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Author Topic: Will Michael revisit ETTR?  (Read 52765 times)
Ray
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« Reply #20 on: August 15, 2011, 07:34:46 PM »
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It will have a green cast, but so what? At least the histogram will be more accurate. Is there some reason you think you need a color-accurate preview on the LCD? It's not like the LCD's are highly calibrated, anyways. If you're judging color based on the LCD preview you're asking for trouble.


So what? No problem at all for me. But some of us interact with others when we go out shooting. I find people often ask to see some of the shots I've taken, especially if the shots are of them.

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Any RAW converter worth using will allow you to set the WB without regard to the in-camera WB setting, so I'm not really sure what you're getting at here.

If there's no problem, then fine. I didn't write there was, just that there might be, and the reason I thought there might be is because in the thread on this subject, that Guillermo linked to above, there was mention that earlier versions of Photoshop did not allow for sufficiently extensive adjustments to be made to correct the WB of an image where a magenta filter had been used to equalise the levels of R, G & B.

I personally I'm not interested in spending more time than I need to when processing images. I usually get satisfactory results with camera set to Auto WB.  If there's a quick and easy method of restoring the correct WB of a UNIWB image, then fine. Tell me. I''m all ears.
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madmanchan
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« Reply #21 on: August 15, 2011, 10:46:25 PM »
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My general guideline is to know how the indicators on the camera relate to raw channel clipping.  Unfortunately this relationship is not clear (this is where the camera vendors could help us).  For my personal 5D Mark II, for example, the spot meter is placed about 3 1/3 stops below the raw green channel clipping, under common illumination conditions.  So to ETTR, I point the spot meter at the brightest area in the scene I wish to photograph, and open up 3 stops more than the suggested exposure value.  That's it.  Then in postprocessing, I adjust the (software) exposure to whatever is needed to achieve the overall desired image brightness.

Technical users (comfortable with dcraw) can do controlled tests to check how their in-camera meter relates to raw channels clipping. 

Non-technical users can also do controlled tests using a fixed scene (a target with tones, such as a ColorChecker or stepwedge can help).  Take a set of bracketed exposures, and note the in-camera spot meter reading for each.  Gradually increase the exposure.  Afterwards, try to postprocess all of them to have the same overall image brightness.  At some point, your brighter exposures will start losing highlight detail (because they clipped) or start having color shifts.  That means you've gone too far.  The brightest exposure that avoids these problems is the +delta you want to use.

Eric
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John.Murray
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« Reply #22 on: August 16, 2011, 12:16:48 AM »
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For my personal 5D Mark II, for example, the spot meter is placed about 3 1/3 stops below the raw green channel clipping, under common illumination conditions.

Now thats sobering.....
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Guillermo Luijk
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« Reply #23 on: August 16, 2011, 03:24:01 AM »
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For my personal 5D Mark II, for example, the spot meter is placed about 3 1/3 stops below the raw green channel clipping, under common illumination conditions.
(...)
Technical users (comfortable with dcraw) can do controlled tests to check how their in-camera meter relates to raw channels clipping.

For my classic Canon 5D I obtained the same headroom as you:


Lighting conditions were not Daylight or any normalized lighting but my sitting room's Tungsten though.

Regards
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Ray
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« Reply #24 on: August 16, 2011, 06:02:13 AM »
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There's another implication of the UNIWB method which seems unhelpful in many situations.

As I understand it, but please correct me if I'm wrong, the UNIWB method allows for a greater exposure, without clipping highlights, than one would usually be able to use.

It's basically like taking a DSLR that has a true base ISO of 100, and giving it a true base ISO of 50. One consequently gets approximately (maybe) a one stop DR and SNR advantage.

Now this is fine if a slow shutter speed is okay. In a sense, it's a method of turning a 35mm DSLR into an MFDB with regard to DR and noise at base ISO.

The problems arise when slow shutter speeds are not appropriate, and one needs to either increase ISO or underexpose.

In these situations, the UNIWB, it seems to me, is of no value.

