JPEG, TIFF or RAW (DNG)? Choosing a file format for your scans

Most scanning services supply JPEG files as standard, with TIFF or RAW available on request. Here is what the difference is, and who each one is for.

JPEG: the everyday standard

JPEG files are compressed to keep them small. At high quality settings the compression is invisible, and every phone, computer, TV and website can open them. A 600dpi scan of a 6×4 print is typically 2 to 4MB as a JPEG.

The one drawback is that JPEG loses a little quality each time a file is edited and saved again. For viewing, sharing and printing, that doesn't matter. For heavy editing it can.

TIFF: for editing and archiving

TIFF files are stored without that lossy compression, and can hold 16 bits of information per colour instead of JPEG's 8. That gives much more room for strong adjustments to brightness and colour without banding in skies and plain walls. You can edit and save a TIFF as many times as you like.

The cost is size: the same 6×4 scan can be 25MB or more. A thousand photos can fill a small hard drive.

RAW (DNG): for film enthusiasts

For negatives and slides, some scanning software can save a RAW file, often in the open DNG format. It holds the scanner's data before most processing, so you can make your own decisions about colour, exposure and inversion of negatives in photo editing software. You can choose a RAW file with dust and scratch removal already applied, or completely untouched.

Be aware: a RAW scan of a colour negative looks just like the negative itself. The colours are reversed and the whole picture has a strong orange tint, which comes from the orange base of the film. It only becomes a natural-looking photo once it has been converted in photo editing or specialist film-scanning software. RAW scans of slides are already positive, so they look much closer to the finished picture.

Can a TIFF be a RAW file?

Yes, and this surprises many people. With film scanners, "raw" describes what has happened to the picture, not the type of file. A raw scan is the data straight off the scanner, with no colour correction, sharpening or dust removal, and for negatives no conversion from the orange negative into a positive. It is usually 16 bits per colour, and sometimes looks very dark and flat until it is processed.

Digital cameras need special raw formats because each pixel on a camera sensor records only red, green or blue, and the full colours have to be worked out later. A film scanner records full colour for every pixel, so its raw data is already a complete image, and a TIFF can hold it perfectly.

That last row is the clever part. Some specialist film-scanning programs can save the scanner's infrared layer inside a raw TIFF, so dust and scratch removal can be applied later, or not at all. You scan once and keep every option open. Everyday scanner software saves processed TIFFs, which are high quality but not raw.

So what is the difference between a raw TIFF and a DNG? For a film scan, the picture data is essentially the same. DNG is a widely used raw format that opens straight into most photo editing programs. A raw TIFF is more widely compatible and is what the specialist scanning programs use to reprocess a scan from the file later.

Which should you choose?

For most families JPEG is the right choice. If you are a keen photographer, see our questions & answers, or just ask Dee.