What base does the author take when taking the log of both sides

Question

I am learning exponential distribution in ThinkStats2 by Allen Downey..
It says that "if you plot the complementary CDF of a dataset that you think is
exponential, you expect to see a function like:
$$
y\approx e^{-\lambda x}
$$
Then, taking the log of both sides yields:"
$$
\log y \approx -\lambda x
$$

My question is what base does the author take when taking the log of both sides? I guess that the author takes log of base 10, but it does not explain why we get $-\lambda x$ on the right side of equation.
Could someone explain this?

Answer 1

The author takes the natural logarithm on both sides (base $e=2.7182818\cdots $)