Spell comparison: Shield vs Mage Armor

Question

Wizards have access to 2 very similar spells: Shield and Mage Armor Both provide you with AC +4 Bonus, but Mage Armor has Range touch (i.e. you can cast it on other party members) and Duration 1 hour/level, while Shield has Range personal and duration 1 min./level

From my point of view Mage Armor is far superior to Shield. In my party Wizard and Monk formed mighty combo, where monk got +4 AC bonus for free every fight. It looks like a gamebreaker.

Question: Why are these spells, which seem so similar in effect, so different in range and duration? Are there any drawbacks or advantages to one spell or the other which I didn't see?

Answer 1

Armor bonuses are more common than shield bonuses.

Mage armor provides an armor bonus to AC, which doesn't stack with other armor bonuses. Since nearly everybody has an armor bonus (even proficiency with light armor gives access to mundane options that match the +4 bonus), this has a limiting effect on its usefulness to non-caster classes. This is why the spell is allowed to be cast on anyone and to last so long: it's not as useful so opening up the target range doesn't have as big an impact on balance.

Shield, on the other hand, provides the much less commonly-used shield bonus to AC (many fewer classes have this proficiency, even fewer of them usually take advantage of it, and getting a mundane shield with a +4 bonus is a build-defining choice). Very few classes run around with a +4 shield bonus, so even those who do use shields would find it very useful... and thus its availability is limited to the caster, to keep that usefulness in check.

It's the same reasoning that makes rings of protection (deflection bonuses) cost twice as much as bracers of armor (armor bonuses) with the same AC value: the less common a bonus is, the more valuable it is because it's less likely to conflict with another existing bonus (which would make it useless).

Two asides

1. It's worth noting that mage armor and shield are both force effects, which have various usefulnesses, but that only shield stops magic missile. It appears Wizards overestimated the value of direct-damage spells, and underestimated how easy ranged-touch-attack spells would be to land, so they thought magic missile would be a cornerstone of wizards everywhere. If that had been true, shield would be much more useful--making its duration/target restrictions more sensible.

2. Exactly how bonuses stack (especially as regards armor bonuses and enhancement bonuses to AC) seems to be rather murky and contradictory in the rules, so if further clarification is needed on that particular aspect of this issue, it should be made a separate question.

Answer 2

Why do you consider it an either-or choice?

Mage Armor is better for casting and leaving on all day. Until you get high enough level that you have Bracers of Armor. In fact, you have a hard decision once you get low grade Bracers of Armor and casting the spell only gives you a +2 to your armor bonus now.

Shield is a great spell to "spike" as a buff when it's big fight time. It stacks with armor/mage armor/bracers of armor, so base mage armor + shield gives you +8 to AC for one fight (plus the magic missile invulnerability), which is pretty darn good, you can do it at level 1 to get a better AC than anyone else can afford at that level!

This is kinda analogous to "Is the monk's AC bonus "better or worse" than his ability to spend a ki point to boost his AC?" Question is meaningless, both are good, even better when combined.

Now certainly I always take mage armor first if I have spells-known limitations (sorcerer or whatever) as it's clearly the more useful baseline (like the monk's AC bonus); then segue to shield as I get a a couple levels and get some bracers of armor. Both spells are great to just have on wands so they don't take up spell slots as their level dependence is pretty mild (more severe with mage armor if you're doing long wandering).

Answer 3

The spells are badly balanced, Mage Armor being stronger

While I agree with BESW's analysis, that a shield bonus is more valuable than an armor bonus, I do not believe it is enough to justify the differences in target selection, and more importantly, duration.

Mage Armor lasts for hours. At low levels, this means it can last through short dungeons. At mid-levels and above, it effectively lasts for an entire adventuring day. Shield is only effective when the caster can specifically "buff up" for the combat, and even then lasts for only one or two combats at most. Even though Shield offers a more useful bonus than Mage Armor does, the spell itself is far less usable.

My experience supports this: Mage Armor is a common spell choice among players of arcanists of all levels, while Shield is a niche spell.

Why this difference? Balance by spell level wasn't a major concern in the development of the third edition of D&D. Pathfinder then took up the spells without modification, so the "problem", if it is one, remains.

If interspell balance was given a thought in this case, I'd guess that the authors badly overestimated the value of Shield rendering the character immune to Magic Missile.