Because of the changing light conditions (the sun was low, it was windy and there was partial cloud cover) it was difficult to get consistent exposures in Aperture prioriy mode with both cameras and both lenses. So I shot a range of bracketed exposures in Manual mode and subsequently selected frames with consistent lighting conditions that had matched correct exposures. Stabilisation was disabled for this tripod-mounted test and Active D-lighting was turned off, which is the default setting.
By comparing frames I judged that the D610 produced the best results at f8 and the D7100 at f5.6. The image above was taken with the Nikon D610 in Manual mode with the aperture set to f8 and the sensitivity set to 100 ISO. My bracketed exposure at 1/200 provided the best exposure with most of the highlight detail retained. The frame from the D7100 was shot at the equivalent exposure of 1/400 at f5.6 also at 100 ISO.
I processed both files in Adobe Camera RAW using identical settings: Sharpening at 70 / 0.5 / 36 / 10, Luminance and Colour Noise Reduction both set to zero, and the Process to 2012 with the Adobe Standard profile. To eliminate the differences in white balance both files were set to 5300k with +6 tint. Just to note that at the time of processing Adobe Camera RAW did not support D610 RAW files and these crops were processed using the ACR 8.3 Release Candidate beta. These settings were chosen to reveal the differences in sensor quality and isolate them from in-camera processing. The high degree of sharpening with a small radius enhances the finest details without causing undesirable artefacts, while the zero noise reduction unveils what's really going on behind the scenes.
Much as I'd like these results to confirm what we saw from the JPEGs, I'm not seeing the sharper edges and improved detail in the D7100 RAW crops compared with those from the D610 - it's almost like the D7100's quality advantage only applies to in-camera JPEGs. That said, they still look very similar. In the first crop the level of detail is actually very similar, and in the second crop, aside from the fringing in the D7100 crop, it's hard to spot a quality difference. The fringing is evident in the third crop too, but again, though the D7100 crop looks slightly harder, I can't see any difference in the levels of detail.
If there's a margin between these two models in the fourth crop, you'd have to look very, very carefully to spot it, but there is one very interesting thing to see here. Look at the roof tiles in the lower left corner of the D7100 crop. Do you notice the slight colour banding there? That's moire - the thing that an optical low-pass filter (which of course, is absent in the D7100) is designed to eliminate. It's very slight and you'd only ever notice it if you were looking for it at 100 percent view, but it's there nonetheless. Having said that it's a fairly minor issue (certainly less of a problem than the fringing) and can easily be dealt with in the shots where it occurs.
Taking these results and the JPEG ones on the previous page together, I think it's fair to say that the quality delivered by the D7100's APS-C sensor is as good as the full-frame D610 at their lowest sensitivities, and the D7100 actually enjoys an advantage in crispness when comparing out-of-camera JPEGs.
Now it's time to examine their high ISO performance, starting with a JPEG comparison with the D7100 in my Nikon D610 Noise results.