[wulinka]

So I have only had my new camara for a couple of weeks, and I swear I have been keeping the front of my lense scrupulously clean, and only had the lense of once, twice at most, for seconds only.

but for a few days I have been seeing this spot in one corner of my pictures. in the example above I have pretty drastically modified the gray levels, to make it best visible. In some pictures it looks much more dense, and somehow even its appearance seems to vary with the focal length I am taking the picture at.

Is this caused by dirt on the sensor? How often do you have to clean your sensor and do you do it yourself? Or what else can cause such a spot?

any advice appreciated!