If you are not sure about this, let's consider a specific example. I need (or want) a shutter speed of 1/200th at F8. The lighting conditions determine that I need to go to ISO 400 to get an ETTR, using Auto WB.

Guillermo's method suggests I can use 1/100th sec exposure at such an aperture and ISO, and consequently achieve lower noise. But 100th sec is too slow. I need to use 1/200th sec.

So, Guillermo's method suggests that I can use 1/200th at ISO 800, without blowing highlights.

Now my question is, why should 1/200th at ISO 800, with UNIWB, be better than 1/200th at ISO 400, using the camera's Auto WB?

I hope no-one is getting a headache reading this.

Or to put it another way, why should 1/200th at ISO 400, with Auto WB and correct ETTR, be worse than 1/200th with UNIWB at ISO 400, underexposed by one stop?
« Last Edit: August 16, 2011, 06:44:55 AM by Ray » Logged
JeffKohn
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« Reply #25 on: August 16, 2011, 08:33:56 AM »
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Ray using UniWB in no way dictates your exposure, in fact you don't have to be using ETTR to benefit from it. UniWB gives you a more accurate indication of the actual RAW exposure for each color channel - nothing more, nothing less. If you don't see the value in that, maybe UniWB isn't for you. But your arguments "against" UniWB in this thread seem more than a little silly to me.

If you're happy with with Auto-WB then by all means keep using it. I would never leave such an important creative decision up to a camera's automatic processing, but that's just me.
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Guillermo Luijk
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« Reply #26 on: August 16, 2011, 08:37:31 AM »
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As I understand it, but please correct me if I'm wrong, the UNIWB method allows for a greater exposure, without clipping highlights, than one would usually be able to use
(...)
Now my question is, why should 1/200th at ISO 800, with UNIWB, be better than 1/200th at ISO 400, using the camera's Auto WB?

Ray, I think you have some misconception about UniWB. UniWB doesn't allow greater exposure, it simply provides more reliable histograms. Shooting at a given ISO, aperture and shutter, will end with the same RAW data no matter if you use UniWB or any other arbitrary WB setting.

Regarding your example: 1/200 at ISO800 (with any WB), will be better than 1/200 at ISO400 (again, with any WB) because the second shot will have less visible noise (not much less though). Using or not using UniWB will simply help you to be aware with more precision if highlights are clipping.

On the other hand using some exposure or another will end in different motion blurring or DOF, and it's the photographer who has to choose exposure paremeters according to the entire application requirements. UniWB is just a tool to more accurately know how much you are exposing, not a guide who decides exposure.

Regards
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BartvanderWolf
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« Reply #27 on: August 16, 2011, 08:47:20 AM »
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If you are not sure about this, let's consider a specific example. I need (or want) a shutter speed of 1/200th at F8. The lighting conditions determine that I need to go to ISO 400 to get an ETTR, using Auto WB.

Fine, but that 'determination' has no ETTR basis.

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Guillermo's method suggests I can use 1/100th sec exposure at such an aperture and ISO, and consequently achieve lower noise. But 100th sec is too slow. I need to use 1/200th sec.

Thus, you won't ETTR but underexpose by 1 stop (relative to ETTR).

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So, Guillermo's method suggests that I can use 1/200th at ISO 800, without blowing highlights.

Yes, but as a second best choice! Best is to ETTR (nothing beats real photons), if you cannot do that then it's better to boost the ISO to 800 than just underexpose at ISO 400.

Do note that with these small 1 EV differences, the improvement can be subtle if any (depending on e.g. camera).
 
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Now my question is, why should 1/200th at ISO 800, with UNIWB, be better than 1/200th at ISO 400, using the camera's Auto WB?

It's not the UNIWB but the resulting decision to safely allow more photons (which requires freedom to increase/maximize photon count) into the equation that makes a difference. The WB is just metadata, and of no real consequence for the Raw conversion.

