Oh dear. Now the real photographers have arrived, all us mere "armchair theoreticians" should bow out
Before I bow out, just so we're all on the same page, I think that everybody on all sides of the ETTR debate would agree with almost all of what you said. Specifically:
- Yes, absolutely, if you underexpose you will get more noise
- Yes, if you are for some reason limited in ISO selection, ETTR is a good idea - if you read the blog post I referenced right at the beginning of this, you'll see that I specifically identify already being at base ISO as the one situation where ETTR makes sense.
But the question is not, to use your ISO 800 example, whether if you take two exposures both at ISO800 one with 2 stops overexposure, one with normal exposure, that you will get lower noise in the overexposed version. Everybody on this thread (so far as I know anyway) agrees that at a single ISO, ETTR makes sense.
The question that's actually being addressed, and I'm afraid your example isn't relevant to, is what if you just changed the ISO setting to 200? Those of us on the "ETTR is over hyped" side of the fence will say that you get the same noise levels, with none of the problems as you would with the "ISO800 plus 2 stops of ETTR".
<Sandy politely bows out of conversation>
Post script to Andrew: If you haven't seen them, I did some articles on hue shifts here: http://chromasoft.blogspot.com/2009/02/adobe-hue-twist.html
The term "hue twist" is actually Eric Chan's. Although I think he now regrets coining the term(!) As regards what raw converter, etc. Certainly most "new generation" Adobe profiles, but not all. Capture One also seen to use hue twists, judging by some of the hue variations I've seen, although not as much as Adobe.
Since you intend to 'bow out', perhaps there's no point in responding. However, for the benefit of others, I shall.
My first reaction to your post was one of confusion. Then I looked at your blog in an earlier post and all is clear. You have failed to distinguish between the two basic methods that cameras of different types handle ISO increases. One type, usually CCD such as many P&S cameras and most MFDBs, are essentially 'one-ISO' cameras.
With such cameras no image quality benefit is derived by using a correct ETTR at a higher ISO than base. One might as well underexpose at base ISO and get the increased assurance that one will at least not overexpose
, even though the LCD review might appear unhelpfully dark.
The other type of camera, usually most DSLRs, and particularly Canon DSLRs with which I have the most experience, provides a distinct IQ advantage when raising the ISO to create an ETTR exposure at the increased ISO setting, as opposed to underexposing at base ISO.
There's no doubt about this. I've tested it, although it has to be said the most dramatic improvement will be observed when the ISO increase is great. For example, an ETTR at ISO 1600 exhibits very significantly less noise, from shadows to upper midtones, than the same exposure at base ISO, which would of course be 4 stops underexposed.
As far as I understand, the reason for this improved IQ when using the same exposure at higher ISOs, is due to a boosting or amplification of the analog signal prior
to A/D conversion and of course prior to all other signal processing further up the chain before the data is eventually written to the memory card.
Such amplification in itself cannot reduce noise. In fact, whatever noise is present in the sensor at the precise time of amplification will also be amplified. The reduced SNR in the final output is not due to a reduction in absolute noise but a reduction in relative noise, that is, the noise introduced after
amplification is smaller as a proportion of the larger signal.
In other words, with DSLRs the higher ISO setting is merely an instruction to the camera to amplify the analog signal by a specific amount, at the earliest possible stage, even before A/D conversion. Whereas, with the other type of sensor, mostly CCDs I believe (and that would include the Canon G10 you used), the ISO setting would appear to be an instruction to the camera's processor to boost the signal at the end
of the processing chain. There's a big difference.
However, with the introduction of Sony's latest sensor used in the Pentax K5 and Nikon D7000, we have a new development whereby it seems
that only one analog boost takes place, and that's at base ISO. This provides no additional advantage at very high ISO, but does result in lower noise at base ISO and lower noise even up to ISO 400 where, remarkably, DR in this cropped format is even higher than that of the 5DMk2 at ISO 400, whether at pixel level or at normalised print sizes.
In summary, unless one is using one of the 'effectively' one-ISO cameras, such as the CCD type, and now the D7000 and Pentax K5, it's always advisable to raise ISO till one gets an ETTR exposure, rather than underexpose at base ISO.
And of course, for maximum image quality across the entire tonal range from deep shadows to highlights, an ETTR at base ISO provides the best image quality the camera can deliver, assuming shutter speed and f/stop are appropriate for the conditions and the composition.
Hue shifts or twists are not a problem for me. If I were to see any, I'd fix them. Most of my images are given individual treatment in Photoshop. I'm changing hues, color saturation, shadows, midtones etc all the time. Let's not create imaginary problems.