Handheld i.e. resting on a balcony handrail iso 4000, 2 secs, @ f4

Lovely clean shot. I like it.

Great picture but can I see a very thin black line running across the image - most obvious on the white face in the centre. Any idea what caused this?

Hi Keith, you're right. I didn't see it when I uploaded it, but have checked and it doesn't appear on the raw file or the high res tiff, but has appeared on the low res jpeg that I've put here. I've no idea what it is.