I am wondering how many stops are there between settings?

For example if I set the control box to say 2 1/2 and then change it to 2 what is the stop difference? Is it a stop / 1/2 stop , etc?

I am trying to make a change in filtration and wanted to know how much I have t o decrease the time fro exposure? I am moving from 2 to 1 1/2...

Thanks,

Kev