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Author Topic: Better than ETTR ?  (Read 17518 times)
ndevlin
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« Reply #20 on: August 05, 2011, 01:58:59 PM »
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Ok, does anyone disagree with the following:

When one exposes a zone II shadow as zone V (to use real photography terms  Wink) and then subsequently places that detail back down into zone II in post-processing, that shadow is much 'cleaner' than if it had simply been exposed at zone II to begin with.  No, there aren't any more tonal values in the shadow areas this way, but there is a qualitative difference.

The same results hold true in a slightly different way for higher tonal values where noise is less of an issue with a 'straight' capture to begin with. But there, too, my anecdotal experience is that the 'extra' data makes a difference.  

If anyone has a different experience of this, I would like to know.

It makes sense that this is a result of obtaining data with a better s/nr at capture. But the 'why' doesn't really matter if the result is as described, does it?

- N.
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Graeme Nattress
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« Reply #21 on: August 05, 2011, 02:36:48 PM »
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Michael, I know what you mean about different cameras behaving differently, and also, you're forced to view the data through a raw converter which may not be performing optimally for that camera. However I don't know off-hand any cameras which have less bit-depth than their noise floor - usually it's the other way around where the bit depth of the ADC exceeds the noise capabilities of the sensor.

Graeme
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« Reply #22 on: August 05, 2011, 02:41:19 PM »
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"But the 'why' doesn't really matter if the result is as described, does it?" - not really. It's results that count :-) However, I always like to get to the bottom of things and I want to know "why?"

Basically - if there's insufficient bit depth (or not enough code values used to describe that "zone") we see banding or posterization. That is a bad situation to be in as we know that the ADC or (lack of) processing precision has lost us valuable image data. This is what the "less code values per stop in linear light data" argument says as to why we should ETTR. I don't know of a camera that performs this way.

The SNR argument says that if you can't see banding or posterization in the shadows, then the bit depth is sufficient given the noise floor of the sensor, and ETTR works by capturing more light and giving a better SNR.

Either way you ETTR - but the "why" is different.

Graeme
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FranciscoDisilvestro
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« Reply #23 on: August 05, 2011, 05:37:47 PM »
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Supporting SNR as the main reason for ETTR:

1.- Suppose (as a reduction to absurd) a ridiculously large bit depth, almost approaching infinity. Then, any stop value will have more than enough values so that the argument about number of values looses validity. The only argument left is SNR.

2.- Take for example a camera like a Nikon DSLR where you could select either 12 or 14 bit RAW. The highest f/stop in 12 bits will have 2048 levels, the same as 2 stops under in 14 bits mode. Do you think you will get the same quality doing ETTR in 12 bits than 2 stops below-ETTR in 14 bits mode, just because you get the same number of levels? I don't think so.


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pedro.silva
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« Reply #24 on: August 06, 2011, 04:13:32 AM »
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I donít think that works as well as you think.

Supporting number of levels as the main reason for ETTR:

1.- Suppose (as a reduction to absurd) a ridiculously small noise, approaching zero. Then, any stop value will have no noise, so that the argument about noise looses validity. The only argument left is number of values (levels).

2.- The highest f/stop would have no noise, but so would the lowest f/stop (and all others in between).  Do you think you would get the same quality doing ETTR as 2 stops below-ETTR mode, just because you get the same absence of noise? I don't think so (you would get a combed histogram, possibly leading to visible banding). 

What we need is sound theory that applies to actual current cameras, and these both produce noise and work with a fairly small number of levels.  The current balance suggests that the main reason ETTR works is by increasing SNR, but that could change in the future. 

Anyway, all of this may become moot if cameras ever expose different sensels for different amounts of time (recording the exposure times of course), for then all sensels would be exposed right (ERô) automagically with the lowest noise possible, and the number of levels would only matter in the final, converted, image. 

On a side note, Guillermo and others seem to think there is no merit in recording noise more accurately.  I think there may be.  The more accurately noise is recorded, the better job noise removers can do. 

Cheers!

Supporting SNR as the main reason for ETTR:

1.- Suppose (as a reduction to absurd) a ridiculously large bit depth, almost approaching infinity. Then, any stop value will have more than enough values so that the argument about number of values looses validity. The only argument left is SNR.

