My son is travelling to Iceland so I’ve lent him a couple of lenses: my 90mm and 28mm. My tripod as well.
He informed me that the little red dot had detached from the 90mm. I was surprised it was not lost. The official name for the part is the [red] index button. It is used as a convenience to facilitate the correct alignment with the mount when attaching a lens. It is not mandatory. After a while, with experience, it becomes unnecessary.
Yet, without it the lens looks naked. Fortunately I had some crazy glue. I hope it holds.
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