Every ISO setting on a digital camera can be classified using four different types. I recommend that you avoid Idiot and Numpty ISO settings whenever possible, and it's good to understand the trade-offs surrounding the use of Bargain ISO. The types are:
- Idiot ISO
- Numpty ISO
- Bargain ISO
- Smart ISO
Most ISO settings on digital cameras are made up of a mix of several types of ISO. For example, on my 5D2:
- ISO 50: Smart ISO
- ISO 125: Idiot ISO
- ISO 160: Bargain and Idiot ISO
- ISO 200: Bargain
- ISO 250+HTP: Bargain, Idiot, and Smart ISO
- ISO 3200: Bargain and Numpty ISO
- ISO 4000: Bargain, Numpty and Idiot ISO
- ISO 5000+HTP: Bargain, Numpty, Idiot, and Smart ISO
The settings on other cameras may be made of different combinations (though tend to be largely similar for a given manufacturer). Here is a detailed example, showing how all four types combine to make ISO 5000 on the 5D2:
- +4 Bargain ISO to get up to 1600.
- +1 Numpty ISO to get from 1600 to 3200.
- -1/3 Idiot ISO to get from 3200 to 2500
- +1 Smart ISO to get from 2500 to 5000
The function of each of the types, briefly:
- Idiot ISO increases the brightness of the data in the raw file using digital gain (math inside the camera) and has no benefit whatsoever.
- Numpty ISO increases the brightness of the data in the raw file using analog gain and has no benefit whatsoever.
- Bargain ISO increases the brightness of the data in the raw file using analog gain, but has benefits/trade-offs.
- Smart ISO doesn't touch the raw file at all, it just changes metadata (similar to setting white balance).
Now a more detailed description of what each of the different types of ISO means.
Idiot ISO is when the brightness of the raw file is increased using math inside the camera. This is similar to using the "EC" slider in your raw converter, except that the changes are written permanently to the raw file. This causes the needless loss of data (clipped highlights or shadows), larger files, and increased quantization error (for non-integer factors).
There is never a reason or excuse for camera manufacturers to use such a braindamaged design when it is in fact *easier* to use Smart ISO than Idiot ISO. This type of ISO implementation has no redeemeing value whatsoever, hence the name.
The next two types of ISO are similar in that they both use an amplifier (analog gain) to increase the brightness of the data in the raw file. The only difference is this: if the resulting amplification has a better Signal-to-Noise ratio (SNR) than Idiot ISO, then I call it "Bargain ISO". If it has the same SNR, then I call it Numpty ISO.
Numpty ISO uses an amplifier (analog gain) to increase the brightness of the data in the raw file. It has only one possible benefit: reduction in quantization error. It has several negative effects, similar to Idiot ISO: loss of data (e.g. clipped highlights) and larger files.
Bargain ISO also uses an amplifier (analog gain) to increase the brightness of the data in the raw file. The only difference is that it results in less noise than using math to do the same thing (digital gain). The reason for this is that some of the noise comes from downstream electronics, such as the ADC (Analog-to-Digital Converter). By amplifying the signal before it gets to those noisy electronics, they are not able to contribute as much noise.
The disadvantages of Bargain ISO are similar to Idiot gain and Numpty gain: loss of data (e.g. clipped highlights) and larger files. But it has a significant advantage: less noise than other methods. This does not always make it the right choice, since it still clips one stop of highlights for every one stop increase in ISO, but at least it has a benefit.
Of course, it would be better if the manufacturers could fix the *real* problem (downstream electronics with high self-noise), but if they can't for whatever reason, this is at least a nice workaround. It allows the photographer to decide between noise and highlight headroom. With the introduction of the D7000, Sony seems to have mostly licked that problem, while Canon is still struggling with it, at least as of Summer 2011.
Smart ISO simply writes the desired change to metadata instead of the raw data itself. Then, the digital gain can be reversed or done differently in post production (such as with a nonlinear, highlight-preserving curve). This is the ideal type of ISO setting and how all of them should work. But until everyone else catches up with Sony's low-ISO read noise levels, Bargain ISO will still be needed.
Idiot and numpty ISO, however, are always braindamaged. They never have any useful purpose. If you shoot raw, then I recommend that you avoid them as much as possible. Choosing to use Bargain ISO or not is a little more difficult, since it's a trade off between a lot of clipped highlights and a slight reduction in noise. If I'm shooting a scene that needs a lot of highlight headroom, I'll often set ISO 800 or even 400 and increase brightness in post rather than use the Bargain ISO 1600, which clips 2 stops of highlight headroom compared to ISO 400 for a fixed exposure.
While we're on the topic, in non-raw cameras (e.g. JPEG-only digicams and most video cameras), there is another important distiction in the types of ISO possible:
- Linear ISO: clips highlights.
- Non-linear: compresses highlights without clipping.
Also, any time a lens reporting to be faster than f/2.8 is used, Canon applies a hidden Idiot ISO. This is Canon's way of falsifying (er, "compensating for") the sensor's poor angle of response.
Keep in mind that this thread is about the ISO *setting* on the camera (which is more properly called "gain" in my view). Other uses of the term ISO are more similar to the meaning of Exposure Index (a better use of the word ISO, IMHO), and are only indirectly related to this thread.