each individual unit (ommatidium) had a calcite(!) lens and likely could focus light fairly well internally. but what do you do with that much data? who knows! i'm not a specialist but i'd hazard you could get good info about things like depth by comparisons across the eye locally.
my favorite reference is this: www.researchgate.net/publication/...
this site has some neat info on trilobites specifically. www.trilobites.info/eyes.htm look at the images here of Erbenochile erbeni! how beautiful. a lot of schizochroal eyes were mounted to give a wide field of view.