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ISO / Sensitivity accuracy

In a new addition to our reviews we are now measuring the actual sensitivity of each indicated ISO sensitivity. This is achieved using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV.

The D80's sensitivity proved to be pretty much exactly as indicated (just like all previous Nikon digital SLRs), with ISO 100 actually being ISO 100. The Sony DSLR-A100 proved to be about a third of a stop more sensitive than indicated (hence ISO 100 is actually more like ISO 125).

Indicated
sensitivity
Nikon D80
(actual sensitivity)
Canon EOS 400D
(actual sensitivity)
Sony DSLR-A100
(actual sensitivity)
ISO 100 ISO 100 ISO 100 ISO 125
ISO 200 ISO 200 ISO 200 ISO 250
ISO 400 ISO 400 ISO 400 ISO 500
ISO 800 ISO 800 ISO 800 ISO 1000
ISO 1600 ISO 1600 ISO 1600 ISO 2000
ISO 3200 ISO 3200 n/a n/a

UPDATE 18/Dec/06: In our original review a mistake was made in the measurement of the ISO sensitivity of the Canon EOS 400D as used for comparison, this has now been corrected and the releveant pages of this review updated.

ISO Sensitivity / Noise levels

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 noise measurement tool (version 1.4 in this review). Click here for more information. (Note that noise values indicated on the graphs here can not be compared to those in other reviews). Room temperature is approximately 22°C (~72°F), simulated daylight lighting.

Nikon D80 vs. Canon EOS 400D (Rebel XTi) vs. Sony Alpha DSLR-A100

  • Nikon D80: Nikkor 50 mm F1.4 D lens, Aperture Priority, Manual WB,
    Default Parameters (Normal), High ISO NR (Normal), JPEG Large / Fine
     
  • Canon EOS 400D: Canon 50 mm F1.4 lens, Aperture Priority, Manual WB,
    Default Parameters (Standard PS), JPEG Large / Fine
     
  • Sony DSLR-A100: Minolta 50 mm F1.4 lens, Aperture Priority, Manual WB,
    Default Parameters (Standard DEC), JPEG Large / Fine
Nikon D80
ISO 100
Canon EOS 400D
ISO 100
Sony DSLR-A100
ISO 100
(125 equiv.)
Nikon D80
ISO 200
Canon EOS 400D
ISO 200
Sony DSLR-A100
ISO 200
(250 equiv.)
Nikon D80
ISO 400
Canon EOS 400D
ISO 400
Sony DSLR-A100
ISO 400
(500 equiv.)
Nikon D80
ISO 800
Canon EOS 400D
ISO 800
Sony DSLR-A100
ISO 800
(1000 equiv.)
Nikon D80
ISO 1600
Canon EOS 400D
ISO 1600
Sony DSLR-A100
ISO 1600
(2000 equiv.)
Nikon D80
ISO 3200
Canon EOS 400D
n/a
Sony DSLR-A100
n/a

From ISO 100 to 400 there is very little difference between these three cameras, noise levels are similar, as is the reproduction of detail (the D80 perhaps being slightly softer than the EOS 400D and DSLR-A100). At ISO 800 the DSLR-A100 looks noisier with noticeable luminance and chroma (color) noise, the D80 maintaining a clean appearance although at the expense of some detail. At ISO 1600 the D80's gray patch looks the cleanest, followed by the EOS 400D and DSLR-A100.

Closer inspection show that this is due to Nikon's superior chroma noise reduction which keeps the mottled color pattern (chroma noise) to a minimum, producing a more 'film like' monochromatic grain. Of the three cameras the EOS 400D still maintains the best detail. The ISO 3200 option (HI 1.0) is unique to the D80 (compared to its peers) and while there's a very clear loss of detail it can still be useful in very low light situations where the choice would be this or not getting the shot at all.

Luminance noise graph

The 'busy' graph below confirms what we've seen above, that noise levels are similar up to ISO 400 after which the D80's noise reduction kicks in. We have also included test results from a six megapixel D70s which has slightly higher noise than the D80 throughout the range. Note that the EOS 400D's black (shadow) noise is lower at almost every sensitivity (except ISO 800).

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

Luminance noise graph (zoomed, only 10 MP cameras)

The graph below is based on the same data as above except it only shows ten megapixel cameras (the D80, EOS 400D and DSLR-A100) and the standard deviation range (vertical axis) has been reduced to a maximum of six. A difference in standard deviation less than one would be quite difficult to distinguish in a normal everyday shot.

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

Chroma (color) noise graph

In addition to measuring luminance noise we now also measure chroma (color) noise. As you can see the D80's chroma-biased noise reduction keeps the visibility of the more digital-looking chroma noise to a minimum, as low at ISO 1600 as the EOS 400D at ISO 800.

Indicated ISO sensitivity is on the horizontal axis of this graph, standard deviation of color on the vertical axis.

Noise reduction differences

Adjusting the D80's 'High ISO Noise Reduction' level appears to only affect the amount of luminance noise reduction applied to the image. If you look at the crop comparisons below the graph you can clearly see the decrease in luminance noise (appearance of grain) as noise reduction is increased, however chroma noise is never an issue even with the 'Off' setting.

NR Off
ISO 800
NR Low
ISO 800
NR Normal
ISO 800
NR High
ISO 800
NR Off
ISO 1600
NR Low
ISO 1600
NR Normal
ISO 1600
NR High
ISO 1600
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