1997 AMC 12 Question 21

For any positive integer $n,$ let $$
f(n)= \begin{cases} \log_8n, & \log_8n\in\mathbb{Q},\\ 0, & \log_8n\notin\mathbb{Q}. \end{cases}
$$ What is $\displaystyle\sum_{n=1}^{1997}f(n)?$