Taken some sunrise photos recently and i am getting a large dark corner in some which does not blend in well at all. Its seems more than a vignette but i cannot work out what is causing it and obviously how to try and prevent it. My set up was a Lee holder with just one grad in the first slot of 2. I have placed a sample below and would appreciate any thoughts on how to prevent it. I'm sure its just my inexperience and lack of technique.