Cheers,
Bart
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Ray
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« Reply #28 on: August 16, 2011, 09:02:26 AM »
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Ray, I think you have some misconception about UniWB. UniWB doesn't allow greater exposure, it simply provides more reliable histograms. Shooting at a given ISO, aperture and shutter, will end with the same RAW data no matter if you use UniWB or any other arbitrary WB setting.

Regarding your example: 1/200 at ISO800 (with any WB), will be better than 1/200 at ISO400 (again, with any WB) because the second shot will have less visible noise (not much less though). Using or not using UniWB will simply help you to be aware with more precision if highlights are clipping.

On the other hand using some exposure or another will end in different motion blurring or DOF, and it's the photographer who has to choose exposure paremeters according to the entire application requirements. UniWB is just a tool to more accurately know how much you are exposing, not a guide who decides exposure.

Regards

Really! So it's all a storm in a tea cup?  I had some sort of concept that due to the application of an automatic white balance, the green channel might clip whilst the red and blue channels were well underexposed.

I thought, by applying a UNIWB, the channel most susceptible to clipping would be brought back into line with the other channels, allowing for a greater exposure.

I had some concept that the blue or red channel might be around one stop underexposed when the green channel was at an ETTR, for example. Or in some cases, as in a sunset, the red channel would be at an ETTR when the other channels would be underexposed.

So I've got it all wrong, then.  Grin
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kwalsh
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« Reply #29 on: August 16, 2011, 09:47:17 AM »
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Ray,

What might be confusing you is that there are a few photographers who will use external color correction filters to equalize the channel histograms and in this case your concerns are valid.  What everyone here is talking about for UniWB is just setting up a WB in the camera to make the in camera histograms better reflect the RAW histograms.  As noted, this doesn't change the RAW data at all.  There is one notable exception, I believe it is the D200 or some other Nikon camera of that era (D2X, D2S?  I can't remember), in which the camera WB actually does affect the analog channel gains in the sensor and so WB does in fact affect RAW data - but that is an odd ball case.

Ken
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Ray
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« Reply #30 on: August 16, 2011, 10:15:28 AM »
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Ray,

What might be confusing you is that there are a few photographers who will use external color correction filters to equalize the channel histograms and in this case your concerns are valid.  What everyone here is talking about for UniWB is just setting up a WB in the camera to make the in camera histograms better reflect the RAW histograms.  As noted, this doesn't change the RAW data at all.  There is one notable exception, I believe it is the D200 or some other Nikon camera of that era (D2X, D2S?  I can't remember), in which the camera WB actually does affect the analog channel gains in the sensor and so WB does in fact affect RAW data - but that is an odd ball case.

Ken

I see. In that case it's all just a 'strom in a tea cup' as far as I'm concerned. For a moment I thought there was a real possibility of extending the dynamic range of the camera, using UNIWB. Can't see what all the fuss is about.
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Guillermo Luijk
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« Reply #31 on: August 16, 2011, 10:26:29 AM »
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I thought, by applying a UNIWB, the channel most susceptible to clipping would be brought back into line with the other channels, allowing for a greater exposure.

That happens only in the JPEG, and that is the principle why UniWB'ed histograms are (or should) be more reliable (and less pesimistic) than those obtained with any other WB. In a UniWB'ed JPEG histogram, R and B channels typically have less exposure than if they were calculated with another WB setting, precisely because any typical WB consists of increasing exposure in the R and B channels.

But the RAW data, and hence (never knew if this is the proper way to use the English word 'hence') what you can obtain in your RAW converter, is exactly the same no matter what WB you used. RAW data only depends on the triad: ISO, shutter and aperture; UniWB is just intended to assist you to be more precise in handling them.

Regards
« Last Edit: August 16, 2011, 10:29:21 AM by Guillermo Luijk » Logged

Ray
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« Reply #32 on: August 16, 2011, 11:21:35 AM »
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That happens only in the JPEG, and that is the principle why UniWB'ed histograms are (or should) be more reliable (and less pesimistic) than those obtained with any other WB. In a UniWB'ed JPEG histogram, R and B channels typically have less exposure than if they were calculated with another WB setting, precisely because any typical WB consists of increasing exposure in the R and B channels.

