Originally posted by Digitalis It could possibly be an oil spot on the sensor itself. Particulate matter inside lenses has to be either at a critical point in the optical path or of an extraordinary size to be visible in photos depending on focal length and aperture.
Sure. Logically though, if it was on the sensor it would appear on photos with other lenses, but it never has.
Quote: Does the spot appear at apertures f/8>= ? If the spot appears at wider apertures, than this it is probably the sensor that is at fault: rather than the lens. The reason for this is the sensor stack is closer to the sensor than the rear element of the lens will ever get. Thus anything on the sensor stack will have a visible effect long before anything in the lens shows up. It is just a simple process of elimination.
I just took a couple of images of the sky, deliberately overexposed by about 3EV, at maximum and minimum f-stop and minimum focal length with the Tamron lens and my Pentax HD DA 55-300 PLM. No sign of the dark spot. If it was dust on the sensor, I'd expect it to be visible clearly at the highest f-stop. I've seen this when there has been dust on the sensor.
Quote: By definition: Flare is non-image forming light, a dark spot indicates an absence of light. So calling it flare is a bit of a misnomer.
I agree to an extent, although logically a lens behaviour could equally cause bright and dark spots relative to the image of the scene - for example if the lens system brightened everything
except a certain spot, you could end up with a dark spot. I was simply looking for a word to describe a variation which is apparently cause by the lens rather than the camera.
Thanks for the suggestions. It's odd, because 99% of the time the lens captures great images (hende why I posted a couple of other examples of good images), and is certainly better than the Pentax DA 16-45 which it replaced. The Tamron does have a fault, which is that sometimes it ceases to function when zoomed to 50mm but readjusting the zoom causes this to go away. There must be a faulty contact inside the lens affected by the zoom. However, the electronic misbehaviour doesn't explain an odd optical behaviour.