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Cameras could do ETTR quicker and more accurately than humans, and without the risk of actually clipping any pixels. The camera could then write a field to the RAW metadata telling the viewer home much to under-expose to restore the natural exposure.

Most cameras don't have an ETTR metering mode. Is there any reason why — anything I'm missing?

mattdm
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Kartick Vaddadi
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    Checkout magiclantern.fm – rob j crowe Jun 27 '15 at 04:54
  • Some recent Nikon bodies have a highlight-weighted metering mode that prevents clipping and can be driven to do ETTR with exposure compensation. Nikons tend not do well with ETTR, but the bodies that have HWM have enough dynamic range that you can pull quite a bit out of the underexposed parts. – Blrfl Jun 28 '15 at 00:51

1 Answers1

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Cameras could do ETTR quicker and more accurately than humans

No, because it is a judgement call what highlights need to be clipped. There is no agreement as to what is photographic dynamic range; and much here depends on the intended use and personal tolerance to noise and artifacts.

Iliah Borg
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    There are different definitions of ETTR. The one I had in mind is increasing exposure until just before the brightest pixel clips. Increasing exposure beyond that is a judgement call. But until this point, it's fine to automate, again as a metering mode, to be used when desired. – Kartick Vaddadi Jun 28 '15 at 07:29
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    @Kartick Vaddadi : sorry, that is a wrong definition, it does not work, and it results in the opposite to what ETTR tries to achieve. However if a camera has a spotmeter, there is no problem in doing it; but at least in doing so one is aware what exactly happens. – Iliah Borg Jun 28 '15 at 14:51
  • I disagree that my definition is wrong. http://photo.stackexchange.com/a/23010/22575 , for example, says: "The technique works provided you don't increase the exposure to the point where it hits the maximum possible value and gets cut off, as this will result in a loss of information (known as clipping/blowing the highlights)." Nothing prevents you from manually increasing the exposure at the cost of clipping. But not wanting to do so, especially automatically, is perfectly valid as well. – Kartick Vaddadi Jul 02 '15 at 04:39
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    @Kartick Vaddadi The goal of ETTR is to decrease noise, not to hold the specular highlights or light sources. What you are suggesting is cross purpose. – Iliah Borg Jul 02 '15 at 15:19
  • I know that the goal of ETTR is to decrease noise, and that's achieved by having the camera automatically increase exposure until a pixel clips. Higher exposure => more light => less noise, assuming that the ISO isn't being increased. Why do you think ETTR (again going by my definition) reduces noise when done manually but not when done automatically? – Kartick Vaddadi Jul 02 '15 at 17:27
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    @ Kartick Vaddadi : > I know that the goal of ETTR is to decrease noise, and that's achieved by having the camera automatically increase exposure until a pixel clips -- No. Specular highlights and light sources mostly need to be clipped. Or the noise is increased. Please familiarize yourself with the reasoning behind exposure meter calibration constants. – Iliah Borg Jul 02 '15 at 17:42
  • I guess I wasn't clear enough in my proposal, which is: 1) Determine exposure using whatever metering algorithm, like matrix. 2) If this exposure results in all pixels having a value < the max, then increase exposure till one pixel becomes = the max value. And write this adjustment factor into the RAW metadata so that viewers can compensate for it. This algorithm is no worse than existing algorithms, like matrix. If you have specular highlights, and they already clip, this algorithm is a no op. It never makes things worse. – Kartick Vaddadi Jul 04 '15 at 03:40