But the RAW data, and hence (never knew if this is the proper way to use the English word 'hence') what you can obtain in your RAW converter, is exactly the same no matter what WB you used. RAW data only depends on the triad: ISO, shutter and aperture; UniWB is just intended to assist you to be more precise in handling them.

Regards


Now I understand why I didn't get involved in that 2008 thread on UNIWB you linked to, involving Panopeeper, John Sheehy and Jonathan Wienke. It didn't make sense. A lot of huffing and puffing about nothing.  Grin
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Guillermo Luijk
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« Reply #33 on: August 16, 2011, 11:26:03 AM »
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But knowing that the camera WB setting doesn't affect RAW data is basics Ray! that is first course Grin
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Ray
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« Reply #34 on: August 16, 2011, 11:35:22 AM »
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But knowing that the camera WB setting doesn't affect RAW data is basics Ray! that is first course Grin

But setting the camera's jpeg to minimum contrast, minimum saturation and minimum sharpness provided a sufficiently accurate histogram for me. Your UNIWB may provide a slightly more accurate histogram, but with an extra degree of trouble that may not be warranted.
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Ray
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« Reply #35 on: August 16, 2011, 07:16:52 PM »
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I'm now getting an isight into how some people view these incessant, obsessive, technical discussions over what may appear to them to be trivial concerns.

Many of us  get excited about a new camera that provides an extra stop of DR or even a 25% increase in resolution, but I don't believe I've ever witnessed before on this site  so much effort and weighty discussion over such a small advantage.

When I saw that 6 page thread on UNIWB, with such detailed technical posts from John Sheehy, Panopeeper and Johnathan Wienke, and links to images that could be downloaded to help one achieve an accurate UNIWB, I thought for a moment there was a real advantage being discussed, and I wondered if I should go to the trouble of using such a procedure myself, under the misapprehension I might eke out an extra 1/2 stop of DR, or so.

I should have realised immediately that changing the WB would not affect the RAW data,  or allow for any increase in exposure before clipping took place.

You certainly fooled me, Guillermo.  Grin
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stamper
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« Reply #36 on: August 17, 2011, 03:42:18 AM »
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I should have realised immediately that changing the WB would not affect the RAW data,  or allow for any increase in exposure before clipping took place.

Unquote

Surely this is semantics. If I import an image into ACR the image has setting > as shot. If I change it to daylight more often than not some clipping will appear shown by the red flashes. Now the RAW data will not be changed but they way I then process it will be changed because I will lower the exposure slider more than I would if I left it at as shot. I like to look at things from a practical sense rather than a purely theoretical one. It seems to me that a lot of the posters on here like to dance on a pin head for the sake of it and argue endlessly theoretical points for the sake of it. I sometimes wonder how good a photographer they are in reality when it comes to producing decent prints? Roll Eyes
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digitaldog
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« Reply #37 on: August 17, 2011, 09:13:05 AM »
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Its not semantics as I see it at all. The WB has zero effect on the raw data. There is a suggestion that is completely separate from the raw, every converter will use it differently or maybe ignore it. You could have some separate metadata that again has no effect on the raw and a proprietary raw converter may use it and produce a visual result that is completely different from another raw converter. And yes, if that suggestion applies some rendering that affects the histogram or clipping, you’ll see it and if you blindly let that apply to the rendering, you’ll get that result. The raw however wasn’t altered a lick.

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I should have realised immediately that changing the WB would not affect the RAW data,  or allow for any increase in exposure before clipping took place.
Correct. What you do in any number of raw converters with any number of rendering controls at this point will of course.
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Andrew Rodney
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Rory
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« Reply #38 on: August 17, 2011, 10:29:04 AM »
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"The WB has zero effect on the raw data."

It does have an impact on your decision making at time of capture, which is the whole point regarding UniWB.
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digitaldog
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« Reply #39 on: August 17, 2011, 01:38:26 PM »
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It does have an impact on your decision making at time of capture, which is the whole point regarding UniWB.

How so?
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Andrew Rodney
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