The arrival of the D5100 appears to signal the completion of Nikons refresh of its non-pro DSLR lineup. Its feature set and pricing mean that it sits very comfortably between the beginner-friendly D3100 and the high-end D7000 - it's clearly aimed to attract the attention of enthusiast photographers without cannibalizing sales of is sister models. A camera maker can judge the success of its market positioning by seeing how many people are wondering 'which model is right for me?' on discussion forums - and we think a D3100/D5100/D7000 progression would minimize such doubts. The official line is that the D90 maintains its position in the range, but both its naming and overlap with other models suggest that its role is now more one of historical interest than future significance.
The D5100 has a very similar 16.2MP CMOS sensor to the excellent one seen in the D7000 but, understandably, loses out on that camera's high-end build and feature-set. So there's no wireless flash control, magnesium alloy build or 39-point AF system but the underlying image quality is all but identical.
As has become standard for a Nikon at this price point, the D5100 offers a single control dial, pentamirror viewfinder and no built-in autofocus motor. However, it gains 1080p video capability (at 30, 25 or 24fps), saved using the efficient H.264/AVC codec, and a 920,000 dot fully articulated LCD panel to help shoot it. These are both significant gains over its predecessor the D5000, and the improvements extend to the D5100 having smaller, neater construction and a more conventional side-mounted hinge for that LCD.
These changes resolve two rather awkward aspects of Nikon's existing lineup: if the D5100 and D7000 end up being neighboring models it will avoid the inelegant overlap that existed between the D5000 and D90. It also ensures a more elegant appearance to the models themselves (the D5000 was many things, but pretty wasn't one of them).
Although the D5100 is listed as having an Expeed 2 processor, it's worth remembering that Nikon doesn't use this naming system to denote any specific components, so the actual chunks of silicon and capabilities aren't necessarily the same as those in the D3100 or D7000. However, in addition to a similarly specced 16.2MP sensor, the D5100 offers the same ISO settings as its big brother - extending up to an equivalent of ISO 25,600. It also inherits 14-bit Raw shooting - one of the factors that helps give the D7000 its impressive dynamic range - which is something Nikon used to offer only on its more expensive models.