If you use legacy lenses you will notice that some of the values are missing in the exif data. Focal length being one of them. You can add some of these with the help of the exiftools [1]. The program is probably available through your distributions repositories.

Examples

To add the focal length go to the folder of your image and then type:
exiftool -FocalLength='50.0mm' blue.jpg
This will add a focal length of 50 mm to your jpg file. In this case the file was the blue bottle. Replace blue.jpg with the filename of your image.

Unfortunately the aperture cannot be written with this tool.

To write all the options of exiftool into a text file do the following:
exiftool > ~/exiftool.txt
This will write the help content which includes the options and examples into a text file in your home directory.

To list all the exif data of your image (you need to be in the folder of your image, or you have to type the path):
exiftool list ‐EXIF:All blue.jpg

There is more you can do, just read the exiftool help. It is all command line stuff though, but that’s a good thing, isn’t it?

Resources

[1] Exiftools www.sno.phy.queensu.ca/~phil/exiftool
It also comes in a Windows and Mac flavor.