The addition of a lens hood can help prevent flares--those circles of light that can accompany images where a strong light source is just outside the fame. These type of lens flares occur when indirect light rays pass through the front lens elements but don't make it all the way through to the image sensor.
Canon's EW-73B Lens Hood is designed for the Canon EF-S 17-85 f/4-5.6 IS USM, EF-S 18-135mm f/3.5-5.6 IS and EF-S 18-135mm f/3.5-5.6 IS STM lenses.
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