In a hypothesis test for the equality of two variances, $H_0: \sigma_1^2 = \sigma_2^2$ vs $H_a: \sigma_1^2 \neq \sigma_2^2$, we use the test statistic $F = \frac{s_1^2}{s_2^2}$. If $n_1=11$ and $n_2=16$, and we assume the samples are independent and normally distributed, what are the degrees of freedom $(df_1, df_2)$ for the $F$-distribution?