Mike (MIKEN) - Thanks for the tip, but I don't think that I have a problem. The banding only shows up on aluminium and not any of the other materials that I do. I think that it is because the aluminium just soaks up any out-of-focus beam, so that only the centre of the focus point is engraving (in printing terms, I am seeing dot-loss instead of the more usual dot-gain).

I've not tried engraving a photo onto aluminium, though I would love to - all the ali I do belongs to my customers and it is all text and vector graphics.
One customer demands incredibly small text - 0.9mm (0.035") high - and it was quite a task finding settings which give good results on the fine stuff without blowing away normal sized text and logos.