Hi,
a player wants to use for his non Tremere magus a focus in certamen.
My first reaction was: "no way". But after rethinking about it... why not?
What would be your opinion on this?
Tremere get it as their free virtue, sure, but they aren't a mystery cult. There's nothing stopping non-Tytalus taking 'Self Confident' or non-Mercere taking 'puissant Creo.' I can't see how this is any different.
All you need is a story or backgrounder for why the character has the virtue. In Tremere, its part and parcel of the apprenticeship training, which focuses in certamen-magic to a degree.
The character could have this focus for any number of reasons:
1.) There's a Merinita tradition that actually delights in punknig Tremere in Transylvania, I think. They might have it. Line of Muj or something, from HoH: MC
2.) Could be part of an Orbus tradition, or be an orbus himself from Tremere. Should have "Tremere wants him... er...out of the way" as a story flaw.
3.) Could be a Tremere apprentice taken by a Bonisagus magus. (Again, "Tremere has special interest" flaw)
4.) Could have joined a Mystery Cult House after gauntlet. (See above... only paranoid if you're wrong.)
5.) Could be a Merecere "Champion" (those who make a life by sticking up for Redcaps bullied by other mages.)
Remember, theoretically there is movement between houses, as mages join Mystery Houses or Societas. This can help move specific House-flaws or House-virtues around the Hermetic map.
Totally ok. As already mentioned, its a normal focus without any sort of restrictions on it. Its just that Tremere has it as standard.
And the magus could just have praticed Certaman a lot...
Or he might just really enjoy it and have a natural talent for it.
EDIT: I guess by RAW Tremeres get their focus as the result of their founders superior understanding and integration of the Nightwalker hedge virtue that Certamen is drawn from. But I think their is more then one way for two characters can arrive at the exact same magical focus.
On a related note can anyone think of a good Major Focus that would reasonably include Certamen. Phantasms perhaps where the focus would apply anytime the mage interacts with immaterial beings like some spirits and faeries as well as straying night walkers and his certamen opponents projections.
Ha! That's wonderful. I'll admit the Merinita didn't catch my interest at first but after hearing about the dissonant nature cult clinging to their origins and this I'm starting to warm up to them. 
Try Astrological Magical Focus (page 37 , 56 in TMRE).
Taurus , Second Decan covers struggle , competition
Mars: Influences war , violence , ambition , and sex , and governs the characteristic Strength.
Awsome I really like the idea of a saga where Tremere can't assume a wholly unchallenged preeminence in Certamen. I can see quit a few Flambeau and Tytalus with a Mars or Taurus Focus. Particularly if you have PC Tremere in a saga who plans to use Certamen often as a tool.
Some Other Ideas
How about a Potent Certamen version of Potent Magic.
Wouldn't it make sense for a NightWalker brought into Ex Misc to have the Focus in Certamen as their Minor Hermetic Virtue.
I thought the corebook was very explicit about the fact that the only focus that applies to certamen is a (minor) focus in certamen.
Yes it actually does say that I didn't notice, thanks. Still I'd be willing to stretch that rule IMO and allow major foci that specifically include the Certamen ritual/contest under their umbrella of effects. What I don't think should be allowed is to apply foci that are compatible with the nature of the phantasms conjured in a specific Certamen contest. If Maga A and Maga B are facing of in a ReAn battle and the phantasms are a couple of eagles locked in an aerial dogfight, Maga A shouldn't get to claim her focus on birds of prey. But A Tremere might want to be leary of throwing down with that quiet Criamon known for his mastery of spirits.
I wouldn't allow major foci to have a part of their focus (for example spirit or mars house) to affect certamen.
Very understandable since it is RAW. I just think allowing it might have some good story potential in specific Saga's.
For instance if you want to highlight the origins of Certamen in a Saga. Allowing a Magus with a focus in spirits to excel in Certamen could provide hints to it's magical roots.
Also it can be a good way to challenge an existing champion who thinks they have Certamen all figured out and is starting to get bored with it.
It could be a way to bring more then one player into the Certamen game or even at least one player if your troupe is like a lot of Ars groups and they don't really care for the system.
You have to balance this with the potential to take away from the specialness of those with the minor Certamen Virtue. But since it takes more then that one one virtue to really build a Certamen master. (Or to really take advantage of any focus) It's unlikely that a Spirit Master will also build up all the other things he needs to horn in on a Certamen masters spotlight.
A character with a Taurus focus who is an obsessive competitor in multiple arenas could also make a really good PC or NPC for some players and sagas. He would probably put alot of effort into mastering Certamen but he would also probably work toward mastering debate/disputio and many other avenues of competition.
The fact is, that for example, spirit focus is already broad enough: it covers your spirit familiar, it covers all interaction with spirits... why add certamen which is worth 1/3 of the focus broad, ... so if it was balanced to start with, it's now 4/3 of major virtue! Not fine.
There is also a possibility that a person without a focus could stumble across it as some integration research (such as nightwalkers).
I would imagine that it would be politically tricky as Tremere would want to guard their (near) monopoly on it.
A
I'm not sure focuses are balanced to start with as a point of fact. By and large a focus in spirits is only useful when spirits are encountered. If you encounter as many dragons in a saga as you do spirits then a minor focus in dragons could be just as valuable as a major focus in spirits. More useful since you don't need an extra virtue to bind a dragon as your familiar. (You just need to find a very small dragon like Maga Pryde did.)
Compared to a Major focus in Disguise, Damage, Stone, or even Necromancy a focus like Spirits is a whole lot harder to apply outside of specific stories. Even following a mystery path into spirit magic or theurgy means multiple seasons of adventures, studying cult lore, and attempting initiations before the character can really start applying his focus in stories the SG hasn't conveniently provided a spirit to cast a spell on.
Even if Spirits is hella useful that doesn't necessarily mean it still shouldn't effect Certamen if it fits for your saga. Some focuses are just more useful then others. How often do you think a Maga focused on Mammals looks at a Magi who is focused on domesticated animals and says. "I wish I was as good with chickens as you are I mean I can do everything else you can do but chickens are hard." (He of course replies with "I can do pigeons and ducks and geese too" To wit she says "Oh good for you, hey you want help with that wolf/mountain goat/lion/bear/whale/aurochs/...")
Well, if you never do Certamen it is worth nothing.
If your saga is centered around dragon, that focus becomes Major. Or if you play a excessively slow realtime saga, any learning Virtue is worthless and all matching Flaws priceless.
Can someone give me a page number for this , as i have failed my Awareness roll to discern the location of such.
Very center of page 90.