Went out today with my new (2nd hand) Pentax K7D and trusty K10D. I wanted to swap the lenses but the K7 wouldn't recognise my Tokina 19-35, yet it continued to work like a charm on the K10? The Pentax 16-45 was happy on either body. After scratching my head and forensically going through the set-up menus to no avail, I hit on cleaning the lens contacts with an eraser, et viola the lens sprang to life. I presume mount tolerances are such that the contacts weren't pushed together quite so well on one body. So I think the eraser will be an essential part of the kit. Fortunately I had a rather hard Staedtler Techniplast eraser which is made of some sort of synthetic compound which doesn't shed bits of rubber to get into the cameras works. So something else to add to the list of things erasers are good for cleaning, along with battery terminals, contacts on memory cards and computer boards.