Look at my lens here:

Do you notice two things? Apart from the fact that this is my favourite long lens, the superb 70-200 f/2.8L IS, I mean.

1. It has a lens hood. Always use the lens hood. Yes, even indoors. It affords protection for both damage and stray light causing “flare”.
2. The dot is on top, even though Canon would rather I put the dot at the bottom. Why do you put the dot on top? Because it is then obvious if the hood is not on all the way (which would lead to vignetting in the corners).

There you go, another little tip to make you a better photographer – if ever so slightly.