# Radix sort and changing bases

I have recently learned about radix sort. I am aware that you can change the base of the numbers you need to sort but I don't really understand why this is good for the radix sort. Radix sort runtime is $O(d(n+k))$ where $d$ is the number of digits in the numbers, and $k$ is the base. So shouldn't there be a permanent ratio between $d$ and $k$ so that the runtime is optimized? How should I choose the base in any other way?