The naked eye
Definition 1 of 1
Pro Tip 1/3Visual Limitation
Refers to what can be seen without help, implying human vision has limits. 
Atoms are too small to be seen with the naked eye.
Pro Tip 2/3Common Phrase
A widely used term in various contexts to highlight natural human vision capabilities. 
The comet is visible with the naked eye tonight.
Pro Tip 3/3No Tools Needed
Stresses the idea that additional visual aids are unnecessary. 
The painting's detail is amazing; it's clear even with the naked eye.