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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
(ISO)
Actual sensitivity
(ISO)
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

Olympus C-7000 Zoom vs. Pentax Optio 750Z

  Olympus C-7000, ISO 80
1/80 sec, F3.5
Pentax 750Z, ISO 80
1/80 sec, F3.5
Crops
  Olympus C-7000, ISO 100
1/100 sec, F3.5
Pentax 750Z, ISO 100
1/100 sec, F3.5
Crops
  Olympus C-7000, ISO 200
1/200 sec, F3.5
Pentax 750Z, ISO 200
1/200 sec, F3.5
Crops
  Olympus C-7000, ISO 400
1/400 sec, F3.5
Pentax 750Z, ISO 400
1/400 sec, F3.5
Crops

With equal sensitivity and equal exposure values the Olympus C-7000 and Pentax 750Z are well matched. Looking down the crops the Olympus starts very clean and only gets noticeably noisy at ISO 200, at ISO 400 noise is noticeable but not too intrusive, although we can see the effects of a noise reduction algorithm. The Pentax is another story, its noise levels were oddly almost identical throughout the sensitivity range (certainly from ISO 80 to 200), we can put this down to a progressive noise reduction, the effects of which we can begin to see at ISO 200 (some softness and loss of detail). Overall the C-7000 is better because of its less intrusive noise reduction and cleaner ISO 80 & ISO 100, the 750Z more consistent but detail loss from noise reduction at higher sensitivities.

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).

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