[Aoi Kurenai]

They wear different kinds of helmets.

There are close up helmets, which offer, at most, a few pinholes of visibility, stunt helmets, which don't offer a ton of visibility, but lets you have an idea of what's around you and then there are this is a stupid dangerous stunt so let's just put big obvious holes in the helmet...helmets, when they wear those, there's a lot of creative editing going on to make sure you don't see it a lot.

They're not always enough though. One of the suit actors on Kamen Rider Dragon Knight knocked himself out when he ran into a wall because he couldn't see out of the helmet. A lot of it is just down to muscle memory and doing a lot of rehearsal with these stunts because you're almost never going to have enough visibility with a Rider helmet on your head.

It's easier to work with Sentai helmets because they're open faced and don't require a lot of techniques to hide holes under mask details.

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Really, my wonder is the how the Mechas fight? Can they see in those warm and cumbersome costumes while prancing around a hot and packed studio?

They can see, but it's still pretty difficult for them. The real issue with mecha suits is just how hot they get in absolutely no time at all. Because of the issues with the suits, they can only ever really film about 30 seconds of footage at a time - stage hands actually have to rush in and take parts of the suit off so that the actor can breath and cool down in between shots.