Why expose ETTR and how to do it?

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Erik Kaffehr
Erik Kaffehr Veteran Member • Posts: 4,974
Why expose ETTR and how to do it?
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There are some more or less bombastic discussions about ETTR. There are often explanations that may be authoritative but not necessarily correct,

At the same time there is a simple explanation, that also happens to be correct. If we illuminate a set of pixels with constant light, their response will vary. The reason is that light arrives in quantums and different numbers of quantums will hit the pixels.

Lets take a ColorChecker and illuminate it with reasonably constant light, take an image of that color checker and open the raw file in RawDigger.

Here you can see the raw data in the raw file. There is a selection placed over the neutral patches and a histogram is shown on the left side showing the distributions of the pixel responses. It would be reasonable that each field would be represented by a single spike. But, what we see are kind of 'bell curves' the brightest patch is very narrow the darker patches are getting broader with a lower central spike. This is the effect of photon arrival statistics.

But, that was the bright side of the image. Movie goers know that there is also a dark side. The image shown here has a very dark side:

Here i have also selected the six neutral patches. The spikes in the histogram are much broader.

Let's choose the fourth brightest neutral patch:

Looking at say the G-channel (green pixels), we can se that the signal varies quite a lot from 7284 to 8067. The right column shows the standard of deviation (stdev). Stdev tells us how much the signal varies. The average value is 7693.5 and sigma is 137.0. We can use the ratio of average divided by stdev as a measure of signal quality. It is called Signal to Noise Ratio. For a clean signal SNR would be high. In this case it is about 56.

Now, let us check the dark side:

Here the average green channel is 169.5 and the standard deviation is 24.4, So, signal to noise ratio is 169 / 24, that is around 7. So the dark side is getting noisy.

What this shows, clearly I hope, that the image gets more noisy when exposure is reduced. For photon arrival statistics SNR is proportional to the square root of the exposure.

What this means is that we want to keep exposure as high as feasible to reduce noise in the darker areas. That is all ETTR is about.

But, increasing exposure means that highlights can be clipped and that can cause ugly artifacts.

So, ETTR is about using maximum feasible exposure without clipping highlights. Significant highlights that is.

So, how to achieve that? Modern digital cameras have some tools to handle that. These tools are:

  • Histograms
  • Blinkies
  • Zebras

Just to say, on most cameras these tools are not really showing raw data.

  • Histograms show the distribution of luminance in the image.
  • Blinkies show parts of the image that is clipped.
  • Zebras show parts of the images that would be overexposed as black and white stripes.

This is a real world image. Here I knew, from experience, that moon is pretty bright. It is just a sunlit block of pretty dark stone 380 000 km away. So, I checked the zebras on the and back down exposure until they went away and that increased exposure until they were just visible.

So, my take is:

  • Download and buy RawDigger, it is a great way to understand how light metering on your camera works.
  • Use zebras, if available. But, keep in mind that they may be conservative.
  • If you have a histogram with little signal on the right side, you can probably improve your IQ exposing more.
  • If you see a spike on the uttermost right side of the histogram, it probably say that there are a lot of pixels clipped.

There is no big reason to overdo ETTR. Most cameras have pretty good SNR at base ISO.
On a camera with a Sony sensor, it may not be a great idea to increase ISO get more ETTR exposure. ETTR is about exposure, not ISO.

Best regards

Erik

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Erik Kaffehr
Website: http://echophoto.dnsalias.net
Magic uses to disappear in controlled experiments…
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