Having previously owned a Canon 20D, I was familiar with traditional ISO values (associated with powers of two), for example: ISO 100, 200, 400, 800, 1600.
Due to my computer background I had assumed that boosting the ISO was achieved through bit-shifting - or digital amplification/gain. It turns out that this is wrong. ISO gain is achieved by analog means prior to analog/digital conversion. That said, ISO expansion modes such as ISO 12800 on the Canon 7D and ISO 3200 on the Canon 20D are achieved by digital gain (bit shifting).
My Canon 7D supports fractional ISO values such as 125, 160, 250, 320 etc. There has been discussions circulating on the Internet saying that the 2/3 of a stop values result in less noise. While there is an apparent loss of noise, this actually a result of over exposing the photo. Specifically:
- ISO 160 is actually photographed as ISO 200 (therefore with analog overexposing) before digitally underexposing by 1/3 of a stop. In this case you are more likely to have hard clipping of highlights.
- ISO 125 is actually photographed as ISO 100 (therefore with analog underexposing) before digitally overexposing by 1/3 of a stop. In this case you are likely to lose some shadow detail and dynamic range.
Both Fractional ISO and Highlight Tone Priority are particularly useful for shooting movies, and JPGs where the image bit-depth is being reduced (in camera) from 14 bits to 8 bits. On the other hand when shooting RAW you want the extra latitude (i.e. bits) to do manual adjustments.
By combining Fractional ISO and Highlight Tone Priority you can get the benefits of extra highlight headroom, and also some reduction of noise. For example, consider shooting at ISO 320 with Highlight Tone Priority enabled. In this scenario the camera will effectively take the shot with ISO set at 200, so it is (analog) underexposing by 2/3 of a stop, then apply the digital gain increase (and avoid highlight clipping).
I believe the Canon 5D mk II behaves the same, but other Canon models or other brands may not. You need to do your own research.
If your camera does behave this way, then by understanding how fractional ISO works, it can help you use it effectively or when appropriate not use it.
- Unfortunately this link is dead: http://www.cryptobola.com/PhotoBola/Canon7D_ISO.htm
- However a Google Search will find many informative pages linking to that site
Read about other things I've learnt here.