Focus in Warframe is an experienced-based resource that can be used to unlock abilities for your Operator. There are five different skill trees, based on the five Tenno schools, that grant your Operator different skills and buffs. To fully unlock one tree, and even partially explore all five, you will need a lot of Focus.
Focus can be earned by using Focus Lenses on weapons and Warframes. Focus Lenses allow you to convert part of your earned experience from killing enemies into Focus. These points can then be spent at the Operator in the back of your Orbiter to unlock and level up skills.
Where to Quickly Farm Focus
Elite Sanctuary Onslaught is currently considered the best place to farm focus quickly. The high numbers of enemies, higher levels of enemies, and speed at which the right build can kill them make it a perfect place for simple Focus farming.
A Saryn with a high-range, the medium-strength build is perfect for this. Saryn can infect a massive number of enemies with her first ability, use her third ability to have her allies help to spread the spores, then use her fourth ability to quickly nuke large groups.
Convergence Orbs will spawn in Elite Sanctuary Onslaught, which will multiply the amount of Focus earned by sixteen. Note, this is twice as much as a Convergence Orb will usually multiple Focus. Affinity Boosters will also impact the amount of Focus that you earn.
If you do not have access to Elite Sanctuary Onslaught yet, you can do high-level Exterminate missions with a stealth Warframe. Just equip Loki, Ash, or Ivara with a Focus Lens of your choice, use a dagger with the Covert Lethality mod, and make your way through the level killing enemies from stealth.
Stealth kills will grant a stealth kill affinity bonus to your earned experience, and also your Focus. This bonus can stack up to five times, for a 500% bonus, as long as you get your next stealth kill within 30 seconds of the previous kill. If you kill an alert enemy or are spotted, the bonus will reset.