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I have one lens hood for a 28-105mm lens that is petal shaped. I have another lens hood for a 50mm lens that is fully round. If the purpose of the lens hood is to block stray light, then wouldn't fully round always block more?

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3 Answers

up vote 10 down vote accepted

As Chills stated, petal shaped hoods are designed to better take into account the wider shape of a camera's film or sensor.

This article on Lens Flare has a good description of lens hoods and how they function.

enter image description here

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This doesn't really answer the question, just link to a article / page that might. Would be better if the answer contained the information, like @Reid's answer. – Håkon K. Olafsen Jul 13 '12 at 11:37
up vote 4 down vote

For wider angle lenses the optimal design is the petal shape. This is because of the rectangular sensor and wide field of view.

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The optimal design for all lenses is petal shaped, but I suppose for longer lenses it would be quite a petal. – Reid Jul 16 '10 at 2:20
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@Reid Preidhorsky As you say, because of the narrower FoV for telephoto lenses, the hood would have to be seriously long before it became necessary to petal-shape it in order to avoid vignetting. – Edd Jul 22 '10 at 15:55
For some fisheyes expected to produce a circular image, I would assume that a round shape is optimal. But in most cases we are using a circular lens to take rectangular pictures on a rectangular sensor or film. – Markus Mikkolainen Apr 27 at 12:46
up vote 14 down vote

The petal design is more effective. Think of what you can see through your lens: it is a pyramid-shaped chunk of space that falls within your view, one that has a rectangular rather than a square base. Now imagine placing a round lens hood atop that pyramid - there will be a large gap on each side, because the corners bump into the round opening first. A petal-shaped hood fills in these gaps, excluding more stray light.

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