2.- Take for example a camera like a Nikon DSLR where you could select either 12 or 14 bit RAW. The highest f/stop in 12 bits will have 2048 levels, the same as 2 stops under in 14 bits mode. Do you think you will get the same quality doing ETTR in 12 bits than 2 stops below-ETTR in 14 bits mode, just because you get the same number of levels? I don't think so.
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ErikKaffehr
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« Reply #25 on: August 06, 2011, 07:09:14 AM »
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Hi!

Most of the noise is coming from the variation of the number of incident photons. So the only way to make a sensor noise free is to make it very large. Whatever you do, there will be noise, however. Even if you detect all photons, it is well possible that there is no photon to detect. For that reason a noise free sensor is not possible, because light by itself has noise. It comes in quantum size packages called photons. If thousands of photons hit each sensor cell we have good statistics, that is low noise, if only a few photons hit the sensel we have poor statistics and noise.

Best regards
Erik

I donít think that works as well as you think.

Supporting number of levels as the main reason for ETTR:

1.- Suppose (as a reduction to absurd) a ridiculously small noise, approaching zero. Then, any stop value will have no noise, so that the argument about noise looses validity. The only argument left is number of values (levels).

2.- The highest f/stop would have no noise, but so would the lowest f/stop (and all others in between).  Do you think you would get the same quality doing ETTR as 2 stops below-ETTR mode, just because you get the same absence of noise? I don't think so (you would get a combed histogram, possibly leading to visible banding). 

What we need is sound theory that applies to actual current cameras, and these both produce noise and work with a fairly small number of levels.  The current balance suggests that the main reason ETTR works is by increasing SNR, but that could change in the future. 

Anyway, all of this may become moot if cameras ever expose different sensels for different amounts of time (recording the exposure times of course), for then all sensels would be exposed right (ERô) automagically with the lowest noise possible, and the number of levels would only matter in the final, converted, image. 

On a side note, Guillermo and others seem to think there is no merit in recording noise more accurately.  I think there may be.  The more accurately noise is recorded, the better job noise removers can do. 

Cheers!

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cunim
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« Reply #26 on: August 06, 2011, 09:06:00 AM »
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At ambient temperatures, the proportion of noise varies with signal level.  It is higher at low signal levels.  That is why we use cooling with high bit depth quantization.  Cooled sensors linearize SNR across a very broad dynamic range so deep shadows have similar noise characteristics to midrange data.  Cryogenic pumps or liquid nitrogen combined with slow readout make for some pretty spectacular images.  Even low levels of cooling help, which is why small scientific grade cameras often use some kind of Peltier cooler.

Photographic cameras aren't cooled, both because of cost/mass issues and because the balance of thermal and read noise is different (can't use slow readout).  So, we look for signal processing approaches to noise reduction.  ETTR is one such (analog) approach.  It moves shadows into the detection range where noise is proportionally lower.  Michael's observations make sense.  Of course, there is no free lunch.  ETTR is a reduction in dynamic range.

Amplification (high iso) vs noise reduction is a whole nuther topic.  With monochrome detectors, noise reduction is better until you get near the photon counting range, and then amplification before quantization becomes more sensitive (though less efficient).  I have never really understood how it works in photographic cameras.
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Graeme Nattress
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« Reply #27 on: August 06, 2011, 09:21:57 AM »
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"ETTR is a reduction in dynamic range" - if you're not clipping then you're maximizing DR.

Graeme
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pedro.silva
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« Reply #28 on: August 06, 2011, 09:38:53 AM »
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No major disagreement there!

I never meant to imply that noise free sensors are possible, any more than Francisco meant that infinite bit depth is. I simply tried to show what I saw as a logical flaw in his reasoning.

However... "the only way to make a sensor noise free is to make it very large": that depends on how you define "noise free".  According to Poisson stats, the larger the value the larger the noise too (only the SNR gets better).  And better quantum efficiency could lead to better SNR too, not just larger sensors, rite? 

Hi!

