hmmm. thanks Ian. That explanation is more troubling since it means the camera is malfunctioning, but it seems more likely to me than flare for the reasons you've stated. So far this is the only image where this has happened. I guess I will keep my eye on it and if it happens again I will have to have the camera inspected. This shot is from a large project and I have also been shooting each image in b/w so it will be interesting to see whether there is a problem with the b/w version once I develop that film. Unfortunately the project will probably end up needing to be in colour, and I won't be back to that location for some time, so it's likely a lost image.