Question: I was looking up some interesting feats to add to my magus build, and ran into intensified spell. I was wondering in which way the feat is actually applied, however. I understand its effects from a mechanic standpoint on a single, but I was wondering how I apply it to my spells.

* Do I choose a single spell, and from that moment on, every time its caster level is +1?
* Do I choose a spell (always the same) and apply it if I want it to, when studying my spells at night?
* Do I choose a single spell (can be different ones, depending on which one I want to prepare) and apply it if I want it to, when studying my spells at night?
* Can I select a number of my spells ((can be different ones, depending on which ones I want to prepare) and apply it if I want it to, when studying my spells at night?

The reason why I ask is because I would feel bad about spending a feat on a spell which I might not use again later on (as in, level 15 - 20).

Answer: 
Metamagic feats are usually more versatile than that
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When a prepared caster like a [magus](http://www.d20pfsrd.com/classes/base-classes/magus) that possesses a [metamagic](http://www.d20pfsrd.com/feats/metamagic-feats) feat like [Intensified Spell](http://www.d20pfsrd.com/feats/metamagic-feats/intensified-spell-metamagic) prepares his spells for the day he can opt to modify any number of his spells—including multiple different iterations of the *same* spell—using that metamagic feat. Preparing a spell modified by a metamagic feat increases the spell slot level the modified spell occupies by the amount listed in the feat's level adjustment. (However, except in the case of, like, the feat [Heightened Spell](http://www.d20pfsrd.com/feats/metamagic-feats/heighten-spell-metamagic---final), this spell slot level increase does *not* change the spell's *actual* level that's used, for example, to determine a spell's saving throw DC.)

A metamagic feat usually is *not* usually limited to, for example, only one spell picked when the feat's gained nor to only one spell in the caster's daily repertoire.

Thus, for example, a magus could prepare the spell [*shocking grasp*](http://www.d20pfsrd.com/magic/all-spells/s/shocking-grasp) modified using the metamagic feat Intensified Spell, and that intensified *shocking grasp* spell occupies one of the magus's 2nd-level spell slots (instead of a 1st-level slot like *shocking grasp* normally occupies). After casting that intensified *shocking grasp*, the magus won't be able to cast that intensified *shocking grasp* again *either* until he prepares intensified *shocking grasp* again the next day *or* unless he's also prepared a second or more intensified *shocking grasp* as a 2nd-level spell.
**Intensify spell**

> 
> Benefit: An intensified spell increases the maximum number of damage dice by 5 levels. You must actually have sufficient caster levels to surpass the maximum in order to benefit from this feat. No other variables of the spell are affected, and spells that inflict damage that is not modified by caster level are not affected by this feat.
> 
> 
> Level Increase: +1 (an intensified spell uses up a spell slot one level higher than the spell’s actual level.)
> 
> 
> 

This feat can be applied to any spell. Every spell that you intensify will take a Spell Slot of one level higher, than original spell.

It doesn't increase the spellcaster level, it increases the spell's limit.

For example: intensified fireball deal maximum 15d6 damage and takes 4th level spell slot.

Furthermore

> 
> Can I select a number of my spells ((can be different ones, depending on which ones I want to prepare) and apply it if I want it to, when studying my spells at night?
> 
> 
> 

This sentence is wrong. First you have to take a rest, then you prepare spells.

> 
> A magus may know any number of spells. He must choose and prepare his spells ahead of time by getting 8 hours of sleep and spending 1 hour studying his spellbook. While studying, the magus decides which spells to prepare.
> 
> 
> 

**Metamagic feats**

> 
> Effects of Metamagic Feats on a Spell: 
> 
> 
> In all ways, a metamagic spell operates at its original spell level,
>  even though it is prepared and cast using a higher-level spell slot.
>  Saving throw modifications are not changed unless stated otherwise in
>  the feat description.
> 
> 
> 

**When you prepare your spells you may decide to apply metamagic feats to them. If you do, you must have sufficent ability score.**

For example maximized and intensified fireball will take 7th level spell slot.