What is the necessary gap for the weaker not to be able to see the stronger?

[withheldforprivacy]

With 1.33x a gap, 3x Kaioken Goku wasn't totally invisible to Vegeta. Therefore, i think...
-1.5x: The weaker cannot see the stronger at all.
-1.75x: The weaker cannot see the stronger, not even with 3 eyes (ex. Ten vs Unweighted Goku)
-2x: The weaker cannot see the stronger, not even with 4 eyes (ex. Guldo vs Krillin/Gohan)

 

[ahill1]

Piccolo couldn't see 3rd form Freeza

 

[Captain Cadaver]

Like any other gap, whatever the plot demands it be at that time.

 

[Let's Go Fearless!]

Numbers doesn't determine the power.

 

[DBZAOTA482]

2x or close to it.