Most of the noise is coming from the variation of the number of incident photons. So the only way to make a sensor noise free is to make it very large. Whatever you do, there will be noise, however. Even if you detect all photons, it is well possible that there is no photon to detect. For that reason a noise free sensor is not possible, because light by itself has noise. It comes in quantum size packages called photons. If thousands of photons hit each sensor cell we have good statistics, that is low noise, if only a few photons hit the sensel we have poor statistics and noise.

Best regards
Erik

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FranciscoDisilvestro
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« Reply #29 on: August 06, 2011, 10:35:31 AM »
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Cooled sensors linearize SNR across a very broad dynamic range so deep shadows have similar noise characteristics to midrange data.  Cryogenic pumps or liquid nitrogen combined with slow readout make for some pretty spectacular images.  Even low levels of cooling help, which is why small scientific grade cameras often use some kind of Peltier cooler.


Correct me if I'm wrong, but as far as I know, cooling the sensor reduces the read noise, but there is nothing you could do about shoot noise, which is an inherent property of light. Since photon noise is proportional to the square root of the photon count, then the higher the photon count, the higher the signal to noise to noise ratio

Even if you achieve zero read noise, you still have shoot noise. ETTR still holds no matter the explanation you prefer: the more levels, the more photons, the better SNR, you can actually derive one from another.

Suppose you could count individual photons, then thatīs your maximum required bith depth, maximum values will be the same as maximum photons and since there is a direct relation between photon count and SNR then it will also be maximum SNR
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cunim
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« Reply #30 on: August 06, 2011, 11:07:03 AM »
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Graeme, no need to get into a discussion DR.  LOTS of that elsewhere.  I will just make two statements and you can decide if one limits DR.

1.  I have 12 bit precision across a 10 stop range.
2.  I have 12 bit precision across 8 stops of a 10 stop range (ETTR).

Francisco, shot noise is a property of the detector package.  You reduce shot noise with cooling.  You reduce read noise in various ways, primarily by slowing the readout.  True 16-bit cameras tend to read out at about 1 kHz.

Really, none of this has much to do with whether or not to use ETTR.  That depends on the exposure lattitude that you need.  I was just suggesting that it is reasonable for shadows to open up better when ETTR is used. 
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BartvanderWolf
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« Reply #31 on: August 06, 2011, 11:44:45 AM »
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Correct me if I'm wrong, but as far as I know, cooling the sensor reduces the read noise, but there is nothing you could do about shoot noise, which is an inherent property of light. Since photon noise is proportional to the square root of the photon count, then the higher the photon count, the higher the signal to noise to noise ratio

Hi Francisco,

No correction needed, you a correct.

Cooling only helps to reduce the spontaneous generation of free electrons, and typically makes sense for longer exposure times than 1 second (assuming normal room temperatures). If the heat build-up inside the camera is significant enough (dark current doubles approx. every 6 degrees Celsius, or Kelvin) it makes sense to cool the sensor if it exhibits extremely low read noise, otherwse the dark current is swamped by the read noise anyway.

Shot noise is purely determined by the statistical properties of the photon flux.

Cheers,
Bart
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PierreVandevenne
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« Reply #32 on: August 06, 2011, 12:06:33 PM »
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_Read_ noise is _marginally_ affected by temperature. Cooling a sensor to extremely low temperature will lower it very slightly, until the sensor simply stops to work :-)

Dark current and read noise are two different things. I am sure you know that Bart, but I find the whole thread a bit imprecise.
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ErikKaffehr
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« Reply #33 on: August 06, 2011, 12:20:59 PM »
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Hi,

Shot noise has nothing to do with sensor, except what is known as full well capacity, the number of incident photons a sensor cell can account for.

ETTR is mainly about maximizing the number of photons detected, that is utilizing full well capacity. Regarding read out noise you are probably right.

The practical effects of ETTR are twofold:

1) Noise in midtones are reduced, that actually may also mean that you can sharpen better as sharpening also enhances noise.

2) SNR (Signal Noise Ratio) in the shadows will be improved, so shadows respond better to "opening up"

As a remark I don't see any benefit of going to full 16 bit signal path. Sensor FWC (full well capacities) are rather going down than up. With present FWCs the lower bits will be dominated by noise anyway. Shoot noise will be significant and readout and other noises will be added to that.

