Wearing products with different SPFs

[MackerelOfFact]

Wasn't sure whether to put this in General Health or S&B.

If I put SPF50 cream on my face and then SPF15 foundation on top of it, does the SPF15 add to the protection of the SPF50 or dilute it?!

Obviously you can slather as much SPF15 on as you like but it'll always be SPF15 (and doesn't become SPF30 if you use twice as much) so I know it doesn't add up like that. But equally it seems better to use the SPF15 on top rather than something without any SPF at all.

Just a bit confused and wondered if anyone knew?

If I put SPF50 cream on my face and then SPF15 foundation on top of it, does the SPF15 add to the protection of the SPF50 or dilute it?!

Neither. You still have SPF 50 if you have put enough on - almost no one does.

Thanks shiny. It's kind of obvious really, but I was confusing myself.