Hi there, thanks for all this advice. Just a quick response to your questions, yes - I shot in manual. The light was constant and was just natural light from the window. I sat my bottom on the same spot, camera same distance to subject with same focal lengths on both lenses, so the variables pretty much didn't change. I think good advice is a series of test shots, especially at different apertures, I will do that as I had not thought of it. Hopefully I can close this thread with some good news that the issue has been discovered. Thank you!