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:

Camera Indicated sensitivity
Actual sensitivity
Canon PowerShot G6 50, 100, 200, 400 100, 160, 320, 640
Casio EX-P700 80, 160, 320, 640 100, 200, 400, 800
Olympus C-7000 Zoom 80, 100, 200, 400 80, 100, 200, 400
Pentax 750Z 80, 100, 200, 400 80, 100, 200, 400
Sony DSC-V3 100, 200, 400, 800 100, 160, 320, 640

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

Casio EXILIM EX-P700 vs. Canon PowerShot G6

  Casio P700, ISO 80
(~ISO 100), 1/80 sec, F4.1
Canon G6, ISO 50
(~ISO 100), 1/60 sec, F4.5
  Casio P700, ISO 160
(~ISO 200), 1/160 sec, F4.1
Canon G6, ISO 100
(~ISO 160), 1/100 sec, F4.5
  Casio P700, ISO 320
(~ISO 400), 1/320 sec, F4.1
Canon G6, ISO 200
(~ISO 320), 1/200 sec, F4.5
  Casio P700, ISO 640
(~ISO 800), 1/640 sec, F4.1
Canon G6, ISO 400
(~ISO 640), 1/400 sec, F4.5

The P700 delivers fairly clean images across its sensitivity range, however at ISO 320 (equiv. ISO 400) the camera's noise reduction system starts to become intrusive and affects the reproduction of detail. At ISO 640 (equiv. ISO 800) the camera also appears to reduce sharpening and the overall result is pretty disappointing with blotchy color noise and poor detail. Casio would have done better aiming less high with the maximum sensitivity and giving the user the option to disable ISO noise reduction.

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).