Best regards
Erik


Francisco, shot noise is a property of the detector package.  You reduce shot noise with cooling.  You reduce read noise in various ways, primarily by slowing the readout.  True 16-bit cameras tend to read out at about 1 kHz.

Really, none of this has much to do with whether or not to use ETTR.  That depends on the exposure lattitude that you need.  I was just suggesting that it is reasonable for shadows to open up better when ETTR is used. 

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John Camp
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« Reply #34 on: August 07, 2011, 12:06:25 PM »
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Ok, does anyone disagree with the following:
When one exposes a zone II shadow as zone V (to use real photography terms  Wink) and then subsequently places that detail back down into zone II in post-processing, that shadow is much 'cleaner' than if it had simply been exposed at zone II to begin with.  No, there aren't any more tonal values in the shadow areas this way, but there is a qualitative difference.
The same results hold true in a slightly different way for higher tonal values where noise is less of an issue with a 'straight' capture to begin with. But there, too, my anecdotal experience is that the 'extra' data makes a difference.
If anyone has a different experience of this, I would like to know.
It makes sense that this is a result of obtaining data with a better s/nr at capture. But the 'why' doesn't really matter if the result is as described, does it?
- N.

"But the 'why' doesn't really matter if the result is as described, does it?" - not really. It's results that count :-) However, I always like to get to the bottom of things and I want to know "why?" <BIG SNIP>Either way you ETTR - but the "why" is different.

Graeme

Okay, this is what I was getting at in the other thread -- the prescriptive mode. "Either way, you ETTR."

So generally, if I ETTR, at least modestly, checking the histogram to try to make sure that I haven't blown any channels, I'll USUALLY be better off than if I just go ahead and plug away on auto-exposure (assuming that I then take the RAW image into LR for adjustment.) Right? I think that's what Michael was saying in the original article, and that's all I was really trying to determine.




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Wayne Fox
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« Reply #35 on: August 07, 2011, 02:30:03 PM »
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Graeme, no need to get into a discussion DR.  LOTS of that elsewhere.  I will just make two statements and you can decide if one limits DR.

1.  I have 12 bit precision across a 10 stop range.
2.  I have 12 bit precision across 8 stops of a 10 stop range (ETTR).


over simplistic and misleading.

ETTR is not about the dynamic range of your sensor.  It is about improving the quality of the data captured by the sensor by utilizing a different methodology in determining exposure settings than those built into cameras, which as Michael pointed out are engineered for use based on the characteristics of film dating back many decades. The dynamic range of the scene and that of the sensor to record the scene are what they are.

If in fact you can open your camera up two stops without clipping any highlights from the exposure setting your camera recommends and you end up with room to the left of your histogram, this simply means the scene itself has less dynamic range than your sensor, it doesn't mean you have limited your cameras ability to capture DR.

Additionally, if you can open your camera up two stops without clipping, and your data still extends all the way to the left of your histogram this implies you have more fully utilized the DR of your sensor, not decreased it, since some of the clipped shadow data is no longer clipped ... in effect you have increased the DR of your capture.
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Guillermo Luijk
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« Reply #36 on: August 07, 2011, 06:12:26 PM »
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On a side note, Guillermo and others seem to think there is no merit in recording noise more accurately.  I think there may be.  The more accurately noise is recorded, the better job noise removers can do.
Can you show us an example where a noise remover did a better job thanks to having more levels?.

On the other hand, if you have more levels through ETTR, you also have better SNR, so talking about noise removers doing a better job is quite surrealistic because you cannot have more levels but the same SNR! Grin

In the article: DO RAW BITS MATTER? I developed some RAW files with a decreasing number of bits (i.e. I rounded RAW numbers before demosaicing to emulate ADC with less bits).

For the Canon 40D, 12bits showed to be enough. The extra 2 bits didn't improve useful information recorded:

Canon 40D, 14 bits RAW development


Canon 40D, 12 bits RAW development



But the Pentax K5, being a much lower noise camera, really needed the 14 bits. A 12-bit RAW development started to display posterization in the deep shadows:

Pentax K5, 14 bits RAW development


Pentax K5, 12 bits RAW development
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cunim
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« Reply #37 on: August 07, 2011, 06:20:02 PM »
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over simplistic and misleading.


