Most digital camera manufacturers have their own proprietary file formats for their digital images when shot in raw (or "unprocessed") form. Raw format is the preferred method of capturing digital images, especially in high-end cameras, because it contains vast amounts of data that allow you to edit the images significantly. Raw formats also prevent the degradation and image noise associated with compressed formats such as JPEG. On Nikon digital cameras, the raw format is called NEF. If you want to convert an NEF file, you will want to convert it to another type of file that is not compressed and will not lose quality.
Open Apple's Preview software. This software comes with all Macs as part of the Mac OS X operating system. It is a general-purpose photo viewer and editor. It does not offer many controls for editing, but is fine for converting NEF files. Go to the "File" menu and select "Open" and navigate to the folder where you stored your NEF files. Select one, several, or all of the files and click the "Open" button.
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Click the "File" menu and select "Save As." In the browser that appears, click on the scroll button next to "Format" and select "TIFF" or "Photoshop", if you intend to work on your images in that program. Another scroll button will appear next to "Depth" for either "TIFF" or "Photoshop." Select "16 bits/Channel" to keep all of the raw data from your NEF files. The "TIFF" choice also has a third scroll button option, "Compression." Select "None" to keep all of the original NEF data. Then click the "Save" button
Open the View NX 2 software that came with your Nikon camera. This, too, is a basic photo editor, though it has more options than Preview. In the View NX 2 browser window, find your NEF image and double-click it. Once it opens, go to the "File" menu and select "Convert." A pop-up dialog box will appear with "JPEG" as the default conversion in the top scroll button. Do not choose this if you want to keep all the data intact. Instead, choose "TIFF (16 Bit)" and do not compress the file. Then click the "Convert" button at the lower right.
Start your version of Adobe Photoshop and select "Open" from the "File" menu. Locate the appropriate NEF file and open it. Instead of going directly to the normal Photoshop editing mode, you will get a special editor for raw files. You can do some editing of the image here, including resizing, and then click "Open."
Go to the "File" menu again and select "Save As." Next to the scroll button marked "Format" in the dialog box, select "Photoshop," "Photoshop EPS" or "TIFF" in order to retain all the image data. Then rename your file and click "Save." This process works essentially the same way in Photoshop Elements.