Dirt specks on the sensor become more visible with smaller aperture that's why I refer you to the links on the how-to. When you check for dirt, shoot a
clear blue sky (if available), at f/20 and I'm pretty sure you'll see the same spot, same location. Try it with a different lens at f/20, if you suspect it's a lens issue. If it persists, it's bound to be sensor dirt. Just because your...