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Why are there both petal and rounded versions?
Because photographs are rectangular in shape, petal hoods offer the best performance by blocking most nonimageforming light. For this reason, it is important to make sure that you mount a petal hood the correct way up, making sure the longest petals are located at the top and bottom of the lens, with the shorter petals to the sides.
Petal hoods are not suitable for all lenses, so simpler rounded versions are also provided. Rounded lens hoods are much more suitable for tilt-shift lenses or any lens where the end rotates when zooming or focussing.