Story of my life.  I plead guilty to being simplistic, but misleading - I don't think so.  

If I may summarize my reading of this thread - ETTR stays below clipping while raising the shadows.  People see an improvement for the shadow data as a result.  This discussion centers on why that improvement occurs.  My own belief is that signal is proportionally higher than noise (various forms of electrical noise, flare, etc.) in the shifted data.  Pretty simple.

The dynamic range compression is peripheral to all this but I will try to make up for the simplistic bit.  Without ETTR a camera might give us a 1000:1 range between white and black clipping - at our desired level of precision.  With ETTR it might be 800:1 because we concede that shadow areas are not as good as we would like.  Of course, the camera response does not change.  It still covers the same range it always did so I can understand the position that the device DR is the same.  However, the image luminance range is not.  It is narrower.  You can't use ETTR with full range data.

Contrast the photographic camera with a scientific imager in which it is possible to tweak various physical parameters to move all of your precision into a narrower range.   It seems to me that ETTR is the same in principle.  We can't alter the response of the camera to achieve higher precision over a limited range, but we can still see some benefits from altering exposures to exclude the bottom of the response curve.  To me, that is a reduction in DR.  I agree that the reduction should not matter as long as our images only contain the norrower range of data.





  
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Ray
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« Reply #38 on: August 07, 2011, 10:39:31 PM »
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Here we are again on a perennially popular topic with an acronym suggestive of aliens from  outer space.  Grin

Getting the best and most appropriate exposure for a particular shot has always been a basic technical requirement for serious photographers.

However, it needs to be stressed that the best exposure is not necessarily the exposure which maximises the photon count and produces the lowest noise in the shadows, ie. an ETTR.

The conditions for an ETTR are generally constrained by DoF requirements, subject movement and the intensity of available light (in the absence of flash).

In fact, I would say that achieving an ETTR, in the sense of maximising the photon count, may be the last consideration.

Choosing the appropriate aperture for the desired DoF, and a shutter speed sufficient to freeze both camera and subject movement, is surely of greater priority.

Only after having selected an appropriate shutter speed and aperture should one then address the implications of ETTR, which may mean increasing shutter speed at base ISO to avoid overexposure, or increasing ISO. With a camera like the D7000 or K5, there's really no need to increase ISO. If the desired aperture and minimum shutter speed for a sharp result, also result in an underexposure at base ISO, then so be it. It can't be helped.

Of course, if one has the luxury of time on one's side, if the subject is static, and the camera is on tripod, then there is surely no problem regarding 'correct' exposure.

The problem of ETTR arises when one doesn't have sufficient time to manually get the settings right for a particular scene because one is trying to 'capture the moment'. In these circumstances, an adjustable feature in the camera that would guarantee an ETTR could be useful.

However, such a feature would also have its own problems. It would be another camera adjustment to get right, and when it wasn't right, the shot might be ruined.

To give you an example. Supposing the automatic ETTR exposure adjustment was set at its maximum setting which ignores very small percentages of the frame, which we would describe as specral highlights. Supposing you are walking through the wilderness and suddenly encounter a rare or interesting bird, or animal, in the shade of some foliage, with significant areas of sky visible through the foliage.

I would suggest there would be two likely results. (1) You forget your camera is set to ignore only specral highlights for an ETTR, take the shot, the bird flies away at the sound of the shutter, and you are left with a lovely shot of deep blue sky shining through the foliage, and a noisy bird in the shadows.

(2) You remember the ETTR setting is not appropriate for the shot, and try to estimate the area of the frame that consists of bright sky which is of secondary importance, so you can make the appropriate adjustment in the camera's ETTR feature. Alas! The bird has noticed you and flies away before you take the shot.

The only solution I see to this problem is to bracket every shot you take, unless you have the luxury of time on your hands to be confident you have made the right settings for an ETTR.

