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Raises critical hit ratio of the holder by one stage.
Playing for critical hits is generally a fool's game, as they will never happen often enough to make them worthwhile over the power you lost by using up your item slot, or using high critical-hit ratio moves. Theoretically, Pokemon who might find Scope Lens (and other similar items) useful are those with the Super Luck ability, as it gives all of their moves a critical hit stage of at least 3, meaning they will get a critical hit roughly one in every four attacks. Pokemon with the Sniper ability, such as Kingdra and Octillery, can also find Scope Lens useful because any critical hits from them will deal three times the normal amount of damage instead of two, although this will only happen once in every eight attacks on average. However, all of these Pokemon will certainly find other items more useful.
Battle Subway, 48 BP.
Castelia City - Talk to Scientist in Battle Company building, 47F.