accomplish this operation properly though. In standard gamma spaces just a linear factor should model a correct exposure correction).

Regarding the main question of this thread, I found my 350D linear in all three channels beyond what the eye can distinguish as long as no RAW channel gets clipped. Once some channel clips, it can affect linearity on the remaining channels, because of the sensor readout electronics I guess.



However in a properly ETTR'ed RAW file there should be no clipping in the RAW file, so this shouldn't be a problem.

These two images have the same tones, just different noise because they were differently exposed:





Regards
« Last Edit: December 15, 2010, 01:53:19 PM by Guillermo Luijk » Logged

joofa
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« Reply #38 on: December 15, 2010, 02:08:33 PM »
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At capture time, there is no color space (with its particular gamut hull) defined. ...Only after all these operations is the data set mapped to a color space (a coordinate system),

A little OT to the OP but there is a notion of color space at the capture time, and there is a defined relationship between the standard color spaces (which you refer to those having a 'corordinate system').  Please see the image below to see the geometry of the setup:



For more information, please see:

http://forums.dpreview.com/forums/read.asp?forum=1018&message=36956797

Joofa
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sandymc
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« Reply #39 on: December 15, 2010, 02:37:15 PM »
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Bart, I see no reason for this as long as the colourspace mapping is linearly done (as in DCRAW and I guess most RAW developers) with a typical 3x3 matrix operation. So there is no reason to expect any saturation/hue change because of exposure adjustments done afterwards.

Yes, completely sound theory.

But the point is, the mapping is not linearly done anymore - three years ago, conversion was linear for most raw converters. It's not anymore - LR and ACR have been non-linear ever since the second generation camera profile with 3D HueSatMap and Look tables that enabled "hue twists" came in. C1 don't publicly document their pipeline, but almost certainly they're the same.

Sandy
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