As I have read the books in Spanish some "lost in translation" could have happened (and seeing the quality of translations, I doubt it didn't happen).
From what I know, the rules state that for each Sphere you require at last one Foci, but nothing sais you cannot have more than one (I believe they even encourage it). But that is not to say, that you can (or need to) use all of them for each Effect you cast.
When you cast an Effect, you describe how you are going to do it, all the components/show/gestures that you put into scene can be described in two ways, each separately is a Foci, all together is the Foci.
- – If your Arete is < 6, you need to use Foci, period. You will receive no bonus for using it.
- If you think they are different Foci then: You will receive a -1 for each Foci you are using (when none are needed) up to -3 that @Kris Wojcik mentioned. But if you think they are all together, one Foci, then you will receive a -1 only. I usually think the latter, since all of it is required for the Effect to work, right? (Investigating this sounds really Hermetic-way).
- Specialized Foci… I believe they are also called Personal Foci (could be?), already gives you a -1, but there is nothing to say that modifiers don’t stack (on the contrary, the book states different in nearly every example) that I am aware of. Unique Foci is not a Specialized Foci, is it more (and I don’t believe you can have a Foci that is both, they are one or the other), by the rues of Unique Foci it seems strange that you could loose the necessity to use the Foci in that Sphere (when having Arete >= 6), but I don’t think there are any rules about that (The Storyteller handbook could have more information about this, since I remember there is a chapter dedicated to this, but I don’t remember reading anything about loosing the need of your Foci and having a Unique Foci), anyway, using a Unique Foci AND a specialized Foci (because your effect uses multiple Foci… if you think in that terms) could grant you -3 (-2 for the Unique and -1 for the specialized).
I think it will be possible to stack a Specialized Foci -1 with the Using Foci when none is needed for another -1. I do not believe that you can do that with the Unique Foci, since it seems that you NEED to use it.
If somehow you convince your storyteller that you don’t need to use your unique Foci (due to having Arete >= 6), it seems also reasonable that you will stack the -1 for using Foci when none is needed.
You have Arete 6 and as a good Hermetic you have decided that Prime, Forces and Matter will not require Foci (I am not sure on the amount of Spheres that you can choose, bet lets say three for the sake of the argument). You then decide to good-old-Fireball something with good-old vulgar magic (Prime 2 Forces 3, lets not start an argument on “are this the Spheres required?”).
You have your platinum crown (this Hermetics must be all rich!), blessed during a full moon cycle in the waters of your birthtown river (that will be the Specialized Foci) and your Obsidian Staff created from The Volcano Spirit and given to your family centuries ago (that will be your Unique Foci).
Depending on your storyteller, the difficulty can range from:
Lowest diff: Highest Sphere (3) + 5 (vulgar magic with witness) -1 for using crown when you don’t need to, -1 for using staff when you don’t need to, -1 for crown Specialized Foci, -2 for staff Unique Foci, for a total of 3 (and some paradox).
Highest diff: Highest Sphere (3) + 5 (vulgar magic with witness), -1 for using Specialized Foci, -2 for using Unique Foci (it could be argued that it is only a -2 since you need both Foci to work, but I usually don’t go that way), you NEED to use your Unique Foci, because it is a Unique Foci and your Effects will simply won’t work without it, so you are not getting any bonus from ‘using Foci when you don’t need to’, for a total of diff 5 (and some paradox).
As you can see, it varies, and it is important. I, as a mage storyteller, try always to make clear how I view the magic (magick? magik?) system so there are no nasty surprises when you try your Effects.