I have difficulty getting coverage when shooting at 10mm with my Sigma 10-20mm with the Pentax flash AF540. I used the inbuilt diffuser; but still the side is not covered. I used bounced flash and still not great. Any suggestion on how to play around with the flash to cover its wide angle? I was taking some Chinese ink paintings for this famous Chinese Professor/artists, and got some of these issues. Some of his paintings are like 10~15 feet long; so I needed the UWA to cover it. I don't have studio lighting condition. Suggestions welcome, if I had to do it again!