What is the easiest way to add data to an EXIF file? For example if I wanted to add a copyright to my images can that be easily added to the EXIF data?
The easiest way to add exif data is to do so in camera. Not all camera models allow for setting the copyright in body however, so the next best way is during image import. I use lightroom as the hub for my digital workflow, and it allows me to specify any and all exif information I want. You'll want to check with your image import utility and verify it has exif support.
Some cameras, support setting an owner name on the camera, which will end up in the EXIF metadata on the picture.
There are thousands of programs for adding extra keywords and other information, and you'll find people rave about their favourite. Personally, I use Microsoft Pro Photo Tools which happens to be free, and allows me to add information as to where the photo was taken ("geotagging") as well.
Cameras generally automatically add EXIF metadata to pictures, such as the name of the person, location, and other useful camera details.
However, when playing with the Exif data, it's difficult to name one favorite program, since there are many helpful tools available that include additional keywords and information. As a Windows-only product, I could not try Microsoft's ProPhoto tool. Nevertheless, in my case (while using macOS), I use AnyEXIF to alter EXIF data in photos since it offers all-in-one features to edit or remove EXIF, GPS, IPTC, or XMP data.