Getting a proper RAW exposure at base ISO is the ideal situation to improve image quality (i.e. reduce noise).
However, in low light conditions, even using the slowest shutter speed and the widest aperture we can afford in our application, we may not get a proper RAW exposure at the lowest ISO setting. In this situation, pushing ISO is recommended to get a better RAW exposure and hence lower noise, but the convenience to do it is not equally critical in all brands.
I have compared the ISO behaviour of the Canon 5D Mark II, the Nikon D700 and the Pentax K5 (with a Sony sensor). Most Canon sensors behave similarly, so as most Nikon sensors, so this can also be considered a brand comparision. The K5 sensor is the same Sony sensor found in the Nikon D7000 and the Sony A380.
Using noise data from Sensorgen.info
for the three cameras under test, collected from DxOMark SNR plots and normalised to an output resolution of 12,7Mpx to make it a fair comparision, the following plots are obtained (the black line represents the improvement in SNR at each ISO increment. The line is plotted gray once improvement becomes negligible):
The improvement in SNR after a 5 stops ISO increment is very different for the 3 types of sensors:
- Canon: SNR improves up to 2.1EV, up to ISO1600, and specially at the lowest ISO values
- Nikon: SNR improves up to 1EV and up to ISO800
- Sony: SNR improves up to 1/3EV and progressively at all ISO values
The conclusion for those 3 different behaviours would be to use each camera in a different way according to:
- Canon: it's strongly recommended to push ISO on a Canon camera as soon as RAW underexposure may happen at base ISO, because improvement in noise will be very important. Pushing ISO beyond ISO1600 becomes useless.
- Nikon: it's not so critical to push ISO to avoid RAW underexposure as in Canon, but still recommended since the there will still be a clear noise reduction. Pushing ISO beyond ISO800 becomes useless.
- Sony: noise reduction through ISO is so slight, that it is not worth to push ISO even if RAW underexposure may take place. By staying at base ISO in all cases, we'll save highlights information (in case the scene has got them) and will not have noisier shadows.
The Sony sensor is close to an ISOless
sensor (the first time I heard this term was from Emil Martinec). This means it could be adjusted at base ISO in any situation, and just concentrate in choosing the most adequate shutter and aperture. Unluckily this will make the camera display look very dark in some situations; camera manufacturers could allow to adjust the display brightness or just consider the ISO setting a metadata parameter that only affects the camera display, not altering RAW data.
I also plotted the dynamic range of the 3 tested cameras (12dB SNR criteria, 12,7Mpx output resolution) vs the real ISO of each sensor. The APS-C sized Pentax clearly beats the two FF cameras at low ISOs, behaves similarly at ISO400 and looses at high ISOs. No clear winner among the FF cameras; the Nikon D700 is slightly better than the 5D2 at low ISOs (Nikon's ISO200 even beats Canon's ISO100), but we must also consider Canon's higher pixel count.