How do you really know when you are ETTR?

[bwana]

thank you for your thoughtful and considerate replies. and SO FAST. i especially appreciate your genuine sharing of your process. yes, getting to know the camera  is all about iteration> capture->development->print.  These articles
http://www.dxomark.com/index.php/Publications/DxOMark-Insights/Noise-characterization
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p2.html#SNR-DR
(specifically figure 12)
is what led to my question. (of course bjanes has even posted a more recent graph). I thought that the curve in the graph was the boundary that the histogram should not cross. But the horizontal axis (abcissa) is not f stops but signal output. And of course the abcissa is a log base 10, not base 2. So there is no obvious relationship between that graph and a histogram. I neglected to read the last sentence of the second article which states:

"Locate the raw value where the S/N ratio meets your threshold for acceptability, then count the number of stops to the saturation raw level; this then is the useful dynamic range. For instance, suppose the minimum acceptable S/N ratio is 4=22; on the 1D3, this S/N ratio is achieved for ISO 200 at roughly S=24.4, 9.5 stops below the saturation value of 13.9 (the max raw value is a bit less than 214-1=16383 due to the bias offset of 1024)."

So if I knew where ~9 stops to the left of the right most clipping value is, then I was thinking that would be my lower threshold (if I accepted a minimum S/N=4). (That might make a good marketing tool for the gullible- imagine Canon advertising 'noiseguard technology' simply by putting a dashed vertical line there!) But as many of you have said, there are other factors-for example, noise reduction algorithms which can selectively be applied (now in lightroom 4) to only the dark areas will spare the detail we want to preserve in the brighter areas. And there are only so many miliseconds you have before capturing 'the moment' - my thoughts should first go to preserving highlight detail than avoiding dark noise.

[Guillermo Luijk]

As long as the only information your camera provides is a JPEG histogram, which is affected by white balance, exposure corrections, tonal curves, colour profile conversions, saturation and sharpness enhancements,... you will not be able to translate any of all those considerations made about stops and SNR in DxO data into your real life worfkflow.

[Nick Walker]

And there are only so many miliseconds you have before capturing 'the moment' - my thoughts should first go to preserving highlight detail than avoiding dark noise.

Whilst charts and graphs provide valuable insights, you have hit the nail on the head re preserving highlight detail - that's what a properly calibrated incident meter (colorchecker) to your choice of RAW converter can offer. Little need for 1 degree spot metering and calculations which are not always an option anyway.

Another method for fast, accurate metering, is to use the cameras metering system (manual exposures) on a nearby surface that doesn't require the narrow angle of a spot meter, provided the surface is under the same lighting as the subject. This method is useful when there is little time to pick up an incident meter, or waving one around in the air might disturb sensitive subjects - golf.

Method

Take an incident reading early on (and occasionally throughout the day when locations/lighting direction and intensity changes), apply the shutter speed and aperture indicated by the incident meter to the camera, noting where the cameras metering diode indicator settles on its exposure scale - from now on use the cameras metering system to mimic the results from the incident light meter.

For a large surface such as grass it doesn't matter which camera metering pattern is used. On grass the cameras meter will often differ from the incident reading - invariably indicating 1/3 - 2/3 rds of a stop under exposed compared to the incident reading - often depends on which way the grass has been mown!

If the light suddenly changes and there is no opportunity to use the light meter, use the cameras meter, adjusting the shutter speed or aperture so the cameras metering diode is returned to the same spot on its exposure scale as initially observed from transferring the incident metering exposure to camera - it could be over or underexposure depending on the surface measured in relation to the initial incident reading used for reference purposes.

In the UK light levels often change very quickly, this system has enabled me to quickly obtain the correct exposure (ETTR) from full sunlight to heavily diffused sunlight seconds prior to sporting moments. It is obviously useless for some sporting scenarios - e.g. rugby player crossing over from sunlight into full shade from sharp shadows cast by the spectators stand.

[Tony Jay]

This question will probably never be settled entirely for the very good reason that the answer varies so much with the type of shooting one does.
Knowing the principles governing ETTR is exceptionally useful as long as it appreciated that it cannot predicate a single "right" method.
Knowing how a camera sensor really is responding to given amounts of light and how that makes an image look is emphatically a trial and error exercise. There is no empiric standard here either as to what constitutes an acceptable result - it really is a "season to taste" approach.

Good luck with your quest.

Regards

Tony Jay

[hjulenissen]

When shooting in liveview against stationary scenes, it ought to be possible for the automatic exposure to make "perfect" exposures in the sense that no sensels are clipped (or at most a user-defined percentage of sensels are clipped) - if artistic choices allows the freedom in selecting aperture and exposure-time (and for some cameras: iso). Anyone know if they do?

-h

[Tony Jay]

None that I know of but this would be an advance on current "technology" as far as exposure goes.
Even better would be a real-time RAW-based histogram that would alter accordingly when changes are made to shutter speed, aperture, and ISO.
I bet the means really does exist already but as best I know a commercial expression of it does not.

Regards

Tony Jay

[RFPhotography]

Perhaps I'm misunderstanding the original question but there's a are more fundamental reason why determining an 'optimal' ETTR exposure for all conditions isn't possible.  That is the dynamic range of the scene itself.  If the scene has a narrow brightness range then it may be possible to push the exposure sufficiently that, in the subjective opinion of the photographer, there's enough signal across the entire range to override the evidence of noise.  But if you're shooting a scene with a 9 stop range using a camera with a 10 stop sensor it's not going to be possible to do that.  You may always have some 'dark' areas where noise will still be too evident; again based on the subjective opinion of the photographer.

[Guillermo Luijk]

Perhaps I'm misunderstanding the original question but there's a are more fundamental reason why determining an 'optimal' ETTR exposure for all conditions isn't possible.  That is the dynamic range of the scene itself.  If the scene has a narrow brightness range then it may be possible to push the exposure sufficiently that, in the subjective opinion of the photographer, there's enough signal across the entire range to override the evidence of noise.  But if you're shooting a scene with a 9 stop range using a camera with a 10 stop sensor it's not going to be possible to do that.  You may always have some 'dark' areas where noise will still be too evident; again based on the subjective opinion of the photographer.

You are right that automatic ETTR can be difficult when the DR of the scene is large, but in those conditions RAW histograms would be the perfect tool to allow the user to decide:



That kind of real time RAW histogram is totally possible, and it allows to find out how much information you would clip with the present shutter/aperture/iso selected (orange bars), and how much information would fall into too noisy areas (dark gray bars). The user would decide which information has to be sacrificed.

The same clipping/noise information could be plotted over the image with blinking areas:



Regards

[RFPhotography]

Oh, I'm not saying camera makers couldn't provide such a tool to photographers, GL.  Not at all.  All I'm saying is (1) with current technology it's very difficult to determine and (2) if the OP is looking for a formula that can be applied to every image he's looking for too much.

Holy CRAP, GL!  We actually agree on something.