The rules make a good job of explaining almost nothing about this, other than it is a condition.

You Discard Focus to change any number of Magnifying Glasses in a roll to Successes, does not cost clues.

Clues spent change 1 Magnifying Glass per Clue spent.

Agreed. Put another way, assume you roll 4 dice and roll 4 clues (spyglasses). Further, assume that you have one Clue token for your investigator and are Focused (have the focus condition).

If you spend your single Clue token, you can change one of the rolled 4 clues to a Success.

If you spend your focus condition, you can change all 4 to Successes.

It just depends on what you feel you may need or know you need to pass the test.

That's brilliant guys thanks, I understand now

Sadly after what happened to 380 lb Bob from accounting at the FFG corporate retreat a few years back, the "team building" part of the retreat became less about "trust your coworker" and more about "focus, focus and focus". Focus now appears in all of FFG's games it seems .... catching a 380 lb man as he leans back, not so much.