Several possibilities cropped up and I am curious...
Min/Maj Magical Focus can be used with Certamen...(Ability)
What other abilities could it be used with...???
Magic Theory?
Penetration?
Concentration?
others etc
Certamen is no longer an ability, but an action, like casting spells. The closest thing would be laboratory activities, like inventing spells or enchanting items, but I don't think you want magical focuses to apply that broadly.
IIRC, they do apply to laboratory activities. Serf's parma.
you can't have a focus who's are is "Laboratory work" because that is too broad;
you can have a Focus "Necromancy" and when you work on necromantic effects in the Lab, you can use your focus.
And what about a focus with charged items?
Invested device?
Lesser device?
Talisman?
Familiar?
Are thei Minor or Major?
May the vis will be with you!
Which is kinda my point: Certamen is VERY broad. It uses all of the Tech/Forms...
Ergo, it seems that you could use it for things that are broad (though it does specifically say "not for charged items)...ie why can't you take a focus with (say) Magic Theory????
{Minor Focus:-if I follow the rule correctly-Double the lowest art score, to the maximum of the highest score involved: Rego-5 Corpus: 9
You would double the Rego score to 10, but the max is 9, therefore the numbers added would be 9+9.}
The net effect would be as follows, when using MT.
Say a scores of Rego/Corpus: 1/15, 5/15, 10/15.=
2+15, 10+15, 15+15. (I feel these score represent a typical range for newer magi).
Now this would only apply to work using MT. It would not apply to casting spells.
Now Major Magical focus would look like this...(If I understand the rule correctly)
1/15, 5,15, 10,15=
2+15, 10+15, 20+15.
I figure this is in line with a Major Virtue.
Now push it to a higher score:
29/30=58+30. 
If this seems a little Too much, you could limit the bonus to the MT score...
...Just some thoughts...
Personally I think focuses are a great addition to Arsmagica's options.
It can add a lot a flavor to a magus... so much that I think that all magi should have one to represent their unique way they channel magic.
Still, it can kill the game if you allow focuses without flavor in your game.
I would move against a focus on MT or any ability since a focus is made to represent the affinity a mage has to work magic in a particular way.
A magus can hence be
-Powerfull at burning to the ground things but not so much when tossing a simple fireball.
-Powerfull at making potions but not creams or other forms of charged items
-Powerfull on personnal magics
-Anything that adds flavor to the magus
When trying to create a focus, think of a circumstance when you want your magus to have an edge over another magus. It can also validate reputations that your caracter has/will gain.
Basically, be carefull in choosing your Focus for it is the most powefull merit you can pick in medium-long term games.
After playing for a while, I can't imaging making a character without some kind of focus. They're just too useful both mechanicaly and descriptively.
Well Tremere are a bit hosed in this regard.
Their House Virtue precludes any other Magical Focus.
Heh, oh yes, my players refer this to the Tremere bonus flaw.
Actually the focus mechanics seem too good sometimes. Especially with ultra specialist characters this can lead to rather absurd lab (and casting) totals quite quickly.
My players (with almost beginning characters) could have pretty much walked through Calebais with the charged items they were able to make due to one well-chosen focus. Those items they made could have affected any ghost in adventure quite easily, as far as penetration was concerned at least. Luckily they only used the items on Pitsdim, and actually role-played the situation with others.
Oh, yeah. We house ruled that.
Decallom:
Can you explain what you were using for your focus???
Curiosity and all that...
Not my character... But the interestingly enough the focus was flames, the character was Flambeau. Being forewarned of the presence of ghosts (don't ask me to explain that), he made a variant (first as a spell, then as a charged item - even the similar spell bonus started to become handy here) of a simple fire blast spell that also affects ghosts -> Mentem requisite.
The plan worked much because the magus in question had about even scores in Creo and Ignem, and was surprisingly good in Mentem as well (and spent several seasons studying mentem before doing anything else).
Hardly the only option. Actually in the same saga we had a Bonisagus who did Perdo Vim effects without a focus quite well enough to affect most ghosts in Calebais. Actually he was my own character, and was a Creo Vim specialist that researched a way to manipulate magic auras with Hermetic magic (and actually completed that original research project during the saga).
Certamen's not "too broad". It does cover all forms and techniques, but only on occassions of magical dueling between consenting parties. I mean, its contextually so narrow that it is almost worthless. It is worthwhile if:
the other guy doesn't have it.
the Tribunal won't just over-rule the whole thing
the loser isn't willing to go to Wizrd's War
both sides can't just hire someone with it.
So, it's a very, very narrow focus...and the opportunity cost is huge. Sure, you are a good duelist in very limited circumstances, for which you gave up what? Healing? Flames? Commanding mortal armies?
What if you took
Lesser Immunity : Certamen ?
Does it count as a Hazard?
No.
You can't be forced into Certamen - you can only perform certamen by agreeing to it, in advance. As such, Immunity; Certamen makes as much sense as "Immunity: not being silly enough to put hand in fires".
What do you think is it possible to count the bonus of the magical focus to get higher level spells at the beginning?
I think it is.
Learning spells is a lab activity and focuses can boost this process.
What do you think about a focus of perfection? It would be a focus to improve the magus' stats and longevity. Giving supernatural abilities (e.g. eyes of the cat) is not a perfection but making tough or more healthy are. Healing and similar things wont be included.
I think it would be a minor focus I compared it to the Self-Transformation and healing foci to rate its level. Should I limit it to perfection of people to remain on the minor level or not?
This seems like a minor to me but I realize that ageing is used as a minor as well. Naturally there are things that one could do witht hteh ageing focus (specifically perdo animal/corpus/herbam) that you can't do with the perfection focus.
Conceptually I'd think that healing would be included but this might push it to a major. Why do you think that healing is not a perfection (or what is your rational for healing not being applicable)?
I have a magus with an unusual focus. In the conversion from 4th Ed, he ended up with 10 points of flaws and 7 of virtues with no major virtue. Looking at the Major virtues, most were completely inappropriate and several were just too complicated to implement, so with troupe consensus I added a Major Focus. We then had to figure out whqat the focus was. It couldn;t be anything all that obvious because a focus should really be something that 'colours' the roleplaying, actions and long term goals of a magus IM(our)O.
So I looked at his history, and noted that something he has done an awful lot of is inventing spells (if he published all the high level spells he has invented he would qualify for the top Bonisagus rep within about 20-30 years of gauntlet). And since he is a bit of an experimental nut in the lab, a lot of those spells not only have unusual effects (and side effects), but have often been invented multiple times (trying to get it right).
So with troupe agreement, I made his focus Similar Spells. In other words his focus is on spells he already knows, usually that means getting them right. I think of it as a case where he is naturally very good at fine tuning existing knowledge.
This is both very broad, covering all Arts and both Lab work and spellcasting, but also very restricted - it only acts when you can get a similar spells bonus.
It also means that he can get exceptionally high lab scores by utilising existing knowledge - need to invent a level 60 spell? Invent a level 20-30 one first, then the 60 can be done in just a season or two. this is proving exceptionally useful in his research project.
On the other hand, the only real use for it in the 'field' is sponting adaptions of spells he already knows. Which is nice for 'down-powering' ihis hoopy big spells, but not really that useful so far.
There's an obvious, game-breaking exploit of this Major Focus: for every spell A that you know, you can invent a similar spell, at as low a level as is useful for you (by lowering Targe, Duration and Range).
And presto: your Focus now applies to spell A - significantly enhancing that spell's casting total and resulting penetration.
I suggest you talk this exploit over with your SG and find a redefinition of your Focus which avoids it.
Kind regards,
Berengar