RF100-500 Purple Dot Dead Center

The last time I was fighting with a purple dot dead center was with a Canon SX260.  (a compact 20X camera)  The purple spot only appeared at certain zooms and focus distance.  I took quite a few pictures of my dog (Mister) when I was trying to figure it out.

I looked at the lens and to my professional eye, it looked clean.   I look at a lot of optics.  But then I thought, well, I should at least do what I tell my technicians to do at work.  Get some alcohol and clean the front lens element before chasing down anything else.   I was very surprised when the purple spot went away.  It certainly was not a lens "full of dirt"   Whatever it was I couldn't even see... maybe I should have looked harder with a bright flashlight... I was looking with overhead lights.   Problem with bright flashlights is you see so much that is not a problem.

With a lens there is the straight through light path that makes an image. But there are also lot of reflections off internal elements and some of them will focus on a sensor.  Some lens designs are known for a problem that if a surface becomes a light source, by scattering light off dirt, there will be some combination of multiple reflections inside the lens that happen to form hot spots on the sensor at certain zoom or focus settings.  When that happening, it is typical that small changes in zoom or focus will make the size of the spot get smaller or bigger on the sensor.    A lens hood sometimes eliminates the problem.   The spot is typically whatever color the AR coatings happen to reflect the most at, and that is typically some blue and some red, making purple.  Knowing that was what made me clean the lens even though it looked clean.

Not sure if that is relevant.  But that is my purple dot story.