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.

What is the real sensitivity?

In our tests both the Minolta DiMAGE A1 and Canon PowerShot G5 proved to be approximately three quarters of a stop (0.7 EV) more sensitive than the selected ISO. The measured light level at the chart was 11.7 EV, at ISO 100 a shutter speed of 1/100 sec should produce an aperture of F5.6, instead both the A1 and G5 used F7.1. The Sony DSC-F717 proved to be spot on; ISO 100, 1/100 sec, F5.6 = 11.7 EV (except at ISO 400 and 800 where it was 0.3 EV slower).

Test notes:

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

Minolta DiMAGE A1 vs. Canon PowerShot G5

	Minolta DiMAGE A1 -	Canon PowerShot G5 ISO 50, 1/50 sec, F7.1
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 100, 1/100 sec, F7.1	Canon PowerShot G5 ISO 100, 1/100 sec, F7.1
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 200, 1/200 sec, F7.1	Canon PowerShot G5 ISO 200, 1/200 sec, F7.1
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 400, 1/400 sec, F7.1	Canon PowerShot G5 ISO 400, 1/400 sec, F7.1
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 800, 1/800 sec, F7.1	Canon PowerShot G5 -
Partial crop
Red Green Blue channels

What's clear from these crops is that the DiMAGE A1 manages to keep noise levels down much lower than the PowerShot G5. Indeed visually the DiMAGE A1's noise levels appear to be the same as the G5 one stop slower. Looking at the pattern of the noise in each channel it does appear as though Minolta have employed a noise reduction algorithm where Canon are simply presenting the image data without noise reduction. The other reason could be the different noise characteristics of the 2/3" type sensor (8.8 x 6.6 mm) used on the DiMAGE A1 versus the 1/1.8" type sensor (7.2 x 5.3 mm) used on the PowerShot G5.

Minolta DiMAGE A1 vs. Sony DSC-F717

	Minolta DiMAGE A1 ISO 100, 1/100 sec, F7.1	Sony DSC-F717 ISO 100, 1/100 sec, F5.6
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 200, 1/200 sec, F7.1	Sony DSC-F717 ISO 200, 1/200 sec, F5.6
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 400, 1/400 sec, F7.1	Sony DSC-F717 ISO 400, 1/320 sec, F5.6
Partial crop
Red Green Blue channels
	Minolta DiMAGE A1 ISO 800, 1/800 sec, F7.1	Sony DSC-F717 ISO 800, 1/640 sec, F5.6
Partial crop
Red Green Blue channels

For each level of sensitivity the DiMAGE A1 and DSC-F717 appear to be relatively close with the visible noise level, that is reflected by the measured noise level shown on the graph below. The thing we must however bear in mind is that the DiMAGE A1 is actually between 0.7 and 1.0 EV more sensitive than the indicated ISO sensitivity, in which case the DiMAGE A1 has a significant advantage.

Luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity (normalized) on the vertical axis. Note that the faint shifted lines better represents the actual sensitivity of the DiMAGE A1 and PowerShot G5.

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.