I don’t remember the exact bit it’s very forgiving. If you’re using the illumination then as long as you keep both eyes open it’s not really that important. With prisms it’s not as easy as red dots but I’ve put the front cover on my prisms before and just used both eyes open. That way the distortion and weird light transfer doesn’t mess with me. Unfortunately you still have the eye box but as long as your optic isn’t mounted far away it’s still usable. In my opinion 1x prisms are pointless. Sure if you’re battery dies you still have a reticle but most optics of decent quality have motion activated on and off, and they also have solar backup. They make fiber wire red dots now that have a reticle that can be seen without illumination. They aren’t as good as prism reticles but they will do the job. As a civilian there is no situation in which you would need at etched reticle. If your battery dies and you don’t have solar backup up, extra batteries weigh nothing and take up almost no space. 1x prisms seem to be a solution no one needed. Now magnified prisms are awesome.