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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 to enable faster shutter speeds and/or better performance in low light. The way this works in a digital camera is by "turning up the volume" (gain) on the CCD's signal amplifiers. Nothing is without its price however and doing so also typically increases visible noise (random speckles visible all over the image).

We are now using a more reliable, repeatable and neutral method for evaluating noise. Shots are taken in daylight lighting in our studio. Noise is measured as the standard deviation of the medium gray patch on a Gretag MacBeth ColorChecker chart. The image is normalized before measurement of noise to remove the possibility of figures being affected by image contrast (one method of masking noise). Note that noise numbers shown on the graphs below can not be compared to those in older reviews.

Test notes:

  • Shots taken at approximately 21°C (~70°F)
  • Lighting was simulated daylight
  • Manual white balance
  • Aperture Priority

Canon PowerShot A70 vs. Minolta DiMAGE Xt

The PowerShot A70 has aperture priority mode which means we can keep the aperture locked and only vary shutter speed, the DiMAGE Xt does not have aperture priority and so at ISO 400 it switched from its 'wide open' aperture of F3.0 to F6.7, hence the sudden drop in shutter speed.

  Canon PowerShot A70
ISO 50
, 1/100 sec, F5.0
Minolta DiMAGE Xt
ISO
50, 1/180 sec, F3.0
 Partial crop
Red|Green|Blue
channels
  Canon PowerShot A70
ISO 100
, 1/250 sec, F5.0
Minolta DiMAGE Xt
ISO
100, 1/350 sec, F3.0
 Partial crop
Red|Green|Blue
channels
  Canon PowerShot A70
ISO 200
, 1/500 sec, F5.0
Minolta DiMAGE Xt
ISO 200, 1/750 sec, F5.0
 Partial crop
Red|Green|Blue
channels
  Canon PowerShot A70
ISO 400
, 1/1000 sec, F5.0
Minolta DiMAGE Xt
ISO
400, 1/180 sec, F6.7
 Partial crop
Red|Green|Blue
channels

As you can see from the graph below the A70 exhibited a steady increase in noise throughout its sensitivity range. Despite the fact that we have plotted the two camera side by side the exposure values tell us that the A70 (like many other Canon PowerShot digital cameras) is actually approximately a stop more sensitive than the Xt at the same indicated ISO, thus the A70's ISO 50 is as sensitive as the Xt's ISO 100.

Luminance noise graph

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

RGB noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of each of the red, green and blue channels (normalized) are on the vertical axis.

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