What's the magnitude of an enchanted item's effect in respect to related Ease Factors (like Second Sight)?
Let's assume a Ring of Invisibility which contains a Perdo Imaginem effect of 25 (Base 4, +1 Touch, +1 Concentration, +1 changing image, +5lvl item maintains concentration, +5lvl 24 uses per day). Do you just divide the effect levels by 5, therefore giving the effect a magnitude of 5 (and thus an Ease Factor of 11 to see through it with Second Sight)? Is the number of uses relevant for the magnitude of the effect, or should it only be magnitude 4? Also, do you round up or down (for example a permanently invisible ring with a PeIm 14 effect)?
It should be the total magnitude of the effect. So in your example, it'd be 5.
In cases like this, items are often better than spells.
Unless it's explicitly spelled out otherwise, you always round any fractions up, in ArM5.
One reason you calculate the magnitude of the effect of an item is to determine if it causes warping. Having the item have multiple uses per day does not increase the level to check for warping (per Legends of Hermes p112). I don't see why you would not use the same rules for determining magnitude with Second Sight.
Now, having the item maintain concentration is not one of the listed examples of exceptions. On the other hand, I read the list as examples, not an exhaustive list.
I would say it was magnitude 3.
"The level of the effect is the equivalent of its spell level. [...] Note the starting level of the effect, for future reference." (ArM5, p.98)
Rereading this I'd also tend to say that the magnitude of an item's effect is calculated without the bonus levels for effect frequency, penetration, or triggers. Seems to make more sense this way.