Does activating eyes of charming take a conscious action, like a standard action? Or is it automatic and passive, more like a gaze attack?
Perhaps you toggle it on and then it is passively aggressive until turned off again?
How do these work exactly?
I am in two minds about this.
The price of the item, seems to imply that it is use-activated:
It uses heighten spell to raise the DC to 16, which implies it is raised to a lvl 4 spell (DC 16 = 10 +4(lvl) +2(minimum casting ability 14)).
Backtracking the price 56000/(4*7)= 2000*spell level*caster level = use-activated.
Furthermore the text implies that it does not take an action. Partly because it states "merely by meeting a target's gaze", but that could also just imply that you cast a spell as normal.
I am more intrigued by the comment "(one target per turn)". If the item simply was 'cast charm person as a standard action', then there was no reason to include this line.
Despite these arguments, I must say that I find the item problematic if it can be used as a free action this way. Allowing lenses to be use-activated to cast spells just makes the spell better than having a quickened spell each turn. The range being based on sight only (subject to GM adjudication) also makes it better than the close range of charm person)
In effect the item should be priced as a Heighten(+3)Quickened(+4)Reach(+1) use-activated Charm Person(CL7) = 9*7*2000=126000gp.
First you put your fingers to your temple.
Then you squint really hard.
Think a happy thought.
Flap your elbows like a chicken, do the hokey pokey and turn yourself about and thats what it's all about (which takes a swift action).