I manually write the metadata for my non-AF lenses and film scans using ExifTool in conjunction with ExifToolGUI. Phil Harvey's ExifTool is an open source command line program offering extremely broad and up-to-date camera and lens support. In addition to being able to read the metadata from almost any camera, it also supports editing of all legal fields along with batch writes. Adding ExifToolGUI to the mix makes the process all the more convenient.
The ExifTool Web site:
ExifTool by Phil Harvey
The Windows executable (all most windows users need):
http://www.sno.phy.queensu.ca/~phil/exiftool/exiftool-10.22.zip
The Web site also provides links to an OS-X installer as well as the Perl library version.
ExifToolGUI is a separate program that provides a handy interface for Windows users. To use ExifToolGUI, you need to also have ExifTool.
ExifToolGUI Web site:
ExifToolGUI
The GUI is officially an inactive project, but the author has been making periodic updates and the program works fine with current versions of ExifTool. I use it on both Windows 10 and Windows 7. There is a download link on the Web site, but finding the actual link requires a bit of drilling beyond that. The real download page is:
Latest ExiftoolGUI version 5.16
Enjoy!
Steve