I agree with your analysis. I was not expectng to photograph a sunset that evening but when I saw near perfect conditions developing I rushed over to one of my favorite spots. There was no time to grab my tripod. As it was I missed the actual setting of the sun. I have quite a few more photos that I have to process from this shoot. The sky was lit up all over the place and not just in the direction of the sun.
The sensor definitely needs to be cleaned. The problem does not usually show up in my photos of kittens and flowers but it is unavoidable in landscapes and in videos.
In these sunset photos I think that it is difficult for the untrained eye to tell the difference between a spec of dirt on the sensor and part of the clutter of the background. I thank you for your efforts.
