Photo enthusiast Chris Malcolm needed a better way to aim his 500mm lens at fast moving subjects (e.g. birds in flight), so he upgraded his lens with a DIY sighting aid by attaching a non-magnified red dot sight:
They’re designed to clamp onto a gun sight wedge mount, so some kind of adapter is required. I played with the hot shoe mount, but it was too flexible — the sight needed re-zeroing at every mount, and was easily knocked out of calibration. The degree of precision required to aim the central focus sensor at the target via the dot also made parallax error a problem on the hot shoe. So I decided to mount it directly on the lens. Least parallax error, plus the geometry of the lens barrel and the sight mount naturally lines it up with the lens. To protect the lens barrel I glued the sight clamp to a cardboard tube slightly too small, slit open to provide a sprung grab on the lens body. The slit also handily accommodates the focus hold button on the lens barrel.
Malcolm reports that the site “works amazingly well”, making it “trivially easy to aim the lens at anything very quickly”.