Antoonio got it right. The bigger the apertures number, the narrower the depth of field (acceptable focus area). So the faster the film, the lower you can set the apertures because faster film needs less light. So if you want a wide depth of field, try a faster film. For less depth of field, use slower film. Of course, this is general, and your lighting conditions and what type of shot you are looking for is a factor too. The more light, the more ability to have a wider depth of field. Less light, less D.O.F. I hope this makes sense.