Attached Link: Lens hoods DIY

Doug is right, the butterfly hoods help prevent vignetting (darkening of the edges of the frame) Most hoods for wide-short telephoto lenses are of the butterfly type, as these offer the best performance in that range, longer lenses usually have a solid hood without fins, as the narrow angle of view doesnt cause the issues with light drop off at the edges that that can be associated with wide lenses. I use a hood always, unless it is not possible because of obstacles. Richard posted a link for DIY lens hoods, which may help if you have lenses without hoods.

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