London System. What to play when Bishop F5??

I assume you mean something like this.

The idea behind Bf5 from Black is to develop and stop White from playing his Bishop to the best square.  The downside?  It leaves b7 undefended, and so White should try to get an attack there.  The easiest way is with a quick c4 and Qb3, which likely forces Black to play the ugly Qc8 or the weakening b6,

Thanks Smithy. I had the same question.

Well yes, but going by the clear intention of the London system of Knight to e5 and then the sequence of Queen to f3 and then h3, I was blitzing one time and I didn't notice that the second idea of Bishop to f5 is to stop your queen from threatening on the h file

tysm this really helped with tryna learn the London system and then they do something like this

1. e4 is a first class move that prevents Bf5. For example: 1. e4 e6 2.d4 Bf5? 3. exf!! +-

Cool!

Very helpful!