ISO Sensitivity / Noise levels

ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. This works by turning up the "volume" (gain) on the sensor's signal amplifiers (remember the sensor is an analogue device). By amplifying the signal you also amplify the noise which becomes more visible at higher ISOs. Many modern cameras also employ noise reduction and / or sharpness reduction at higher sensitivities.

To measure noise levels we take a sequence of images of a GretagMacBeth ColorChecker chart (controlled artificial daylight lighting). The exposure is matched to the ISO (i.e. ISO 200, 1/200 sec for consistency of exposure between cameras). The image sequence is run through our own proprietary noise measurement tool (version 1.5 in this review). Click here for more information. Room temperature is approximately 22°C (~72°F), simulated daylight lighting.

Sony DSLR-A550 vs. Canon EOS 500D vs. Nikon D5000 vs. Olympus E-620

• Sony DSLR-550: Sony 50 mm F1.4 lens, Manual exposure, Manual WB,
  Default Parameters (Standard), High ISO NR Normal (default), JPEG Large / Fine
  
  
• Canon EOS 500D: Canon 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters (Standard PS), High ISO NR (Default; Standard), JPEG Large / Fine
   
• Nikon D5000: Nikkor 50 mm F1.4G lens, Aperture Priority, Manual WB, ADL off
  Default Parameters (Normal), High ISO NR (Default; Norm) JPEG Large / Fine
   
• Pentax K-7: Pentax 50 mm F1.4 lens, Manual Exposure, Manual WB,
  Default Parameters (Bright), High ISO NR default (Medium), JPEG Large / Premium
  

	Sony DSLR-A550	Pentax K-7	Nikon D5000	Canon EOS 500D
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
ISO 3200
ISO 6400
ISO 12800

There's not a big difference at the lower ISO settings and as usual most of it is down to the amount of noise reduction each camera is applying in-camera. In the ISO 800-3200 range the Canon EOS 500D is the clear winner for JPEGs (Sony uses too much noise reduction, Pentax, arguably, doesn't use enough). At ISO 6400 and 12,800 the A550's (and the Canon's) output is so blurry that it falls firmly into the 'only for emergencies' category - fine for small prints, but don't expect miracles.

Noise graphs

The graphs confirm that the JPEG noise has lots of chroma noise reduction, though it's interesting that the shadow noise is a little higher than the Canon or Nikon at most ISO settings (confirming something we saw in our real-life images).

	Sony DSLR-A550 JPEG noise graphs
Chroma
Black
Gray

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis.

RAW noise

Finally let's take a look at the A550's RAW output next to the competition. Removing any in-camera noise reduction and processing the images using Adobe Camera Raw (V5.6 in this case, all NR set to 0) gives us the nearest thing to a 'level playing field' for assessing the relative noise levels of the four cameras' sensors.

	Sony DSLR-A550 Raw	Pentax K-7 RAW	Nikon D5000 RAW	Canon EOS 500D RAW
ISO 100
ISO 200
ISO 400
ISO 800
ISO 1600
ISO 3200
ISO 6400
ISO 12800

Looking at the crops it's clear that the noise levels across the three higher resolution cameras here (Sony, Canon, Pentax) are very similar, though to our eyes the Sony looks like there's a little bit of noise reduction in the raw files. The Nikon D5000 is a little less noisy, presumably thanks to its marginally lower pixel count.

Raw Noise graphs

As you can see the A550's noise is very similar to the Nikon D5000 and is the lowest overall in the group. We know the Bionz processor does some noise reduction on raw files, and - though the others may well have NR in the raw pipeline too - it's the only one that looks, just a tiny bit, noise-reduced at settings above ISO 800.

	Sony DSLR-A550 Raw noise graphs
Chroma
Black
Gray

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity on the vertical axis.