Zenphoto version 1.5.9
Graphics support: PHP Imagick library 3.7.0
ImageMagick 6.9.12-40 Q16 x86_64 2022-02-16 https://imagemagick.org
supporting: bmp, bmp2, bmp3, gif, gif87, jpeg, jpg, png, png24, png32, png8, tiff, tiff64, webp

Only the the files with .tiff extension are supported, the .tif files don't show up. Renaming the file works but i would like that both file extensions to be supported.
Any advice ?

Zenphoto actually would support both variantes but apparently Imagick itself does not. This is server side and outside of Zenphoto's responsibility. Best ask your host about that.

One more thing I don't understand.
In phpinfo(), imagick shows a long list (244) of supported formats.
In ZenPhoto admin overview imagick shows a small subset (14) of formats supported.
What is going on ? Is it just a display problem ?

Zenphoto is for websites the focus lies on web image formats that browser actually can display.

Found out why only 14 formats are supported in imagick
they are hardcoded in lib-Imagick.php in $_imagick_format_whitelist
Still need to find out why the filename .tif is not supported and .tiff is.

Found out why only 14 formats are supported in imagick
they are hardcoded in lib-Imagick.php in $_imagick_format_whitelist

Yes, as said, web image formats. If I am not mistaken, supported by Imagick does not mean it actually can render them like image.

You can use any flle format in albums using the anyfile plugin. It will just not render any sizes.

Still need to find out why the filename .tif is not supported and .tiff is.

As said this is a Imagick core thing. I tried and while it lists both within an album it only generates only sizesf or the .tiff one. Which is the "official" suffix though.