Why TIFF files are hard to share
TIFF was made at Aldus in 1986 for desktop publishing, and its last full revision, 6.0, dates from June 1992. It is a container: one file can hold one picture or several, stored without compression or with one of many schemes, from black and white to 16 bit colour and CMYK. That flexibility is why scanners, fax software and print workflows still write it, and why few programs open every variant.
MDN says that other than Safari, browsers do not natively support TIFF images in web content, and lists the format among those to avoid on the web. A classic TIFF also stops at about 4 GiB. BigTIFF, the version with 64 bit offsets, is not supported here, and a file of that kind gets a message that says so.
How much smaller the JPG is, and when it is not
The saving depends on how the TIFF was stored. A 2048 by 1536 photo took 9.0 MB uncompressed, 5.5 MB with LZW and the horizontal predictor, and 5.2 MB with ZIP and the predictor. At quality 90 each of them gave the same 683 KB JPG: 93 percent smaller than the uncompressed file and 88 percent smaller than the LZW one. The JPG differed from the TIFF by 1.9 levels out of 255 on average, a PSNR of 39.0 dB.
Scans behave differently. A grey scan of an A4 page at 200 dpi, 1654 by 2339 pixels, took 3.7 MB uncompressed and 292 KB with LZW. The JPG was 350 KB: 91 percent smaller than the first file and 20 percent larger than the second. The list under your files says larger when that happens, so you can keep the TIFF in those cases.
Black and white scans: use PNG instead
Fax software and document scanners often store one bit per pixel with CCITT Group 4 compression, which is very compact: our 1654 by 2339 test page took 36 KB. A JPG has no way to say black or white. It stored the page in 388 KB, 979 percent larger, and 2.5 percent of its pixels were shades of grey that the original never had (PSNR 35.0 dB).
The same page as a PNG was 37.9 KB and identical to the TIFF pixel for pixel. TIFF to PNG is the better choice for text, line drawings and anything that is not a photograph.
What changes inside the picture
- Colour depth. A JPG holds 8 bits per colour, so a 16 bit TIFF is reduced to that.
- RGB colour profiles. Adobe RGB, Display P3, DCI-P3, ProPhoto RGB, Rec. 2020 and Generic RGB profiles are converted to sRGB. We compared six against a LittleCMS conversion: the average difference was 0.01 of a level and the largest 1. The JPG carries no profile.
- CMYK. CMYK files are converted with a plain formula and an embedded CMYK profile is not used. Against a conversion with the Generic CMYK profile ours differed by 18 levels on average, up to 118, so judge print colours in a program with colour management.
- Transparency. JPG cannot hold it, so transparent areas are filled with the colour you choose, white by default. TIFF files with premultiplied (associated) alpha are handled too.
- Pages and turning. Only the largest page is converted, or the first of pages of equal size. The orientation tag is applied: all eight values gave the right picture.
- Print size and text fields. The pixels are all kept, but the JPG does not say how big to print them, and fields such as artist and copyright stay behind, even with Keep selected under Metadata.