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Nikon D-5100 Digital Camera
The Nikon D5100 is a 16.2 megapixel DX format DSLR F-mount camera announced by Nikon on April 5, 2011. It features the same 16.2-megapixel CMOS sensor as the D7000 with 14-bit resolution, while delivering Full HD 1080p video mode at either 24, 25 or 30fps. The D5100 is the first Nikon DSLR to offer 1080p video at a choice of frame rates; previous Nikon DSLRs that recorded 1080p only did so at 24 fps. It replaces the D5000 and is replaced by the D5200.
Nikon
D-5100
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Nikon
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Like Nikon's other consumer level DSLRs, the D5100 has no in-body autofocus motor, and fully automatic autofocus requires one of the currently 162 lenses with an integrated autofocus motor. With any other lenses the camera's electronic rangefinder (which indicates if the subject inside the selected focus point is in focus or not) can be used to manually adjust focus.
Can mount unmodified A-lenses (also called Non-AI, Pre-AI or F-type) with support of the electronic rangefinder and without metering.
if you just want a video mode that you can easily jump to without interrupting your still shooting, the D5100's design inherits the D7000's intelligence. The switch on the side of the mode dial toggles between regular and Live View/Video mode, so you don't have to use an awkwardly placed mode on the dial. And the record button is in a great spot by the shutter; it's easily reachable with your forefinger, but not in a spot where you're likely to hit it by accident.
Wikipedia