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Provides a 10% accuracy boost to all the user's moves. For example, a 50% accuracy move becomes 55% accurate (.5 x 1.1 = .55). As a further example, moves with at least 90% accuracy, like Rock Slide, will have an effective 99% accuracy assuming no evasion/accuracy modifiers, whereas the normally 85% accurate Fire Blast will only reach 93.5% accuracy (.85 x 1.1 = .935).
Wide Lens is normally used on frail Pokemon who use somewhat inaccurate moves; Gengar for example. Missing a Hypnosis or Focus Blast with Gengar could be crucial because of its frailty. While Wide Lens's accuracy boost isn't huge, it is large enough to merit use if you find yourself losing a lot of battles because of missed attacks. There are a few downsides to using Wide Lens in lieu of a different item. For one, Wide Lens doesn't boost your accuracy to 100%; crucial misses still happen even when holding the Wide Lens. Also, choosing to use Wide Lens means you have to forgo the healing benefits of Leftovers or the boost of a Choice item or Life Orb.