An in-depth case study of the use of DxOMark dataTuesday June 09 2009 Sensor Insight
The second main difference between the Canon EOS 500D and the Nikon D5000 pertains to dynamic range (DR) measurement (11.5 vs 12.5 after normalization). The DR score represents the best results of DR measurements obtained for the entire ISO range.
DR results for both the Canon and the Nikon:
Dynamic range is the ratio between the highest and the lowest gray luminance a sensor can capture. The highest gray luminance value is easily computed as the minimum luminance required to saturate the sensor; the lowest gray luminance is the gray luminance for which the SNR is larger than 0.
Once again, Nikon takes the lead in low ISO settings. Canon behaves very well in high ISO.
One important difference between the Canon EOS 500D and the Nikon D5000 is their respective behaviors in low ISO in the darkest part of the dynamic (see the full SNR results for each sensor in “further information” below).
The readout noise of Canon impacts its DR measurement; further, note that this sensor does not take advantage of its 14-bit mode, as the maximum DR available is only 11.5 eV.
Below are the Full SNR tabulations for the two sensors:
The theoretical SNR curve of an ideal sensor (i.e., no readout noise, only photonic noise) should be linear on a log/log scale along the entire dynamic.
In reality, we observe an increasing influence of readout noise when gray level decreases: the curve falls more than expected for the dark gray level. So when the curve presents an inflection, it denotes the darkest grayscale observed without the influence of readout noise. The readout noise will influence mostly the low ISO settings.
For the D5000, it is fairly hard to distinguish any inflection in the curve, denoting a very slight increase for readout noise (photonic noise is still dominant even in the darker part of the dynamic). This is not the case for the Canon 500D.
For the Nikon D5000, the ISO 100 and ISO 200 noise curves are close along the whole sensor dynamic because the real ISO sensitivities are very close:
ISO measured (ISO 100) = 119
ISO measured (ISO 200) = 140
The Nikon D5000’s DR does not gain a full stop between ISO 200 and ISO 100, but this is due in this case to the real sensor sensitivity, which does not evolve as expected.
It is not the same story for the Canon EOS 500D:
- The real ISO sensitivity shows one full stop between ISO 100 and ISO 200;
- The Full SNR curves confirm this measurement, as the difference between the two curves is 3dB, as expected for the other 90% of the dynamic—the bright part;
- For the darkest value of the dynamic, the curve at ISO 100 falls very quickly.
The Canon EOS 500D behaves the same way as many other Canon cameras (see Dynamic range and ISO sensitivity)— that is, the readout noise becomes predominant and thus the lowest gray level, such that SNR=0dB is independent from ISO. This explains why the Canon EOS 500D’s DR does not increase between ISO 200 and ISO 100 and obtains only 11.5 eV as its maximum DR measurement.