Shards are just upgrade materials, they don't really have anything to do with light. Upgrading the defense on each piece of armor increases the light for that said piece. Max is 27 for legendaries, 30 for exotics and raid gear. With 3 leg' & 1 ex you'll be 29.25
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I was just mentioning there are 3 more defensive upgrades costing 18 shards and didn't seem like the less than 100 defense would get me to 29.