I use two tools:
, a Mac OS app that's available through the Apple App Store or directly from the author's website
. This app allows you to add/change much of the EXIF, including the lens information without opening LR. The ExifTool utility is the underlying engine of ExifChanger. The UI of ExifChanger is much more user friendly than using ExifTool directly. The user can create presets for frequently used metadata.
plugin for LR that you can use to add lens information, including working aperture. LensTagger also allows the user to create presets that can be used for future shoots.
Hope this helps.