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...)
Answer
Some film era lenses could produce this effect on digital cameras, it's called a "hot spot". The reason is that unlike film emulsion, which is matte, digital sensor is glossy (as it's composed of thousands of micro lenses) and it causes the light to bounce back to the rear lens element. If this element is flat or if it's curvature is not enough to effectively scatter this bounced light, the light then bounces back to the sensor which, again, bounces it back to the lens and so on. This results in a hot spot.
As this hot spot is an internal lens flare, it depends not on your front element being in the light, but on the subject matter. High contrast, back lighted or highly reflective scenes usually produce a more noticeable spot.
One of the well known lenses suffering from this effect is Tamron Adaptall-2 90mm f/2.5 macro lens. It has a flat rear element and the hot spot has a purple tint because of the color of the lens coating. It also gets much more prominent as you close the aperture.
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