Lens hood difference?

[Invictus]

So after purchasing a new lens recently and finding a circular deep profile lens hood in the packaging, I started to wonder. What is the difference between a circular hood and a flower/petal type hood, other than shape and size?

 

[Tiberius47]

The circular ones are really for lenses where the front element rotates, wheras petal hoods are for lenses with front elements that do not rotate.

The reason is that as the lens projects a circular image onto the sensor plane, the corners of the frame are closer to the boundary of the circle that the middle of each side. Because of this, the hood can't extend as far in front of the lens at the corners as it can on the top/bottom/sides.

So, if the front element rotates, any part of the hood could be at the corners of the frame, so it all has to be short. On a lens that doesn't rotate, the parts of the hood on the top and bottom and sides are always going to be there, so they can extend longer before appearing in the image, and thus they do a better job at sheilding the front element from stray light.

 

[Invictus]

Thank you, I understand very well.