ISO Sensitivity / Noise levels

Standard Test

ISO equivalence on a digital camera is the ability to increase the sensitivity of the sensor. The 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 ISO's. 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 image sequence is run through our own proprietary measurement tool which measures the standard deviation (normalized) of the middle gray patch (indicated by the red rectangle above). Additionally we now have a 'detail crop', this is currently a postage stamp. Note that the detail crops below are not intended for comparison between cameras but to test for noise reduction (different cameras require different aperture settings to achieve the same shutter speed and hence may be more or less sharp).

Actual vs. Indicated sensitivity

One thing not to fall foul of when performing this kind of test is the difference between the indicated sensitivity and the actual sensitivity. The actual sensitivity is easily calculated by comparing the exposure used by the camera to that measured by a high quality light meter (ensuring the middle gray patch is equal for all samples and cameras). By doing this we managed to produce the following difference table:

Hence, the Canon G6 is roughly two thirds of a stop more sensitive than its indicated ISO, the Casio EX-P700 is a third of a stop more sensitive than its indicated ISO and the Sony DSC-V3 is about a third of a stop less sensitive than its indicated ISO (from ISO 200 up).

Test notes

• Shots taken at approximately 23°C (~73°F)
• Lighting was artificial daylight
• Manual white balance
• Aperture Priority

Canon PowerShot G6 vs. Sony DSC-V3

	Canon G6, ISO 50 (~ISO 100), 1/60 sec, F4.5	Sony DSC-V3, ISO 100 (~ISO 100), 1/100 sec, F3.5
Crops
	Canon G6, ISO 100 (~ISO 160), 1/100 sec, F4.5	Sony DSC-V3, ISO 200 (~ISO 160), 1/160 sec, F3.5
Crops
	Canon G6, ISO 200 (~ISO 320), 1/200 sec, F4.5	Sony DSC-V3, ISO 400 (~ISO 320), 1/320 sec, F3.5
Crops
	Canon G6, ISO 400 (~ISO 640), 1/400 sec, F4.5	Sony DSC-V3, ISO 800 (~ISO 640), 1/640 sec, F3.5
Crops

Despite a jump of two megapixels and a significant reduction in photosite size the PowerShot G6 delivers cleaner images than its five megapixel 'older brother' the G5. Compared to the other seven megapixel digital cameras in our group test the G6 performs well, especially between ISO 50 and 200 (ISO 100 to ISO 320 equiv.). The G6 also appears to not use noise reduction to achieve this (although it may be subtly turning down sharpening).

Luminance noise graph (actual ISO sensitivity)

ISO sensitivity is on the horizontal axis of this graph, standard deviation of each of the red, green and blue channels (normalized image) are on the vertical axis. (Click here to compare this to the indicated ISO sensitivity noise graph).