Body & Design
The most obvious difference between the D5100 and its predecessor is the addition of a side-hinged, fully articulated LCD screen. The D5000's LCD screen was articulated, but slightly awkwardly, via a hinge at the base of the camera's rear. This caused problems in some shooting positions, especially when the camera was mounted on a tripod. The D5100's screen is hinged in a more conventional way, along its side, which allows it to fold out from the left hand side of the camera. It is fully articulated, which means that the display can also be folded inwards, for protection.
The redesign of the LCD screen has forced some pretty major changes to the D5100's rear control layout. Because of the incorporation of a vertical hinge on its left hand side, the buttons that occupied this position on the D5000 have been moved to the right of the LCD. The play and magnification buttons are now ranged close to the 4-way controller, and the menu and delete buttons have swapped places. The 'i' button, which used to sit to the left of the LCD screen on the rear of the D5000 has moved too, and joins the AE-L/AF-L button on the top right of the camera, alongside the control dial.
The D5100 uses the same EN-EL14 battery as the D3100. It's a 7.6Wh battery that the D5100 manages to make last for 660 shots, according to CIPA standard tests (which compares favorably against the 550 achieved by the D3100).
Viewfinder specs and view
The D5100 has a very typical viewfinder specification for its class of camera. It offers 0.78x magnification and a 95% field of view, which is pretty standard. As you can see from the illustration below, the D5100's viewfinder is pretty much the same size as the Canon EOS 600D/Rebel T3i and Canon's entry-level EOS 1100D/Rebel T3.
One figure hidden away in every SLR's spec is the size of the viewfinder (often in a format that makes comparison between competing models impossible). The size of the viewfinder is a key factor in the usability of an SLR - the bigger it is, the easier it is to frame and focus your shots, and the more enjoyable and involving process it is.
Because of the way viewfinders are measured (using a fixed lens, rather than a lens of equivalent magnification), you also need to take the sensor size into account, so the numbers in the diagram below are the manufacturer's specified magnifications divided by the respective 'crop factors'.
|The Nikon D5100's viewfinder magnification is 0.78x, putting it in-line with its immediate peers once the crop-factor has been taken into account. This is not bad for an optical viewfinder at this price, but is much smaller than the 1.1x electronic viewfinder in Sony's competing A55 (Which would appear as 0.73x on this diagram).|
Most cameras at this level crop the frame slightly when you look through the viewfinder - in other words you get slightly more in the final picture than you see through the viewfinder. In common with most of its competitors the D5100 only shows 95% (vertically and horizontally) of the frame. What this actually means is that a small portion of the captured image doesn't show up in the viewfinder. The 'missing' 5% is illustrated below.
|Nikon D5100: 95% viewfinder.|