Thursday, August 4, 2011

ETTR for the fourth time........

Umm, yes, scary but true.

Michael Reichmann has a new article on ETTR. It's called Optimizing Exposure, and you can find it on the Luminous Landscape site. The article itself doesn't have anything new - it's just a rehash of Michael's old arguments, "levels", etc, but it is vintage MR stuff, with references to important and highly competent people that Michael knows and that support what he's saying. Only he never actually mentions their names!!!

There's no point in my going over the ETTR stuff again - those who want a point-by-point commentary on what Michael wrote can look in the LL forum discussion of the article - other have already pointed out all the issues.

But two interesting things did come out of the article. Firstly, Michael makes the point that exposure on modern DSLR's in actually pretty primitive. Divorced from all the ETTR mumbo-jumbo, it's a valid point, and worth taking a look at the article for.

The second thing of interest that came out was an additional potential reason for using ETTR.

Quick recap - in my previous ETTR posts, I showed that ETTR was mostly a waste of time except in very particular circumstances - e.g., to effectively synthesize a lower ISO than your camera comes equipped with.

However, some of the folks in the forum pointed to an article by Emil Martinec of the University of Chicago, "The Consequences of Noise". It's fairly technical, and quite theoretical, but here's a short summary as it applies to ETTR:

Firstly, Emil destroys Michael Michael Reichmann's "levels" argument for ETTR. This is what he says:
"In particular, the idea that the benefit of ETTR comes from the "number of available levels" suggests that the image quality would be one stop worse for the ISO 1600 image than it is for the ISO 3200 image, since by being one stop down from the right edge of the histogram, fully half the available levels are not being used. However, as we have seen, noise is much more than two levels in all exposure zones at these ISO's, so the extra levels used in the ISO 3200 image simply go into digitizing the noise, and are thus of no benefit in improving image quality. In fact, the quality of the two images will be very nearly the same (rather than one stop different)"
Sound familiar? It's the same answer I came to previously.

But then Emil goes on to make the point that that noise in a digital image actually comes from more than just the sensor, it also comes from the electronics around it (the amplifiers, etc). This is known as "read noise". In a modern camera, generally the read noise is quite low, and you're only really worried about the sensor noise. But Emil's argument is that you can minimize the read noise by shooting at higher ISO. Specifically he says that:
"Somewhat counter-intuitively, for fixed aperture/shutter speed, it is best to use the highest possible ISO"
Now that might sound somewhat crazy when you hear it the first time. But think of it this way:

  • Sensor noise is basically fixed for any combination of aperture and shutter speed (the point I made in the previous posts on ETTR, and the reason why you should just adjust ISO, not mess with ETTR). If the same number of photons hit the sensor in the same time, you're going to have the same sensor noise.
  • However, because of the gain of the amplifiers in the electronics, effective read noise (the noise from the electronics) is lower at high ISO. 
So there you have it - there is another situation in which ETTR will benefit you in addition to the ones I laid out in the previous posts. That situation is the one in which the read noise (the noise from the camera electronics rather than the sensor) is a significant portion of the image noise.

The question, of course, is does that happen in practice? ETTR has a number of downsides, such as the color shifts I showed previously, so you won't want to use ETTR unless there's a real advantage. 

The answer to that question unfortunately depends on your camera. Personally, I've never seen a practical shooting situation with a modern camera where read noise was an issue relative to sensor noise. But I can't rule out it happening.

LATE EDIT: So, ok, here's the proof - as predicted by the theory above - courtesy of Guillermo Luijk. For some cameras, going to higher ISO really can reduce noise for the same shutter speed/aperture.  However, note that it took four stops of overexposure (ISO 100 to ISO 1600) on an old Canon 350D to get Guillermo's results, and Guillermo specifically says that many cameras don't do this at all. Interesting, but in my view, four stops is a bit impractical. 


    Dave Wujcik said...

    The one place I make the most use of ETTR is when shooting stage, where I need a faster shutter speed to control motion blur. This normally means pushing iso up a bit to increase shutter, but then I sometimes over-expose to try and get some shadow detail in faces.

    -- Dave

    olamid said...

    Another analogy that might clarify the idea behind ETTR is recording audio.

    All competent recording engineers practice ETTR. They record a signal that is as hot as their equipment can accept without clipping, so as to minimize the ADC and quantization noise which adds hiss and muddies up bass (analogous to shadow details).

    Exposing to the right is exactly the same principle; you don't have to be four stops (or 12dB in audio) too low to be able to see the shadow degradation (or hiss in audio) that results from too low of a recorded signal.

    The straw man argument that the blogger seems to be examining is analagous to arguing that quantization noise is not important in audio recording. Yes, quantization noise is much more subtle than hiss and clipping, and it may be something that requires 12dB difference to hear a significant difference.

    But ETTR isn't about quantization noise for me. It's about avoiding hiss due to recording "in the mud" (at low levels). Hiss is audible with a 3dB increase in digital level, and shadow underexposure due to failing to ETTR is visible even if the underexposure is one stop or less.

    Sandy said...

    Yes. But point is, on a camera you can change ISO to reduce the "hiss" as much as you want. No need for ETTR.