In a related question I had asked when you must choose if you are using a lower caster level when casting a spell.
If for some reason you want to cast a spell at a lower CL and are forced to make a concentration check, such as casting defensively, what CL do you use? The one you are casting the spell with or your actual CL?
You can cast a spell at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the same caster level.
I am asking this question in a general sense so that it works for Pathfinder. The specifics of why I am asking are for Spheres of Power (SoP). In it I am playing a caster who has the focus drawback. I have lost my focus and to successfully cast a spell is Concentration (1d20+ability mod+CL) VS DC 20 + 1/2CL. The 1/2 CL is directly tied to the spell being cast, but is concentration? By casting weaker spells I can have useful effects but with easier to manage DC's.
Your magic requires you to use an item such as a wand, holy symbol, ring, or staff to create magic. Using magic without your focus requires you to make a concentration check (DC 20 + 1/2 the caster level) to produce the desired effect. Failure means time (and any spell points) are spent, but no effect happens. If a focus is lost, stolen, or broken, the caster must create a new focus by securing the necessary item and spending 8 hours bonding with it. At the GM’s discretion, however, another caster’s focus may be used instead.
If a caster chooses to use an effect at a lower caster level than his maximum, the DC is also lowered.