Given the start of our new saga, a lot of character building is going on around these parts. Everyone has played Ars before and there are several book-savvy veterans amongst our group. This leads to the inevitable finetuning in order to make your build as effective as possible. Magical Focuses are thus basically obligatory. But trying to balance the scope of the different focuses has revealed some muddy waters when it comes to these Virtues (one being the Major Magical Focus: Faerie), but I made a separate thread for that for a reason).
Time for an example: One player created a Nightwalker who joined the Order via House Ex Miscellanea and later went on to join House Criamon. He picked up the Mysteries of Inscription on the Soul and Spirit Familiar. The player was looking for a Magical Focus to cover his talisman (himself, or actually his spirit), his familiar (ghost of a family member), and his Phantasticum form. He initially hoped to cover this by a Major Magical Focus: Spirits, but I was reluctant to let his own spirit fall under that for talisman attunement. After some shifting around, we settled on a compromise: a Major Magical Focus in "Own Family (Corpus/Mentem)" as an analogue to "Necromancy (Corpus/Mentem)" - so this would cover the Arts of Corpus and Mentem.
Other characters have equivalent Focuses as well, but in further discussing the options it soon became clear there were quite different opinions on the subject matter. So let's get to the meat of it...
It seems to be accepted as canon that a Magical Focus in a given topic (that is supposed to "be slightly narrower than a single Technique and Form combination, although it may include restricted areas of several such combinations" (ArM5, p.46) is actually a universal Magical Focus in everything when the topic matter becomes the medium for enchantments. The prime example is the Minor Magical Focus in "Wands" that allows the magus to benefit from his Magical Focus on any effect he enchants into a wand, no matter the Technique/Form combination, even though the focus is supposedly narrow. Someone with a Major Magical Focus in Necromancy (that according to the text insert is restricted to the Arts of Corpus and Mentem) could thus have an animated corpse as a talisman and benefit from his Magical Focus on a Pilum of Fire effect cast by an enchantment on it (thus working around the established limit on the Arts). This seems to be extremely powerful to me, and I have to wonder if this was the actual intention.
Core Question #1: Should a Magical Focus cover any type of effect as long as the medium of the enchantment falls under the Magical Focus?
Next (based on the Faerie Focus discussion) we were wondering how narrow the focus of a Magical Focus actually is yet again. Take a magus with a Minor Magical Focus in "Birds of Prey" for example. He obviously gets the benefit of this virtue when healing, detecting, or controlling the respective animals. Assuming the previously established canon he will also benefit from the Magical Focus when binding such an animal as his familiar and enchanting effects into the bond. Now assume he enchanted an invisibility effect for his familiar into the bond and wants to dispel it. Does it fall under his Magical Focus because its an effect created by the bond with the animal? What if another magus cast an ongoing Creo Ignem effect on the familiar - would dispelling the effect from the animal fall under the Magical Focus? What if he were to cast a Pilum of Fire at a bird of prey himself?
Core Question #2: Is the target of an effect relevant when it comes to deciding if an Magical Focus applies, or is only the effect in itself relevant for this consideration?
Finally, a magus with a Minor Magical Focus in "Disease", which is supposedly restricted to Creo/Perdo Corpus/Animal effects that affect diseases, intends to bind a spirit or daimon of disease as a spirit familiar. While there is analogy, it is a spirit and not a disease in itself, and in itself more gravitating towards Vim effects. While Corpus may obviously be substituted as the applicable Art in this case, the base effects for dealing with it are still different from the ones that are intended to target "Diseases".
Core Question #3: How flexible is a Magical Focus as to what effects (or targets as per question #2) it can support as long as there is some thematical connection?
... these questions may aim at the same core problem, but approaching each separately might make for interesting revelations. I'm really eager to hear some opinions.