Robert’s Photography Blog

Or perhaps I should say, ‘be careful using the in-between ISO settings’. One highly touted feature of Canon EOS 30D over the EOS 20D is the addition of 1/3 stop ISO settings. There have been forums posts for months questioning the validity of the these settings. Now quantitative data have been posted on dpreview.com by John Sheehy.

The primary ISO settings (200, 400, 800, 1600) result from amplifier gain while the in-between ISO settings (125, 160, 250, 320, 500, 640, 1000, 1250) are calculated on the DIGIC chip from the nearest primary setting. Yikes! No math in the camera please! Isn’t that why we use RAW? We want to do the post-processing properly on a computer!

And you thought that ISO 125 is the next lowest in noise ISO setting after 100? Nope! ISO 125 has as much shadow noise as ISO 640. Read on!

According to John, “When the camera is set to 160 and EC 0, it is just like the camera is set to 200 and EC +0.3, except that the 160 damages the RAW data by posterizing it, and completely discards the top 1/3 stop of highlights. For the JPEG user, the latter doesn’t matter because JPEGs always throw away those highlights, anyway”.

Then he goes on to say, “ISO 200 is the basis for the camera’s ISOs 160 and 250. The camera just over- and underexposes by 1/3 stop to achieve them, and then divides and multiplies the digitized results by about 1.25. The higher one (250) is just an underexposure of 200. Under-exposure is more inductive to blackframe noise than amplification of the same magnitude.”

For those that have read this far, this is a non-trival issue. Pushing a primary ISO by 1/3 stop adds nosie while ‘pulling’ a primary ISO by 1/3 stop reduces dynamic range (albeit only slightly). Here are tips:

Tip 1: If you shoot RAW (EC=0), stick with the primary (100, 200, 400, 800, 1600) ISO settings. If you tend to expose to the right, there is a slight benefit to using ISO settings from the 1/3 stop group (160, 320, 640, 1250). This benefit is negated if you use a RAW converter that can recover overexposed highlights (ex. Bibble).

Tip 2: If you shoot JPEG only, use the primary ISO settings or one from the +1/3 stop group (160, 320, 640, 1250).

Tip 3: Avoid at all times, ISO settings from the -1/3 stop group (125, 250, 500, 1000).

Why has Canon chosen to do it this way? There might be limitations with variable amplification of the photosites on the sensor with the current technology.

If you have time, you should read through the John’s lengthy thread on dpreview.com. At least, take a look at John’s graph at the beginning of the thread. I decided not to reproduce the graph here because of copyright issues.