I have a simple scene. three objects on a table. One has a glass shader assigned. None of the others do. If I set the output file type to .iff the glass object renders correctly. If I just change the output file to .tif compressed the output file has a pure white hole where the glass object was - looks like what an alpha chanel would look like.
I also tried changing the output to .tga and it renders correctly. Any ideas why it treats tif differently ?