[sun care] relationship between % active ingredient and SPF rating?

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My current sunscreen, Invisible Zinc Daily Moisturiser SPF30, claims to be 20% zinc oxide. The SPF50 sunscreen from that same brand apparently has 25% zinc oxide.My current Avene mineral sunscreen I think has something like 6% zinc, but claims SPF50. I emailed Missha asking what the percent zinc was in their All Around Safe Block Waterproof SUN SPF50 PA+++, and a rep told me it was 6.9%.I don't know anything really about the science involved, so can somebody pls help clarify how products with a much lower % zinc can claim a higher SPF? Or any helpful links? Would also love it if anyone could shed some light on this topic generally, about how formulation impacts SPF :)If it's relevant, the Invisible Zinc products use micro zinc, Avene uses nano, and I'm unsure about Missha. (Edited to add the SPF on my Avene sunscreen)
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Submitted by wintersux_summer4eva