The Nikon RAW image format is Nikon Electronic Format (NEF). Photos taken in RAW mode are stored in files with the .NEF extension. With each new camera, the RAW data changes, and software must be updated accordingly to be able to read the file. The reason for this is that a RAW (.NEF) file contains the output taken directly from the camera sensor as well as the camera specific settings you use to take the shot. A new camera means a new sensor and new data and settings. If you try to open the D7100 .NEF file using a program made before the D7100 was released, it will not be able to understand the format.
You have two basic choices: either update your program to a new version, or use a converter to change the format.
The Nikon D7100 .NEF files can be opened using Nikon ViewNX2. D7100 support was added as of version 2.7.3 and 2.7.4 is the newest version out now. ViewNX2 allows only basic editing capabilities. You can use this program to save the RAW file as a TIFF and open in the editor of your choice (such as Adobe Photoshop). Nikon ViewNX2 is included with the D7100 or you can download it from Nikon.
You can open and edit D7100 .NEF files using CaptureNX2 latest version 2.4.1. This commercial software allows advanced editing and high performance 64-bit compatibility for use on modern operating systems. Current CaptureNX2 users can update to the latest version with a free update. New users can download a 60-day free trial or purchase CaputreNX2 full version on Amazon, while owners of the previous version can upgrade from Capture NX 1.x.
Adobe Lightroom 4.4 and Lightroom 5 and Photoshop CS6 support for Nikon D7100 RAW (.NEF) files as of April 3, 2013.
Adobe Photoshop CS6: Adobe Camera Raw 7.4 You can open D7100 files with Adobe Photoshop CS6 with the newest Adobe Camera RAW plugin version 7.4. Earlier versions of Photoshop (CS5 and older) can not be updated to the newest Camera Raw version and will not be able to open Nikon D7100 .NEF files. You will need CS6 with the updated Camera Raw to open D7100 .NEF files. For earlier versions, use the DNG converter (see next item below).
Adobe DNG Converter: You can convert D7100 NEF files to Adobe Digital Negative Gallery (DNG) format using the Adobe DNG Converter version 7.4. After converting to DNG format, you can open .DNG files with earlier versions of Adobe Photoshop and other programs that can read DNG format.
Adobe Lightroom: Open D7100 RAW (.NEF) files using Adobe Photoshop Lightroom 5. You can also use Adobe Photoshop Lightroom Version 4.4. Lightroom versions before 4.4 will NOT read the D7100 RAW .NEF file. Lightroom 3 can not be updated to read the files. You must have the current Lightroom 4.x branch or use the Adobe DNG converter described above.
NOTE: If you are still unable to open the D7100 .NEF files, make sure you are NOT using Nikon Transfer (standalone program) to copy the images. Old versions of the Nikon Transfer program can corrupt the D7100 .NEF files and can cause problems opening them. If you have files corrupted by Nikon Transfer, you can try this tool to fix them. In the future, copy the files using your operating system, or use the transfer function in the newest version of View NX2.