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 exposure is matched to the ISO (ie. ISO 200, 1/200 sec for consistency of exposure between cameras).

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 (lots of fine detail) but we may replace this at a future date. Note that noise values indicated on the graphs below should not be compared to those in other reviews.

Nikon D2X High ISO Noise Reduction

The D2X features adjustable High ISO Noise Reduction. By default this is set to 'Normal' (as used for the crops shown below) but can also be set to 'High' and 'Off'. A description of each mode is shown below (reproduced from the Nikon D2X user manual with permission).

Test notes:

• Shots taken at approximately 22°C (~72°F)
• Lighting was simulated daylight measured as 10.1 EV (at ISO 100)

Nikon D2X vs. Canon EOS 1Ds Mark II

• Nikon D2X : Nikkor 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters, High ISO Noise Reduction 'Normal', JPEG Large / Fine
  
• Canon EOS 1Ds Mark II: Canon 50 mm F1.4 lens, Aperture Priority, Manual WB,
  Default Parameters, JPEG Large / Fine

	Nikon D2X n/a	Canon EOS 1Ds Mark II ISO 50, 1/50 sec, F4
Crops
	Nikon D2X ISO 100, 1/100 sec, F3.5	Canon EOS 1Ds Mark II ISO 100, 1/100 sec, F4
Crops
	Nikon D2X ISO 200, 1/200 sec, F3.5	Canon EOS 1Ds Mark II ISO 200, 1/200 sec, F4
Crops
	Nikon D2X ISO 400, 1/400 sec, F3.5	Canon EOS 1Ds Mark II ISO 400, 1/400 sec, F4
Crops
	Nikon D2X ISO 800, 1/800 sec, F3.5	Canon EOS 1Ds Mark II ISO 800, 1/800 sec, F4
Crops
	Nikon D2X (HI-1) ISO 1600, 1/1600 sec, F3.5	Canon EOS 1Ds Mark II ISO 1600, 1/1600 sec, F4
Crops
	Nikon D2X (HI-2) ISO 3200, 1/3200 sec, F3.5	Canon EOS 1Ds Mark II ISO 3200, 1/3200 sec, F4
Crops

Visibly there's little difference between the gray patches up to ISO 1600, at ISO 3200 the D2X's noise reduction is more effective at managing the noise. A check of the detail crop however shows that at ISO 1600 and 3200 we're losing detail to the smoothing effect of noise reduction. A quick scan of the graph below shows that noise levels between these cameras are actually quite similar up to ISO 800 (with or without noise reduction), with noise reduction switched Off the D2X's ISO 1600 noise is marginally higher than that of the EOS 1Ds Mark II. Set noise reduction to High and you get the cleanest looking images and lowest measurable noise levels but at the expense of image detail (see samples below graphs).

Luminance noise graph

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of luminosity (normalized image) 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 image) are on the vertical axis. In this graph the D2X High ISO Noise Reduction was set to 'Normal'.

Effect of High ISO Noise Reduction

As we've seen above the three options give you the option to control the amount of noise reduction applied to the image (which is refreshing as few digital SLR's allow control of NR). Below you can see crops from ISO 400 - 3200 images at each noise reduction setting. Personally I'd opt for 'Off' and stay below ISO 3200.

	Off	Normal	High
ISO 400
ISO 800
ISO 1600 (HI-1)
ISO 3200 (HI-2)