+1 to the prior poster in that planning is a must with these purchases, but the lenses are more important to me than the filters.
It's all about fighting vignetting to me, and to do that, you have to spec the right type/size of holder to do that job.
The first rule of these filtering systems is to assess two things:
1) What is the widest FL you want to use these filters with?
2) Does that particular lens have a front filter ring?
From that, you go to any reputable manufacturer's site and they will have the vignetting / FL information you need to make a buying decision. Note that your 10-18 will 'register' as a 16mm FF focal length on the wide end for whatever tables they use.
For instance, Lee's is below (taken from their 100mm system manual PDF). I'm sure Cokin, Fotodiox/Wonderpana and Formatt-Hitech have either similar guides or a customer service person that could tell you.
- A