Also, do you always do cr.mk XX wp.srk[light punch shoryuken] when u see focus? Is that safe if they backash? Are there characters with a quicker back[d]ash that can punish it?
Here are the different situations in which I perform cr.mk xx lp.srk:
1) The opponent is focusing in my face in attempt to punish a fireball on reaction (or anticipating a fireball)
2) The opponent loves to focus
Here are the pro's and con's of doing cr.mk xx lp.srk to punish focus attacks:
1) The opponent dashes forward and you punish them for a good amount of life
2) Opponent back dashes and is not able to punish your whiffed lp.srk giving you lots of space
1) They back dash and are able to punish your whiffed lp.srk (for example, Akuma/Gouki)
2) Opponent is able to charge focus into a level 2/3 and punish your whiffed lp.srk
Now that you know the different situations this will give you an idea of what my options are when I see a focus attack.
Generally speaking, if I see a person focus attack, I will test them to see if they want to bait me out. If Ryu does low forward while the opponent is focusing he still can get crumpled. In order to avoid this situation Ryu must special cancel into special move like a fireball or shoryuken (thus punishing the opponents focus attack).
In the attempts to punish the focus given you've pressed cr.mk you have a few options depending on when your low forward connects with their focus attack.
cr.mk xx nothing - You can retract your poke and block. The opponent may also forward or back dash. If cr.mk is timed as soon as the start of a focus attack you may or may not be able to block in time depending on which character is focusing (for example, Fei Long's focus attack has long range and speed).
cr.mk xx srk - You punish their focus attack if they forward dash. However, the opponent may back dash or charge their focus attack to punish your cr.mk xx srk. This isn't always the best answer.
cr.mk xx fireball - I occasionally do this when I notice opponents focusing my cr.mk from far away. This gives me extra space to for my fireball to come out. If the opponent is focusing much closer it's better to use dp to punish since fireball might get stuffed.
cr.mk xx ex fb - Ex fireball has a faster start up and recover time compared to regular fireball. I ocassionally do this and get count-hit out of ex fireball, but that's because I'm timing it wrong. I use this in consideration to how far the opponent is from my low forward as well as when my cr.mk hit their focus attack.
cr.mk xx solar plexus (f.hp) - There are times when doing low forward from max distance and throwing a fireball is unsafe (given the opponent can ultra through the fireball at max range). However, if you have already hit their focus attack it isn't very likely that you'll have enough time to do solar plexus. Only in some occasions you will hit the opponent, but it's very risky.
cr.mk xx sweep - You're thinking, "Sweep?" It isn't a combo, but it's a block string that punishes people focusing and dashing forward. It's a great tool to use in the moment, but not as muscle memory. I use it on reaction if I see opponents moving forward after their focus attack. Be careful as some characters may be able to recover in time and block your sweep after their focus attack recovery frames.
cr.mk xx delayed fb/ex fb - Depending on the player and spacing, I like to delay my punish if the opponent has baited out cr.mk xx lp.srk. The delay gives you enough time to react. If they; 1) back dash they eat a fireball/ex fireball (chip or full damage), or 2) forward dash you have enough time to decide to use fireball/ex fireball or nothing at all.
My buddy Javier (Gouki) baits my cr.mk xx lp.srk by focusing and back dashing. I eventually whiff the dp while he back dashes and Gouki/Akuma is able to walk forward and sweep me for a punish (as well as follow up with a mix-up).
Be careful on which punish you decide to use when your cr.mk meets a focus attack. Always analyze the situation and use your greater intuition. But most of all, do not hesitate. Make a decision and learn from it whether you punish the focus attack or not. :)