Improved Critical doubles your crit range but says it doesn't stack with anything that modifies your range.
Critical Focus adds to your die roll to confirm a critical. It effectively modifies your range but seems to be written to function otherwise.
Strictly speaking IC lowers the threshold and CF gives you a boost.
Do these feats stack?
I'm leaning towards they do.
Sigurd
Actually, Crit Focus doesn't effectively increase your crit range. It gives a bonus to confirm crits, so it's more of a situation "to hit" bonus. Confirming a critical still requires that you hit the opponent's AC, not that you simply crit again. Only Natural 20 is an auto-hit, not any critical.
Anyway, yes, they stack. You double the crit range and then, should you crit, you get a +4 in addition to any other "to hit" modifiers on the critical confirmation roll.
Ah, Ok. Different rolls. Still looks like a decent combo - if you hit.
From what I can tell, Crit Focus is sort of a feat tax to get into the rather nice crit feats. Fighters finally being able to impose useful status conditions beyond "prone"? Yes please.
From what I can tell, Crit Focus is sort of a feat tax to get into the rather nice crit feats.
I'm not certain it's a feat tax. On your second or third iterative, most or all of your hits will also be critical threats. Even with things like a Keen rapier, you're still rolling a 15 which probably hits. Critical focus means that you'll confirm a lot more of those hits.
To quantize the effect, let
h - the probability that you hit on an attack
c - the fraction of the time you threaten a critical
d - average non-critical damage is d
N - critical multiplier
I'm going to assume that every critical threat is a hit and ignore the 1/20 auto-miss/auto-hit issue.
Without Critical Focus, your expected damage is
(h-c)d + chNd + c(1-h)d = hd + Nhcd - hcd = hd + c(N-1)hd
The first term is the damage from hits that don't threaten, the second is the damage from confirmed criticals and the last is the damage from critical threats that are not confirmed.
With Critical Focus, your expected damage is
(h-c)d + c(h+0.2)Nd + c(1-(h+0.2))d =
hd + hcNd + 0.2cNd - hcd - 0.2cd =
hd + c(N-1)(h+0.2)d
So critical focus increases your expected damage by 0.2c(N-1)d, regardless of how likely you are to hit. Assuming you have optimized and have Keen/Improved Critical and an 18-20 or x4 weapon, that is a 6% increase in your average damage. Until you can average 34 damage on a non-critical hit, that's better than weapon specialization.
As an added bonus, you also have less disappointments when you fail to confirm your critical threats. :-)
ah-ha, this one knows his crit-fu...
I stand corrected. Nonetheless, it still feels as much a feat tax as Mobility is for Spring Attack (in a skirmishing build) or Improved Unarmed Strike for a (non-monk) grapple build. Yes, it gives you something that isn't valueless, but it's still a bit lacking compared to other feats.
I usually skip right to 1 Level of Monk as a Multi-Class Tax if you want a Grappling Build... :-)
(Though Fighters do have enough Feats now to go ahead and get both Feats themselves)
Yeah. A first level human fighter is better at grappling than a first level (non-human) monk. Plus a true class fighter doesn't miss out on a level of favored class or (eventually) the capstone.