Learnings (Part 1): Fractional ISO

The ISO setting on a digital camera controls the gain applied to a digital sensor (making it appear more sensitive to light) - in the same way that ASA film speed specified the film sensitivity.

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.
Highlight Tone Priority (or HTP) is a another camera feature which alters ISO behavior.  This is enabled through Custom Function II-3 (on the 7D) and is often recommended for video work.  In this mode, camera will intentionally underexpose (analog) by one stop.  For example if you choose ISO 200 (the minimum), then the camera actually shoots at ISO 100 (therefore halving the exposure).  In a (digital) post processing step, the image has digital gain applied (normalising the histogram), but this time with smarts to better handle highlight clipping.  Or as Canon says "The graduation between greys and highlights becomes smoother".

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.

Additional References:
  • 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.


William said…
You should really send some of your experiences and advice into a photography site such as http://digital-photography-school.com/
Alex Megremis said…
FIY, the supporting link you've provided is dead.
Mark Stead said…
@Alex Megremis: Thanks, unfortunately that link had some very detailed analysis. The best I can do is replace it with a search link for pages that refer to that site.

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