To correct Alex
, a petal hood has different lengths of petal to match the orientation of the film/sensor. A petal hood has a wider spread on the top and bottom leaves, and relatively stubby leaves on the sides. This allows the hood to give maximum shielding without causing vignetting. Circular hoods are most often found on longer length lenses where the field of view is so narrow that the length of the petals is by and large irrelevant.