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Canon 40D:

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Canon ISO Implementation:

 It is a somewhat complicated issue.

The 40D, like the 30D before it, implements intermediate ISO’s by software multiplication after digitization of the raw data. So there aren’t really intermediate ISO’s, as far as the camera electronics is concerned; instead, the camera overexposes by 1/3 stop at ISO 160-320-640 etc, and underexposes by 1/3 stop at ISO 125-250-500-1000 etc., using the nearest “main” ISO 100-200-400 etc. That is, it internally only has the main ISO’s; it adjusts the metering by 1/3 stop; and then after the capture it multiplies or divides the raw values by 1.25 to make the data look as though it came from the indicated ISO.

What does this do for noise? Well, exposing to the right (ETTR) is a technique for reducing noise, because to push the histogram to the right you must increase the light gathered, and the more light gathered the less apparent the noise is relative to the signal. Similarly, underexposing increases noise by making the signal stand out less above the noise. What the camera is doing at ISO 160-320-640 etc is overexposing by 1/3 stop at ISO 200-400-800 etc and dividing by 1.25 after the fact. But this also removes 1/3 stop of highlight headroom, because internally the camera uses a higher ISO (200, say) that amplifies the signal more than (say) ISO 160 and so pushes 1/3 stop of highlights past the saturation point of the electronics (the A/D converter, specifically) which are then lost. Dividing by 1.25 will not bring them back since they are already clipped.

Conversely, the other set of intermediate ISO’s (125-250-500 etc) underexpose the image by 1/3 stop and then push the exposure by 1/3 stop before writing the raw data. Like any underexposure, this increases the noise.

Bottom line: on the 40D, ISO 125-250-500 etc are underexposing at ISO 100-200-400 etc and are no different than using ISO 100-200-400 etc and manually dialing in -1/3 EV of exposure compensation. ISO 160-320-640 etc are overexposing at ISO 200-400-800 etc, and are no different than using ISO 200-400-800 etc and manually dialing in +1/3 EV of exposure compensation. The only difference will be that you will have to apply appropriate exposure adjustment during raw conversion if you stick to the main ISO’s, if you use the intermediate ones the camera will do it for you internally. I personally use the custom function on my 40D that turns off the intermediate ISO’s; I prefer to set exposure compensation myself, knowing that I have the same amount of highlight headroom available at each main ISO, rather than having to remember that I have an extra 1/3 stop at some ISO’s and 1/3 stop less at some others, etc, when using the intermediate ISO’s.

Now for the 1D2n. In its pro series cameras (including the 5D and presumably the 5D2), Canon does use hardware amplification to implement the intermediate ISO’s, rather than using software multiplication of the raw data after the fact as in the 30D/40D (and presumably the 50D as well). However, the implementation is by a separate amplifier that amplifies the signal before digitization, using the output of a previous stage of amplification at the next lowest main ISO. For instance, both ISO 250 and 320 are derived from an additional stage of amplification from an earlier stage of amplification at ISO 200. However, this extra amplifier is so noisy, that it provides little or no advantage over underexposing by 1/3 or 2/3 stop at the next lower main ISO — there is little or no gain noise-wise (essentially none above ISO 400, and very little below ISO 400), but there is again a loss of 1/3 or 2/3 stop of highlight headroom due to the extra amplification pushing highlights past the saturation point of the A/D converter. And again, I have the custom function set on my 1D3 that turns off the intermediate ISO’s; I would prefer to have the extra highlight headroom of underexposing at the next lower ISO, if I need the faster shutter that the higher intermediate ISO meters for.

And for all you Nikon users, the D300, D3 and D700 all implement intermediate ISO’s as hardware amplification in a single stage; there are no drawbacks to using them, no convoluted chain of logic needed 8)