There can be other advantages of bracketing exposure (or ISO) when trying to capture the moment. One of the other shots, either the overexposure or the underexposure, may be preferred irrespective of its degree of closeness to the ideal ETTR, simply because the subject has moved to a more interesting position, or adopted a more interesting facial expression during that one whole second that many cameras need for 3 frames.
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Wayne Fox
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« Reply #39 on: August 08, 2011, 12:04:48 AM »
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The dynamic range compression is peripheral to all this but I will try to make up for the simplistic bit.  Without ETTR a camera might give us a 1000:1 range between white and black clipping - at our desired level of precision.  With ETTR it might be 800:1 because we concede that shadow areas are not as good as we would like.  Of course, the camera response does not change.  It still covers the same range it always did so I can understand the position that the device DR is the same.  However, the image luminance range is not.  It is narrower.  You can't use ETTR with full range data.
It is no secret that at black clipping we have weak data.  If our dynamic range is less than the sensor, moving all the data to the right records that data with more precision and less noise.  We do not lose any data in the highlights.  The data is not full range, that's why you can use EttR.  No one ever stated that you should use EttR with every image you take, and I believe anyone who uses it will say it has little use if the DR of the scene equals or exceeds the DR of the sensor.  However, in a great many shooting situations with sensor dynamic ranges of todays cameras, you will have plenty of head room to move your data to the right.  Yes, there may be no data at the black clipping point, but so what? You still have the same relationship of the white  point of the scene to the black  point of the scene.  The fact they don't match the clip points of the sensor really doesn't matter.



Here we are again on a perennially popular topic with an acronym suggestive of aliens from  outer space.  Grin

Getting the best and most appropriate exposure for a particular shot has always been a basic technical requirement for serious photographers.

However, it needs to be stressed that the best exposure is not necessarily the exposure which maximises the photon count and produces the lowest noise in the shadows, ie. an ETTR.

The conditions for an ETTR are generally constrained by DoF requirements, subject movement and the intensity of available light (in the absence of flash).

In fact, I would say that achieving an ETTR, in the sense of maximising the photon count, may be the last consideration.

Choosing the appropriate aperture for the desired DoF, and a shutter speed sufficient to freeze both camera and subject movement, is surely of greater priority.

Only after having selected an appropriate shutter speed and aperture should one then address the implications of ETTR, which may mean increasing shutter speed at base ISO to avoid overexposure, or increasing ISO. With a camera like the D7000 or K5, there's really no need to increase ISO. If the desired aperture and minimum shutter speed for a sharp result, also result in an underexposure at base ISO, then so be it. It can't be helped.

Of course, if one has the luxury of time on one's side, if the subject is static, and the camera is on tripod, then there is surely no problem regarding 'correct' exposure.

The problem of ETTR arises when one doesn't have sufficient time to manually get the settings right for a particular scene because one is trying to 'capture the moment'. In these circumstances, an adjustable feature in the camera that would guarantee an ETTR could be useful.

However, such a feature would also have its own problems. It would be another camera adjustment to get right, and when it wasn't right, the shot might be ruined.
I don't think anyone suggests that EttR has replaced aperture/shutter speed/ISO as the priority settings in determining an exposure.  All that is suggested is that an exposure based on EttR often will leave you with better raw data than the setting chosen by your camera, if you have the headroom and don't have to compromise those other areas.

Unfortunately there is absolutely no easy way to use EttR with current cameras.  You have to shoot, examine histo, and adjust (and your guessing at that because the histo isn't telling you enough about the raw data).  But using metering/exposure techniques designed for non-linear film is also not optimal.  It works because it almost always errs on the underexposure side, and certainly there are times and photographic situations where speed is critical.  But even in somewhat time sensitive shooting situations, an exposure system that based it's estimate on EttR would make it effective in more situations than one might think.

It does seem that designing a metering system based on EttR might have been challenging until LiveView was introduced. But it seems quite possible now, and as suggested in the article could certainly be included as an option.  Cameras with live histograms make EttR really quite easy, seems some engineer could analyze the histogram in firmware and recommend or choose a setting based on that much like I can by viewing the histo and changing the settings on my point and shoot. Of course first the camera makers need to understand that histogram's based on the raw data are important, and at least provide that as an option.
« Last Edit: August 08, 2011, 03:33:58 AM by Wayne Fox » Logged

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