The Nikon D90 was tested using a Nikkor 18-105 F3.5-5.6 VR lens and compared to the Pentax K20D using a DA* 50-135 F2.8 lens. Images below show the performance of these cameras on a typical scene at each ISO sensitivity. These are all 100% crops from maximum quality full-resolution JPEG images taken at night in manual mode with automatic white-balance and set ISO sensitivity.

What can be seen below is that the D90, unlike the K20D, applies noticeably more noise-reduction as sensitivity is increased. Noise levels in both cameras increases steadily between shots but the Nikon D90's noise-reduction is clearly paying off. While the Pentax shows visibly more noise starting at ISO 800 and keeps its images quite sharp throughout its usable range, the D90 produces relatively smoother images. The Nikon D90 can pull-off a mid-size prints at ISO 3200 where the K20D would barely make a usable small print. A small print can still be made from the D90's ISO 6400 mode, this cannot be said of the K20D though.

Here the Nikon was at its default color rendition which is warmer than reality. The K20D produces a more natural rendition of color. It cannot be seen from these crops but the overall scene consists mostly of snow which caused the Pentax K20D to meter one-stop under. Positive compensation of +1 EV was required. The Nikon D90 metered things perfectly using its Matrix metering mode, besting what most cameras would do.

Another factor for the differences in softness at low ISO comes from the lenses used. The Nikon lens used is not so great, particularly compared to the excellent Pentax DA* 50-135 F2.8. Unfortunately, we did not have similar lenses on hand. Note that a lens at a given focal length and aperture does not change softness with ISO. So any change in detail between ISOs is caused by internal image processing.