https://www.youtube.com/watch?v=PHCq7mwMRm4 In a word, well four actually, ‘don’t waste your money’…for this lens anyway. No doubt they work with longer focal length lenses, but I do not have one. They come in two lengths 10 & 16mm. With the lens set a 40mm a magnification of approx. 0.3 was achieved with a working distance (end of lens to subject, hood removed) of approx. 60mm. With the 10mm tube fitted a magnification of approx. 0.2 was achieved with a working distance (end of lens to subject, hood removed) of approx. 110mm. Coincidentally this was the same magnification/working distance of the lens on it's own, without the 10mm tube fitted. With the 10mm or the 16mm tube fitted a magnification of approx. 0.54/.58 was possible with a working distance (end of lens to subject, hood removed) of approx. ZERO, end of lens almost touching the subject. As I said……IMVHO Kenko Tubes are NOT for the 12-40, they have gone back to Amazon. If others decide to replicate my test and find errors, please don't shoot the messenger, I was just trying to establish whether it was worth keeping the tubes......for use with the 12-40.