They actually can see blurry image and dim light if im not mistaken
Some said they use moonlight to navigate their way 

Light and vibration(air&ground)

I was just joking when I say their 8 eyes are for show. In daylight they can only see blurred images with other senses like sensory hair on their anatomy to assist but by night time the outer layer on it's eyes are retracted making them 10,000 times more sensitive and can even see with just the light from the stars which they use to navigate their way. And the most remarkable thing is they can see 360 degrees. Watch BBC Animal Planet on an episode named one sensitive scorpion, it contains a lot of information you'll find useful, me being a lazy reader watch a lot hahahaha.