Today’s Question: Regarding the base ISO setting, someone told me that for Canon cameras it was 160. True?
Tim’s Quick Answer: I would say that in the case of Canon cameras the base ISO is generally 100, but you may see less noise at 160 ISO compared to 100 ISO. The same concept would generally apply for higher ISO settings as well.
More Detail: My understanding is that many (or most) digital cameras only use hardware amplification for certain ISO settings. These are often in one-stop increments, so that hardware amplification is employed for 100 ISO, 200 ISO, 400 ISO, and so on.
For “in between” ISO settings, the nearest hardware amplification value would be used, and then the signal is processed after the capture to increase or decrease the effective amplification with software.
What that means is that a capture at an ISO setting of 160 is really a capture at 200 ISO with the resulting image darkened by one-third of a stop. The result is that while there may have been more noise in the initial capture, that noise was masked by the image having been darkened. By contrast, at an ISO setting of 125, the lower setting of 100 ISO would be used for the actual capture, but then the signal is amplified by one-third of a stop after the capture by software.
The key here is that in general software amplification is inferior to hardware amplification. This is why you might see more noise at 125 ISO than you would at 160 ISO or potentially even 200 ISO.
The specific results will vary from one camera to the next, based on a wide variety of factors related to base sensor sensitivity, the nature of the various types of hardware and software amplification applied to the signal, noise reduction applied to the resulting data, and more.
To be sure, you will often see that noise levels do not increase in a linear manner as you raise the ISO setting. Furthermore, you may see less noise at a higher ISO setting in some cases, as noted above. This is part of the reason I strongly recommend doing some testing on your own to get a better sense of the overall noise behavior for your camera. This is especially important if you tend to use very high ISO settings on a regular basis in your photography.