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JPG
What is a JPG file?
The universal photo format. Small files, no transparency, quality drops each time it is saved.
What JPG is
JPG is a binary format, meaningful only to a program that knows it. It stores a grid of pixels, so enlarging it past its own resolution softens it. It is used for photography, the web, email and handing a finished file over.
The extension is .jpg, and the full name is JPEG Image. Both matter less than what the file can hold, which is what the rest of this page is about.
Where JPG came from
Joint Photographic Experts Group published it in 1992. The specification is ITU-T T.81.
A format that has been readable for that long is a format worth trusting with something you want back in ten years.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
Detail is traded for size
JPG discards detail to stay small, and what it discards does not come back. That is the right trade for something being looked at and the wrong one for something being edited: each re-save takes another bite.
It cannot be transparent
JPG has nowhere to record transparency. Anything transparent in the source is filled with a solid colour on the way in — white, unless something chooses otherwise.
Colour it can describe
Up to 8 bits per channel. It can work in RGB, greyscale and YCbCr, the model video and JPEG compress in.
Eight bits per channel is what a screen shows and what nearly every delivery format carries.
There is a size limit worth knowing
JPG cannot describe an image larger than 65,535 pixels on a side. That is far beyond an ordinary photograph and well within reach of a stitched panorama or a high-resolution scan, which is exactly when people meet it.
It goes by other names too
Also known as JPEG. Worth knowing when searching for a program that handles it: the name on the specification and the name on the menu are not always the same one.
What opens JPG
Adobe Photoshop, GIMP and Preview read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
What it can carry besides the content
JPG can hold EXIF, XMP, IPTC, ICC profiles and GPS coordinates.
The GPS coordinates are the field worth thinking about before sending a file anywhere: they say where the photograph was taken, and almost nothing warns you that they are still in there.
It is a delivery format
JPG is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
What goes wrong with it
The recurring complaints: it degrades a little every time it is re-saved.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .jpg, .jpeg, .jpe |
|---|---|
| Media type | image/jpeg |
| Published by | Joint Photographic Experts Group |
| First published | 1992 |
| Specification | ITU-T T.81 |
Converting JPG to something else
Converting something else to JPG
Formats worth considering instead of JPG
Where these figures come from
The claims on this page are checkable, and these are the documents that settle them.