Aperture does not provide tools to modify the exif tags, that are supposed to be set by the camera, like lens information, camera make, etc. This is fine, as long as an image has these tags, and they are correct. But what if these tags are wrong? For example, if you scan slides, the scanned image will show the exif data describing the scanner and not the camera data you used to take the picture. I prefer the image tags to describe the camera that I used to take the shot, and not the scanner that was used to scan the slide.
To put the camera information into scanned images I use the commandline utility exiftool. You can install it from here:
Once it is installed, I use the Terminal to add lens data and camera make to scanned images.
The original master image files need to be accessible to the terminal commands. So I relocate the originals to a folder, where I can access them from the Finder, if the originals are not already referenced. To relocate, select the images in the Browser and use "File > Relocate Originals".
Then I open a Terminal window (From Applications > Utilities) and enter for example the command(s) - to set the lens on my old Nikon F2 scans:
exiftool -overwrite_original_in_place -preserve "-Make=Nikon"/mydirectorypath
exiftool -overwrite_original_in_place -preserve "-Model=Nikon F2" /mydirectorypath
exiftool -overwrite_original_in_place -preserve "-Lens=Nikkor 1:3,5 f=55 mm" /mydirectorypath
Note: The Terminal commands given above are actually only four lines of code - even if they appear wrapped around over more lines. It should look like this in the Terminal window:
The Nikon strings have to be replaced by the make, model, and lens of your camera.
/mydirectorypath is the full path to the folder containing the relocated originals, for example "~/Pictures/originals", if you relocate the images to a folder "originals" in your Pictures folder. To enter the path to the folder into the Terminal window you can simply drag the folder with the relocated masters into the Terminal window, after entering the beginning of the command and a space character.
The final "touch" command is important. It will tell Aperture, that the originals have been modified, and magically the new tags will be visible on the versions in Aperture as well.
After adding the tags I consolidate the images again, if I want to keep my library managed.
Here is more documentation on exiftool. exiftool Application Documentation