After using a Pentax film SLR for 26 years, last summer I switched to a digital SLR (Nikon D90). I am aware that with a DSLR I should swap lenses quickly and carefully to stop dust getting on the sensor. The result? Out walking last week, I had a reasonable view of a heron. I carefully removed the rear lens cap from my telephoto zoom and positioned it with the white spot showing, switched off the D90, pointed it downwards, removed the mid-range zoom and attached the telephoto, and then put the rear lens cap on my mid-range zoom. (With my film SLR I used to remove the lens first, and gave priority to moving the rear lens cap, leaving the camera body look after itself.) I lifted the camera, only to see a large silhouette of an insect in the viewfinder. I switched back to the mid-range zoom, but again saw the insect. After looking more thoughtfully, I decided that the insect must be on the viewfinder screen itself. By now the heron had disappeared. I moved away to a spot where I could sit down, removed the lens again, and after a few blasts of air into the mirror box I could no longer see the insect. I hope it has gone, not just hidden in a corner of the mirror box. I never seem able to change lenses on my Nikon as slickly as I used to manage on my Pentax. I regularly get caught out by the unconventional action, equivalent to a left hand thread, contrary to the overwhelming majority of “push and twist” mechanical connections. I might make more progress in getting used to it if each lens change was not accompanied by reversing lens hoods, which mount with a conventional right hand thread action. (But thank you Nikon for supplying the lens hoods as standard, rather than only making them available as accessories, at excessive cost.) I would be interested to hear if anyone knows why Nikon use a left hand thread action on their lens mount? Was it chosen to match the mechanics of earlier Nikon cameras? Was it simply an oversight? Or was it a deliberate choice, perverse though it seems to me?