Pushed ISO: Let's make it clear

Friday February 04 2011

Sensor Insight
ISO and exposure | RAW ISO values: The fundamental basis for accurate comparisons | RAW ISO measures are inferior to manufacturer ISOs: is this a problem?

RAW ISO measures are inferior to manufacturer ISOs: is this a problem?

The RAW ISO measured value is almost always inferior to the ISO that you decided to use with your camera. Take a Canon EOS 60D, for instance. When you select ISO 200, the measured RAW ISO sensitivity is 160. At ISO 800, the measured value is 632… and so on. This simple fact compels some users to complain that the manufacturers are cheating [lying?]. Sure, this is a trick, and we are about to explain it, but it is not cheating.

ISO sensitivity compared: In grey, the nominal sensitivity. Some cameras, such as the Nikon D70 (red dots), have a RAW ISO corresponding to the JPEG value. Many others, such as the Canon EOS 60D (orange dots), have a much lower RAW ISO than the nominal value.

In fact, it is precisely the JPEG ISO value that all the manufacturers publish. They do so because JPEG (or any RGB) output is the visible output that photographers use. So when you select ISO 800 on your camera, you’ll have a JPEG ISO at 800, but the RAW ISO will be at (for instance) 550. The JPEG results are achieved by playing with the tone curve shape. This is absolutely legitimate: the ISO standard allows manufacturers to use this JPEG value. They are not cheating.

An old custom

Not cheating—okay. A trick? Maybe. The RAW-to-JPEG conversion allows cameras to achieve improved speeds, and to boost ISO values. And this is nothing new. What happens today with digital cameras also happened in the past with film cameras. When manufacturers produced an 800 ISO film, for example, they often used revamped 400 ISO film, and simply “asked” that it be processed differently. So when lab operators received a film labelled 800 ISO, they treated it differently (keeping it twice as long in the chemicals, for instance) in order to produce a stronger signal on the film. Digital camera manufacturers have simply implemented this same strategy in the camera body, and so could be seen as a “good old tradition.”

Moreover, underexposing the RAW file allows manufacturers to use their own complex algorithms to obtain a better output for the highlights while retaining good medium tones.

And this is also the reason why everyone should use genuinely efficient RAW converter software.