Is this a crop? If it is, well, it looks just like the kind of dust spot I so frequently have to deal with on my sensor. I often find that I cannot actually see the spot itself when I examine the sensor itself with the mirror up - but dut it must be as the spot invariably disappears from images once I have cleaned the sensor in one way or the other.

What you see is the ~30-pixel wide (200 microns or so) shadow of the piece of dust, which sits quite a way above the actual sensor.  If you stop down from f/11 to 16 or 22 the spot will become smaller and darker (and you'll probably see a bunch of others too).  They can be invisible to the eye but create noticeable shadows. Lots of threads here and elsewhere about "sensor cleaning" to help you get rid of them

Good luck

Andy

Can anyone speculate as to what this mark might be?

http://homepage.eircom.net/~ThePlayer/IMG_2941.jpg

It's the circular, faintly grey spot, near the upper right hand corner.

It looks transparent. The texture of the wall seems to be visible through it.

I have taken the same shot with a couple of lenses, so it is definitely the sensor.

Also, I have examined the sensor closely and I can't see anything on it.

Very annoying. Even more annoying is to discover that this camera, bought in B&H in December, is not covered under warranty here in Europe. A major black mark against Canon, in my opinion.

D.

Yes, it is a crop.

I guess it must be dust. Can't see it, on the sensor, but it's visible to the naked eye on the camera's screen, so it ain't that small! ;-)

Thanks.

D.