Can anyone look at the attached photo and tell me what gives (and/or what I am doing wrong).

I have just come back from Nepal taking shots with a Canon D60 and EFS 18-135 mm and EF 50mm f1.4. Many shots (only shots with plenty of sky) have this curve in the left hand corner of varying thickness and density (see attached email-sized jpeg). It didnt seem to matter whether I was using a lens cap or not , or whether I was using the Circular Polariser or ND filter or not , most photos with the Himalayan sky have the curve in some form.

I hadnt noticed this in any shots taken in the previous 6 months in Australia with the same camera/lens.

Any advice welcome, Many thanks.

Patan Sky.JPG