How to read the common EXIF fields
EXIF stores values the way a camera produces them, so some need a little translating. The viewer does that for you, and this is what the common ones mean.
- Exposure time is how long the shutter was open. 1/250 s is four thousandths of a second: short enough to freeze most everyday movement. Longer times such as 0.8 s or 4 s call for a tripod or give a blurry subject.
- Aperture (f-number) such as f/1.8 describes the lens opening. A smaller number is a wider opening: more light and a blurrier background.
- ISO speed is the sensor sensitivity. Higher numbers brighten dark scenes and add noise.
- Focal length in millimetres is the lens zoom. The 35 mm equivalent value lets you compare cameras with different sensor sizes.
- Flash is a code that combines whether the flash fired, whether it was forced or automatic, and red-eye reduction. The viewer spells it out, and shows the number in brackets.
- Orientation is a number from 1 to 8. 1 means upright. 6 means the picture must be turned 90 degrees clockwise to look right, which is how a phone held upright often saves it.
- Colour space tells programs how to read the colours. sRGB is the usual one.
- Software, Artist and Copyright are free text written by whatever saved the file or by a person. They are easy to edit, so they show what the file says, not what is true.
Reading GPS coordinates
EXIF stores a position as three fractions for degrees, minutes and seconds, plus a letter for the hemisphere: N or S for latitude, E or W for longitude. The viewer shows both forms. In decimal degrees, south latitudes and west longitudes are negative, so -33.8688, 151.2093 is 33 degrees south and 151 degrees east, in Sydney.
The same block can hold altitude (above or below sea level), the direction the camera faced, speed, and the time and date of the GPS fix in UTC. Phones usually write a position only when the camera app has permission to use location. Many cameras write none at all.
A coordinate is as accurate as the receiver that produced it. Indoors, or just after the phone woke up, it can be off by a street or more. The viewer shows what the file says and cannot judge how good the fix was. XMP and IPTC can also hold a place: a city, a state, a country name. Those are listed under Location too, with their source.
Dates, time zones and why a photo can have several
A photo can carry three EXIF dates. Date taken is when the shutter was pressed. Date digitised is when the picture was turned into a digital file, which is the same moment for most cameras and different for scans. Date modified is when a program last saved the tags. Newer files add a time zone offset such as +11:00, and fractions of a second. Older files give a clock time with no zone, which is only the camera's own clock setting.
XMP and PNG files can hold dates of their own, in a different format, and the file system adds its own created and modified dates outside the file. They do not have to agree.
XMP, IPTC, thumbnails and maker notes
- XMP is an XML packet used by editing programs and some phones. It repeats many EXIF values and adds creator, rights, keywords, ratings and an editing history. Each property is listed with its source.
- IPTC is the older caption standard used by news and stock workflows: headline, caption, keywords, credit, copyright and place fields.
- The thumbnail is a small copy of the picture stored inside EXIF. Some programs forget to update it, so it can show a photo before it was cropped. The viewer shows it with its size.
- The maker note is private data from the camera maker. It is listed by size and the maker when the tool can tell, and it is never interpreted.