hi guys, my gf just came back from france and she borrowed my tokina 12-24 + 400D. she took certain photos with the tokina inside a museum and it turned out like the one below. there are dark diagonal corners.
1) no CPL was used
2) no hood was used
3) her hand wasn't blocking the lens
4) she's used to using DSLRs so it's not a question of noob mistakes
5) it only happened in pics taken in the museum and no where else.
what could have caused this? she's as clueless as i am. thanks (: