In the rules for Major and Minor Magical Foci it is written that the virtue applies
When you cast a spell or generate a Lab Total within your focus […]
Does this mean the focus kicks in when the cast is within the focus or the spell is within the focus?
To give an example, consider
a magus with a focus in birds, who casts Phantasmal Animal to create an image of a bird,
or a magus with a focus on daemonslaying, who casts Winters Icy Touch on a manifested daemon.
Would their foci apply, or would they need to invent more specific versions of the spells in question, i.e. Phantasmal Bird or Snuffing out the Daemonflame?
I feel that a focus implies a particular understanding of a group of effects, rather than particular ability in applying effects to a given target - thus it’d only apply if the spell was wholly within the focus.
On the flip-side, ruling per-spell would make listed foci like ‘men’ or ‘women’ particularly useless, requiring two versions of each corpus spell and so on, not to speak of ‘Enemies of Persia’.
You always “invent” a spell, whether you use a Lab Text, invent it from scratch, or learn from a teacher. In all cases, your focus will apply.
The only case I remember where there are 2+ variants of the same spell is from Potent Magic, where you can impose the use of a casting item to add a shape/material bonus.
The mage might still be interested in inventing a spell such as Phantasmal Bird if it's pushing the limits of their Arts. The focus might make the spell quicker to invent, or it might be completely beyond them to create the broader spell.
When a magus invents a spell, his Magical Focus will apply to his Lab Total if the spell's scope is completely included within the Focus. So if he has a Minor Magical Focus with wolves, the Lab Total of a spell which makes a wolf's fur fall off will benefits from the Focus, while a more general spell that makes an animal's fur fall off will not benefit from it.
The Focus will apply to the Casting Total for the first spell all the time, since it always falls within the scope of that spell (it was invented that way). The Focus will also apply to the second spell when its Target is a wolf (within the scope of the Focus), but not if its Target is a different animal like a cat (outside the scope of the Focus).
Note that I use Target instead of target, because casting a Pilum of Fire against a wolf would not benefit from the Focus, since the spell creates a fire and directing that fire at a wolf is just incidental.
Arthur has the right of it, and a good description of it as well.
The small addition that made me decide to post as well - the stricter reading of the rules are pretty clear that the focus should generally apply within a specific form. Arthur’s Focus in Wolves would probably not apply to creating an illusion of a wolf, because the Focus should apply to Animal, not Imaginem. Creating a Creo Terram statue of a wolf should also not apply, even if it’s all wolf-y. Turning a wolf into a statue or a statue into a wolf with Muto should apply, IMO.
That is what I meant with my note about the Target vs target.
The Target of a Creo Imaginem illusionary wolf is an image, not a wolf. The Target of the Creo Terram statue in the shape of a wolf is a piece of stone, not a wolf.