Forgot to add - and on the topic of custom functions - this is WELL worth a look and read
Noise at Fractional ISO
I turned off fractional ISO after reading this test.
Reality is.. I dont need it anyway.. I am perfectly happy with ISO 50, 100, 200, 400, 800, 1600 and 3200
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The 1D* series and the 5D use a simple amplifier before the ADC to get the 125/250/etc and 160/320/etc groups of ISOs, so, basically, all they are doing is under-exposing and then scaling up the signal at an analog stage. This gives consistency in highlight headroom, relative to metering, but brings up the read noise very high relative to the general trend of the "main" ISOs. This is only an issue with Canons, and the D3 if it does something similar, and does not affect most digital cameras, especially ones with CCDs.
The difference is that CMOS cameras with high-ISO optimizations have read noise at their main ISOs which does not increase in proportion to the ISO. ISO 100 and 200 have almost exactly the same read noise on most Canons (the 10D was the last camera where they were really much different), so when you under-expose ISO 100 and scale up the data, the read noise gets scaled up higher than it is at ISO 200. As you approach the highest ISOs, the relationship between ISO and read noise is closer to linear, and the in-between ISOs are not as detrimental, but it is important to remember that when you are in a range of ISOs where the read noise is proportional to ISO, then you might benefit more from using a lower "main" ISO and under-exposing yourself, to get the highlight headroom if needed.
Most traditional designs, including most CCD cameras, have a nearly linear relationship between ISO and read noise, made only slightly non-linear by the relationship of ADC-generated noise to the signal, so there is no special loss due to intermediate ISOs, but still, you might benefit from the extra highlight headroom by shooting at a lower ISO with the same absolute exposure, especially if you shoot RAW. The traditional method of ISO gain will give about 15x as much read noise at ISO 1600 as at ISO 100. Most Canons, however, only have 2.25x - 2.5x as much read noise at ISO 1600 as they do at ISO 100, and most of that gain is in the range above ISO 400, as ISO 400 has only a small amount of extra read noise over ISO 100 on some models, like the 1Dmk3.
The other main noise component, shot noise, has nothing to do directly with ISO settings, but is directly tied to the actual exposure you are getting (Av and Tv value in the given lighting).
The Canon APS-C cameras that offer in-between ISOs work on a slightly different principle; rather than having an additional late-stage amplifier, they simply under-expose ISO 100 for 125, and over-expose ISO 200 for 160, and just manipulate the original RAW capture arithmetically, to push and pull the RAW file back to a normal exposure. In this case, ISO 125/250/etc are ISOs to be skipped over, but in the case of the 30D and 40D, the 160/320/etc group actually have less read noise. In the 30D and 40D, ISOs 125 and 640 have approximately the same read noise. On the 30D, highlight headroom varies between the groups by a good amount, and ISO 160 actually has more DR than 200, from which it is derived (because the camera clips some of 200's capture before writing the RAW file - very stupid), but it is preserved when scaled down to ISO 160. ISO 160 also has slightly more DR than ISO 100 in the 30D.
It's one big mess, I tell you. A pitiful lack of standards.
