"Interface evolves toward transparency. The one you have to devote the least conscious effort to, survives, prospers. This is true for interface hardware as well, so that the cranial jacks and brain inserts and bolts in the neck, all the transitional sci-fi hardware of the sci-fi cyborg, already looks slightly quaint. The real cyborg, the global organism, is so splendidly invasive that these things already seem medieval." -Gibson
That's where things tend to become more complicated. It's dependent on what you're writing and when it's occurring. It's a big leap from retinal displays and discrete leds to full on eyes. Neuroprosthetics especially the Bionic eye are a more complicated. There are biological and technical factors that play a part. Often this is ignored but you kind of can't really do that.