Specifically, this photo.
This was taken with a Canon 1000D, with a 1985-ish Admiral/Panagor 55-225mm manual telephoto/macro that I picked up for cheap at a charity shop; it's a Minolta mount, so it's connected through a teleconverter. In macro mode the lens works fine, but in telephoto mode you get this smudge. It comes and goes depending what you're looking at but it's always there.
I don't think it's a lens flare; it happens when the entire camera is in shadow. I have another couple of similar manual lenses, and it shows up there, too, but those lenses are fouled with fungus so I just assumed that was the problem, but this lens looks pristinely clean to my ability to inspect it (I'm certainly not going to attempt to dismantle it!). Needless to say, my usual Canon electronic lens doesn't show anything like this.
Does anyone recognise this artifact? Is it a problem with the lenses? The teleconverter? My technique? (I'm hoping for the last, because it's more likely to be fixable...)