If the slower ISO is "pushed" during processing I can well understand this. I'll try to illustrate my original question with a concrete example. Lets say the camera's lightmeter indicated a "standard" exposure of 1/250th at f8 using ISO400. The ETTR shot might then be 1/125th at f8, also at ISO400. In ACR/Lightroom you would then back off the ETTR exposure to give a processed image with lower noise than the "standard" shot. However, how would this processed ETTR image compare to an alternative "standard" shot of 1/125th at f8, but at ISO200?
There is no short answer; it depends on the camera. What camera do you use?
The main point of ETTR is that increasing exposure decreases image noise. And it is important to note that ISO setting is not
part of exposure -- exposure is the light intensity times exposure time of the capture, the total light captured, and so depends only on shutter speed and aperture. ISO is an amplification of the electronic signal after capture, and therefore has nothing to do with the amount of light that was captured. There are two main sources of image noise: (1) photon noise, statistical fluctuations in the number of photons in the light arriving at the sensor, and (2) electronic read noise, noise in the camera electronics which processes the signal during and after capture. The larger the exposure, the more light there is, and as a result the photon noise fluctuations are smaller relative to the amount of light captured; this is the main point of ETTR -- that noise relative to signal is smaller as exposure is increased. So if the ISO is fixed, expose to the right to improve the image noise characteristics.
Now, how does changing the ISO affect things? As far as photon noise is concerned, not at all, if exposure is kept fixed and ISO is varied; photon noise depends only on exposure, and ISO is not part of exposure. But the read noise is a property of the camera electronics, that depends on the camera design details. The second component of noise, the read noise, is
ISO dependent on many cameras. Since the exposure is about light, which is made up of photons, it is useful to measure the read noise in photon equivalent units. Since the photons are converted into electrons during the capture process in the pixels, the unit of measure 'electron equivalents' is conventionally used. Here are the read noise characteristics in 'photo-electrons' (e-) as a function of ISO of a variety of cameras:
The first point is ISO 100, and each successive point is one stop higher ISO. So, on the 5D2 and D3s, read noise drops steadily until about ISO 1600 or 3200 (there is no true ISO 100 on the D3s, rather it uses ISO 200 metered for ISO 100, which is why the ISO 100 and 200 read noises are the same on this camera). So for these cameras (and all Canons, and Nikons with CMOS sensors designed by Nikon) the read noise is lowered by raising
the ISO, up to about ISO 1600 or so, provided the exposure is kept fixed
. Please note that this is always
a secondary consideration to simply increasing the exposure, which will always lower the noise more than keeping the exposure fixed and raising the ISO. Only if you are exposure limited should you consider ETTR via increasing the ISO for cameras such as the D3/D3s/D700 or any Canon.
A new breed of cameras such as the D7000 use the Sony 'Exmor' CMOS sensor family. These turn out to have an essentially flat read noise characteristic. This means that there is no advantage to raising the ISO to reduce noise. Similarly for any camera with a CCD sensor, such as older generations of Nikon such as the D2x, or any MFDB such as the P65+ (I am ignoring here the pixel binning features of the more recent Phase offerings, which lower image read noise at the cost of reduced resolution). For all these cameras, ISO is essentially irrelevant; the only thing that controls image noise is the exposure. In fact, there is little reason to do anything other than peg the camera at or near base ISO, and set the largest exposure you can subject to the constraints of motion blur and needed DOF. Raising the ISO will do nothing but chop off highlight headroom. The meter will say you are underexposing in many cases, but a properly designed raw converter should be able to amplify the raw data accurately in the same manner as raising the camera ISO setting (note however that Adobe changes internal settings on its raw conversion based on the ISO indicated in metadata, so one will have to play around a bit to undo that; see also a parallel thread on DPP
). The difference is that one can put a shoulder on the tone curve that rolls off highlights more delicately. Canon and Nikon already offer a limited version of this with Highlight Tone Priority/Active De-Lighting