A couple of months ago I managed to find a used DSLR body for a very good price at a local flea market. I had been thinking of getting a proper DSLR for some time now (so far I’ve been using a “bridge” camera, the Fujifilm S9500), so for only £30 it was worth the risk. What I got was a Nikon D100. This is a fairly old, 6 megapixel DSLR, which cost about $2,000 when it came out in 2002. I primarily got the camera to use it together with my telescope for astrophotography, and for that it didn’t need to do much. However, now that I had the camera body, I also wanted a lens. With Nikon being one of the two big DSLR manufacturers (Canon being the other one), it didn’t take long to find an inexpensive used lens on ebay. When I tested the lens with my camera body, it became clear that something didn’t work properly. While autofocus worked fine with the lens, whenever I wanted to use even a slightly smaller aperture (with F-numbers as low as 5 or 6) the camera displayed an error, and depending on how high an F number I had dialed in, the image tended to over-expose. I decided to investigate.