TIFF - Tagged Image File Format

TIFF stands for Tagged Image File Format. It was created by Aldus, with input from Microsoft, so that scanners and desktop publishing software would have one format they could all agree on, and Adobe has looked after it since acquiring Aldus. The word tagged is the whole idea: instead of a fixed layout, a TIFF is a directory of numbered tags, each saying what a piece of the file means. That makes it enormously flexible, which is why it became the standard for scanning, archiving, print production and fax, and why two TIFFs can differ so much that a program reads one happily and refuses the other.

How a TIFF stores its picture

A TIFF opens with a tiny header stating the byte order, meaning which end of a number is written first, marked II or MM, and pointing at the first image file directory. That directory is a list of tags; some hold their value directly, others the position in the file where the real data sits. Each directory can point to another, so one TIFF file can hold many images, which is how multi-page scans and faxes work.

The pixels themselves can be stored in several ways, and a tag says which:

• Uncompressed, for maximum simplicity and size.
• LZW, Deflate or PackBits, all lossless, and the usual choice for photographic masters.
• CCITT Group 3 and Group 4, lossless methods designed for black-and-white fax and document scans.
• JPEG, which makes that particular TIFF lossy.

So a TIFF is lossless in normal use, but not by definition: it depends on the compression tag. The format also reaches beyond ordinary pictures: 16 and 32 bits per channel, CMYK and Lab colour for print, alpha channels, tiles for very large images, and private tags where programs keep their own extras, including Photoshop's layers.

What metadata a TIFF carries

More than any other common image format, because in a TIFF the metadata and the image are stored the same way, as tags.

• Baseline TIFF tags read like a document header: ImageDescription, Make, Model, Software, DateTime, Artist, Copyright, HostComputer, Orientation and the resolution tags.
• EXIF, the camera metadata standard, lives in a private directory that a tag points to. That is no coincidence: EXIF was designed on top of TIFF's structure, which is why the EXIF block inside a JPG is itself a miniature TIFF. Expect DateTimeOriginal, ExposureTime, FNumber, ISOSpeedRatings, FocalLength, LensModel and often a camera or lens serial number.
• GPS tags occupy a directory of their own, with latitude, longitude, altitude and a timestamp in UTC.
• XMP, Adobe's XML metadata, takes up a single tag: titles, descriptions, keywords, star ratings, colour labels and an edit history naming the software that touched the file.
• IPTC, the publishing standard, takes up another, with by-line, caption, keywords, city and credit fields, while an ICC colour profile and a Photoshop resource block each have a tag as well.

Several of those fields are personal, and a TIFF tends to keep them, because TIFFs are masters that get copied and archived rather than re-exported. Artist, Copyright and the XMP creator field can carry a name. HostComputer can name the machine that saved the file. Serial numbers identify a camera or lens; GPS tags identify a place. Scanned documents are worth a second look: the scanner's make, model and driver version are routinely recorded alongside the page.

When to use TIFF

TIFF is a format for originals rather than for delivery: a photographic master you will keep editing, an archived scan, artwork going to a printer in CMYK, or a multi-page document. Lossless compression means you can re-save it without accumulating damage, and its bit depth and colour options survive professional workflows that JPG cannot.

The costs are size and reach. A TIFF master is many times larger than the JPG exported from it, browsers generally will not display TIFF at all, and an unusual TIFF can be rejected by software that reads ordinary ones perfectly well. For photographs straight from a camera, a raw file or a DNG keeps more latitude; for multi-page documents, a PDF is easier for other people to open. Keep the TIFF as the source and hand out something else.