Editing JPEG File Interchange Format (section)

=== Resolution and aspect ratio ===
The JPEG standard does not include any method of coding the resolution or aspect ratio of an image. JFIF provides resolution or aspect ratio information using an application segment extension to JPEG. It uses Application Segment #0, with a segment header consisting of the [[null-terminated string]] spelling "JFIF" in [[ASCII]] followed by a byte equal to 0, and specifies that this must be the first segment in the file, hence making it simple to recognize a JFIF file. [[Exif]] images recorded by digital cameras generally do not include this segment, but typically comply in all other respects with the JFIF standard.