[Sun Care] Why some physical suncreens with lower percentage of Zinc Oxide having higher spf?

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I'm looking for a physical suncreen for my on-treatment skin (chemical suncreens always gave me CCs) so i need the suncreen to have high protection. But i found out the spf of sunblock products not really depend on their percentage of UV filters. For example, Drunk Elephant Umbra Sheer Physical Daily Defence Spf 30 which has 20℅ of Zinc Oxide have lower spf than SkinCeuticals Physical Fusion Uv Defense Spf50 (5% Zinc Oxide, 6% Titanium Dioxide). If i want better UV profection, should i choose sunblock base on their spf or their percentage of UV filters?. Thanks for your advices.
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Submitted by Annathenotsopretty