Is Jupiter the Karaka for focus? Being the lord of the house of the Celestial Archer?
I think the archer symbolism is more about aiming for the future with faith and optimism. A symbol of victory and auspiciousness.
I think Mars and Sun are stronger karakas of focus, especially Mars.
Mars is will-power and "won't-power". It forces us to use out logic in the present moment to solve intense problems. It also rules our brain and eyesight and with using our vision to focus. This would be crucial to archery and also hunting (as Mars rules fight or flight response as well, which jolts us into the present moment).
The Sun gives power, vitality, but most important for focus ... consistency - the steady motivation to see things through with out getting weighed down or distracted by burdens.
S.
I love using medical astrology for these kinds of questions because it gives us something concrete to work with. Impaired focus is a real issue that can cause people a lot of problems. It is readily observed in individuals (if you spend enough time with them) so there should be some clear/specific/concise ways of determining one's ability to focus (and there is)!
I put a lot of time into finding specific/reliable combinations for ADHD using Ernst's system of medical astrology. The short version is that Mars and the signs of Mars (Aries and Scorpio) rule the nervous system and thus have to be afflicted to impair focus. But, they need to be afflicted in specific ways. There are other things that give us a sense of direction, inspiration, or vision, but if that concrete ability to focus (as indicated by Mars and its signs) is afflicted, it can cause a lot of issues with turning that vision and direction into anything.
If you'd like a more detailed/specific explanation, maybe I'll write an article on it.