DNG - Adobe Digital Negative

DNG stands for Digital Negative. It is a raw photo format published by Adobe as an open, documented alternative to the private raw formats every camera maker invents for itself. A raw file is not a picture yet: it is the almost untouched reading from the camera's sensor, which a program must develop into an image, much as a film negative had to be printed. Because each manufacturer's raw format is a trade secret, software has to be taught every camera model one at a time; DNG exists to end that. Some cameras, phones and drones write DNG directly, and Adobe's free converter turns other makers' raw files into it.

How a DNG stores its data

A DNG is a TIFF file underneath, with the same tag-and-directory structure. Inside, the important part is the sensor data. A camera sensor does not see colour: it is a grid of light sensors under a pattern of red, green and blue filters, so each position records one colour. A DNG keeps that mosaic as read, usually at 12, 14 or 16 bits of precision, before it is interpolated into full-colour pixels. Compression is lossless by default, although a lossy option exists for smaller archives.

A DNG also carries a ready-made JPEG preview so viewers have something to show, plus the numbers a converter needs to interpret this camera's colours: colour matrices, the light source they were measured under, black and white levels, and the white balance as shot. Those calibration tags are the trick: a proprietary raw file assumes the software already knows the camera, while a DNG describes itself, so any converter can render it correctly. Settings such as white balance, contrast and lens corrections are stored as instructions rather than applied, which is why raw editing is non-destructive: you are changing notes, not the picture.

What metadata a DNG carries

A great deal, because a DNG is both a photograph and a working document.

• TIFF tags for the basics: Make, Model, Software, DateTime, Artist and Copyright.
• EXIF, the camera standard: DateTimeOriginal, ExposureTime, FNumber, ISOSpeedRatings, FocalLength, LensModel and frequently a body and lens serial number.
• GPS tags with latitude, longitude, altitude and a UTC timestamp, when the camera or a paired phone supplied them.
• DNG's own tags: the format version, a unique camera model string, an identifier for the raw data, a camera serial number, and OriginalRawFileName, the name of the file the DNG was converted from, which preserves the original .CR2 or .NEF name and its numbering.
• XMP, where editing software keeps the human side and the develop settings: title, description, keywords, star rating, colour label, rights statement, and the crop and tone adjustments you have made. Because the XMP lives inside the DNG, those edits travel with the file rather than in a companion file.
• Maker notes copied from the original raw file, holding manufacturer-specific details.

All of that makes a DNG one of the most personally revealing files a photographer handles: GPS coordinates, camera and lens serial numbers, an owner name if the camera stores one, the original filename, the editing history and the software's name can all sit in one file. Keep the embedded preview in mind too: it is a separate picture, and if written before a crop it may still show the wider frame.

When to use DNG

DNG suits archiving, and any workflow that would rather deal with one raw format than a dozen. It keeps a raw file's full latitude, so you can recover a blown sky, correct a white balance or lift deep shadows, while being openly documented, usually a little smaller than the manufacturer's original, and self-contained, with edits and keywords stored inside it.

Against the camera's own raw format, the trade is control against completeness: converting to DNG can drop or obscure manufacturer-specific extras, the maker's own software may refuse the converted file, and it adds a step to every import, so photographers who rely on the camera maker's software usually keep the original. Against JPG, a raw file must be developed before anyone can use it and is many times larger, so keep the DNG as the negative and export a JPG when a picture has to be shared.