As we all know, formula for mutual information is as follows:
$ I(X, Y) = H(Y) - H(Y|X) $
In case of Binary Symmetric Channel with flip probability equal to $f$ and both alphabets of size $2$:
$ I(X, Y) = H(Y) - H(Y|X) = log2 - H(f) = 1 - H(f) $
I'm confused about the equality $H(Y) = log2$ a little bit. Is it always the case that entropy of output is equal to $log$ of size of alphabet? If not, could you give me an example of such a channel?