Like the core spell on page 139: The Treacherous Spear.
I think it is worth separating out that there are two types of foci being discussed. One is shape and one is material and I feel like one is 100% worth saving, while the other I can take or leave.
A focus in a shape, such as swords, wands, boots, rings, or ships let you make cool, flavourful magical items that do a variety of things. A focus in a material, like metal or wood is not as cool and flavourful, though I personally think its fine as well.
If you're making a magical sword then you get the bonus, it doesn't matter if the magic is to shoot lightning, detect dogs, turn you invisible or just something as pedestrian as making the sword a little bit sharper. The sword doesn't give you a shape bonus to any of those effects so the foci is just going to be replacing the bonus you could have got elsewhere while also making you a cool weird wizard that only makes magical swords.
Is a sword that casts DEO fine under the ruling of shape foci not counting? Even at Touch range its not affecting the sword directly. If we're doing a ruling that means enchanting a sword to be a demon-slaying sword does not get the bonus from a Focus in Swords then I am very much against that. Likewise a focus in castles should apply when you enchant it with Chamber of Spring Breezes even though that only affects the air in a room in the castle and not the castle directly.
If a focus doesn't apply to all powers put in an item then a focus in wooden wands seem pretty useless in general and very weak as far as enchanting goes and it means that the majority of magical items are going to end up being weapons and armour because they are the ones that benefit the most from being directly affected and that is extremely dull compared to everything else in Ars Magica. Not all things are created equal in Ars, but Roland and Himnis shouldn't end up at one of them producing the finest swords in existence and the other is a joke that is a waste of a Focus, of which you can only have one.
If it must be changed then I feel that Shape foci should apply, but Material foci shouldn't. Or a new virtue that gives you a focus in a shape for enchanting should exist because of the aforementioned demon-slaying sword (or a sword that will cause a light wound when someone touching it's blade lies , a PeCo effect attached to a InMe effect) or whatever cool magical stuff you want the sword to do. Material foci aren't as cool so while I think they are fine I wouldn't mind them becoming a "they only apply if it would apply when inventing the effect as a spell" or whatever.
If balance is the primary concern then if an enchanting Virtue is created then it should probably lock out a normal Minor/Major Focus because otherwise you end up with double dipping, eg: they have a focus in fire and an enchanting focus in swords and if they make a sword that shoots a fireball we're now tripling the lowest art.
tl:dr: Foci: Swords/Wands/Ships/Trees are great and flavourful, Foci: Metal/Wood/Glass I think are fine too, but I'm not willing to die on that hill unlike the one for shapes.
I'm strongly-opposed to an enchanting-specific focus. Magi built for enchantment are plenty strong enough already; I would like magi who are not built for enchanting, but are powerful specialists in a thing, to be powerful at enchanting that thing too. I don't want every verditius to pick up Minor Enchanting Focus:Wooden Wands or Silver Rings and get to double their lowest art. I do want your classic spellcasting specialist to be really good at enchantments in their specialty.
Dividing it between Shape and Material bonuses feels weird to me. I don't hate it, but it doesn't feel particularly thematic. And it advantages certain types of foci over others.
The way we've always handled this (which I realise from this thread is an inadvertent house rule...), and this specifically applies to the "Minor Magical focus: Swords" thing for the confraternity of Roland because that's what the character in our saga it first applied to had, is that the magical focus can apply based on the shape of the item, but only if the effect involves using the item as a tool.
Basically if whatever you do to trigger/use the enchanted effect would qualify as "us[ing it]
in an appropriate manner" for the purposes of an item of quality then it's valid. Which makes sense, considering the confraternity of Roland also teach the item of quality mystery.
So for a sword making the sword unbreakable is valid because the effect involves swords (standard magical focus). An effect which does something harmful to someone at touch range is valid, because causing harm when you hit people with it is the essential nature of a sword. An effect which shoots a bolt of lightning is not valid, because it's not in the essential nature of a sword to harm people from far away.
As for a wand making it unbreakable is still valid. An effect which does something harmful to someone at touch range is not valid, because that's not in the essential nature of a wand. Shooting a bolt of lightning is valid, however, because projecting bolts/missiles is in the essential nature of a wand (according to the shape bonus, anyway).
This is loosely tying the validity of the focus to the shape bonus of the object, but the "using it as a tool" thing adds some leeway for edge cases. An example in our saga would be Curtana, part of the English royal regalia. We had it as a creation of the confraternity of Roland but as it's intended use/essential nature is as a ceremonial sword it has effects related to majesty and awe, rather than combat.
The "harm others on contact" effect described above would not have been valid uses of the magical focus on this sword because its essential nature is to be worn in ceremonies, not used in battle.
My conclusion from reading this thread is that the existing situation is actually OK. People are interpreting the rules in different ways, but this is not something that needs to be fixed with errata.
If we actually had a ticket system, I would be looking at marking this WONTFIX.
I'm wondering if, like some posters suggested, this could be made into 2 foci, "Casting" foci and "Enchanting" foci
Then, we consider the following.
As a rule of thumb, "Casting" foci can be considered to have a 1-word descriptor / restriction for Major foci, such as Fire, Men, Plants, Necromancy, Stone... For a minor focus, you add 1 restriction, such as Firemen, Growing plants, Corporeal Necromancy, moving stone.
What if Enchanting foci had 1 more descriptor?
I like the shape + matter as a base for Major foci, and we could add 1 more limitation, often (but not necessarily: 2 major foci could do, as an exemple) technique-related for minor foci.
This would mean that canon foci, if Enchanting Foci, would be Major virtues, but I can live with that.
So we could have Major Enchanting Foci such as Golden Rings, Metal Armor, Wooden Wands, Linen Clothes, Stone Swords... And minor Enchanting Foci like Golden Rings of Creation, Metal Armor of Control, Wooden Wands of Destruction, Linen Clothes of Illusions and Health, Stone Swords of Fire and Steel.
Just a random thought, not very well thought-out, I admit .