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Nikon D3 ISO speed linearity

The Nikon D3 has speed settings from ISO 50 to an incredible ISO 25,600. We are interested to know if these settings correctly reflect the sensitivity to light of the camera.

Method

This test used the data obtained for the mid-grey signal-to-noise ratio test, see here.

That test photographed a white board using aperture priority mode, so the camera would use its matrix metering to place the subject in the centre of its histogram. A shot was taken at each ISO rating from 200 to 25,600. The mean value of the resulting raw DNs was calculated. The NEF files were also developed in Adobe Lightroom and a note was taken of where the peak was on Lightroom's histogram.

Results

It was found that the mean value of the raw DNs increased by a small amount as the ISO speed increased. Accordingly, the peak on the Lightroom histogram moved slightly to the right as the ISO increased, but remained approximately in the centre.

The exposure set by the camera halved exactly for each doubling of the ISO, except for the top speed of ISO 25,600 where a slightly longer exposure was set.

The graph takes both of these factors into account by plotting ISO speed against the "appropriate exposure" which is the actual exposure the camera set adjusted by the small factor necessary to keep the mean value of the DNs the same as it was at ISO 200.

Provided one is confident that the D3 ISO speed is correctly rated at ISO 200 - and although I neglected to compare it directly with an exposure meter, I did find the D3 set exactly the same exposures as my Canon EOS 5D did - these results show that all the higher speed ratings of the D3 are correct.

Peter Facey, Winchester, England
20080207 updated
20080119 originated