ISO files store only the user data from each sector on an optical disc ignoring the control headers and error correction data, and are therefore slightly smaller than a raw disc image of optical media. udf file extension is sometimes used to indicate that the file system inside the ISO image is actually UDF and not ISO 9660. img extension, tend to have slightly different contents. img extension can also be found on some ISO image files, such as in some images from Microsoft DreamSpark however, IMG files, which also use the. iso file extension is the one most commonly used for this type of disc images. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created. Other than ISO 9660 media, an ISO image might also contain a UDF (ISO/IEC 13346) file system (commonly used by DVDs and Blu-ray Discs), including the data in its files in binary format, copied exactly as they were stored on the disc. Optical disc images are uncompressed and do not use a particular container format they are a sector-by-sector copy of the data on an optical disc, stored inside a binary file. And like any other ISO image, it may be written to an optical disc such as CD, DVD and Blu-Ray.

Software distributed on bootable discs is often available for download in ISO image format. ISO images can be created from optical discs by disk imaging software, or from a collection of files by optical disc authoring software, or from a different disk image file by means of conversion.

ISO images are expected to contain the binary image of an optical media file system (usually ISO 9660 and its extensions or UDF), including the data in its files in binary format, copied exactly as they were stored on the disc. Īn optical disc image (or ISO image, from the ISO 9660 file system used with CD-ROM media) is a disk image that contains everything that would be written to an optical disc, disk sector by disc sector, including the optical disc file system.

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