Strange dark spots in picture

[Ants Koel]

I went out testing my new Tamron SP AF 10-24 F/3,5-4,5 Di II today.
Unfortunately I got some strange dark spots in pictures that revealed themselves afterwords, on bigger screen.

I could not reproduce these spots back at home (but the light conditions were different).
The spots reveal themselves with light coming from back or form side.

In the following example you can see the spots in the upper part, at 12.



In a picture taken towards the sun they seem to be gone:




Any ideas what could that be?
Some strange type of flare?
Dirt on sensor (but then why couldn't I reproduce it at home)?

[baycruisers]

My guess is dust on the sensor. Stop down to f16 or f22 and shoot a several photos of a plain bright sky and I'll bet you see the spots in the same places in each image. You may not have been able to reproduce it at home if you were shooting wide open (e.g., f4).

[OsmosisStudios]

Yup, sensor dust.