Sometimes, when looking at eBay or other sites, I see a lens with inches in the focal length description rather than mm. How should this be translated? I have always used 25mm per inch when working with Artillery. A French 75mm is just about the same bore diameter as a 3 inch Naval Gun. Will the same formula work with lenses? If not, why not? Thanks, Dennis

One inch = 25.4 mm whether you are measuring guns, lenses, or the length of your ... nose. So to convert inches to mm, multiply inches by 25.4. Conversely, to convert mm to inches, divide mm by 25.4. A 101mm focal length lens = 3.97xxxxx, or 4 inches (101/25.4). Have fun!

Cool. Thanks. I'll remember that if I'm measuring my lens, or my...nose!