When in doubt, go to the databook I say. >.>
Source:
https://mangahelpers.com/forum/showthread.php/45246-ShounenSuki-s-Databook-3-translations-(all-jutsu-4-characters-12-misc)
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Mind's Eye of the Kagura (神楽心眼, Kagura Shingan)
Ninjutsu, Hiden, No rank, Supplementary, All ranges
User: Karin
With the mind's eye, sense those yonder and survey a thousand riBecause Karin has excellent perception abilities, it is possible for her to use this enemy-searching technique across an extremely vast range.
By closing her eyes and opening her mind's eye, she can perceive abnormal chakra activity within a radius of dozens of kilometres. Also, if she has a specific chakra in mind, she can perceive its location and movements in minute detail.
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So there you have it folks, you're suppose to close your eyes. Also, what's interesting to point out is that Karin's can sense people beyond 10km. For some reason, I read it as the 10 km being her limit.
From the Wikia, "By focusing and opening the mind's eye, Karin is able to find and track an individual's chakra over a vast distance that
exceeds ten kilometres. "
That statement with the '...survey a thousand ri.' It could be an expression but ....
1 ri = 3927.27 m = 3.927 km
So 1000 ri = 3927 km = 2440.12 miles
But that may be going a bit too far. >.>
However for those who mentioned Byakugan, I have evidence that its limit also exceeds 10 km.
No. Referring to the fact people use senjutsu as an excuse to "evolve" their techniques simply by entering the mode (no training), and I don't mean make em more potent, I mean add completely new, often nonsensical effects. Stuff like say water spit no jutsu? Add senjutsu and it just becomes a bigger water spit, simple really. But no, people do stuff like suddenly its super potent acid that melts things worse than xenomorph blood, and that'd be from the milder end of things.
....that makes no sense.
However, I do want people's opinion on how Sage Mode would affect a doujutsu though, if at all. o.o