How restrictive are visors to your vision?

Generally, when someone wears a helmet in cartoons, they shout, "Hey! Who turned the lights off...?" then proceed to bump into the nearest wall. I can't imagine anyone wearing something that blinding.

So, with the various visors, how well can you see in the worst cases, and in the common cases?

Since the vision slits are close to your eyes, you can see better than you might think.
Also, it's possible visors were raised in battle, mainly being lowered in lance charges or for arrows. (This would explain the lack of straps on period visors.)

Well, I made my own great helm years ago, with no training, and I have nearly 180 degree peripheral vision while wearing it. I can easily see a man from head to toe until he is within 5 or 6 feet of me, at which point it's pretty much head to knees without moving my head. As Jojo says, the closer to your eyes the slits are, the better your vision.

Matthew

Worst helmet for vision that I ever wore was a great helm, not mine, and not padded to fit my head. Sat poorly. Eyeslot was a long way from my eyes, and in the wrong position. With my opponent in sword range, I could see him from neck to waist. Since he had a glaive, and I couldn't see either end of the weapon, this was sub-optimal.

A good well-fitted helmet with narrow eyeslot should let you see your opponent head to hips. Note that you can see through the "breathing" holes, too. On a typical pigface, these will let you see your feet. Some pigfaces have a "mouth" slot too, which would be good for foot vision.

Reduced vision, greatly reduced hearing, and lack of fresh air will all motivate opening the visor, or wearing a more open helmet. Would the future Henry V have been hit in the face by an arrow if there weren't good reasons to have one's visor open?

But, with a good helmet, not too bad, especially against a single opponent. The reduced vision (and hearing) will be a bigger handicap with multiple opponents (but then, you might benefit from the extra protection more often).

One has to wonder if the percentage of people with vision problems was similar to today's population. Without contacts, eyeglasses, or Lasik, could 8%-15% of those wearing armor not see better than 20/200 anyway?

Eyeglasses were a medieval invention. (13th century)

A C.J. Chivers article "The Weaknesses of Taliban Marksmanship" describes the high rate of vision problems amongst the Afghans today (aggravated by the high altitude, which causes high UV, which causes cataracts). I think I remember reading that "longhouse" cultures in NE North America had a high rate of eye problems due to spending all winter in smoky dwellings. I'd be interested to know what medieval medical authorities had to say about vision problems.

Jojo Zerach wrote:

Eyeglasses were a medieval invention. (13th century)

But earpieces, let alone glasses tough enough to wear into battle, came much later. Medieval glasses were for reading.

We do have the case of King John (the Blind) of Bohemia being led to his death at Crecy at the age of 50, having lost his sight 10 years earlier.

One thing to note is that many modern helmets are made with the visor too far from the eyes, resulting in poor vision, which also results in many modern myths about the visibility of helmets.

I do have glasses in the medium short sighted range and if I take them off, I still can still hit someone on specific spots of his body and ride a bicycle. Our requirement for glasses has a lot to do with the necessity to long range read and identify symbols and in warfare we have optically very demanding long range shooting.
Other than protheses like glasses, you need to keep in mind that we can as well influence our vision by intentionally squeezing muscles around the eyeball that through external pressure alter the shape and vision range characteristics. If you need more demanding long distance seeing only for short durations of time training of this capability might suffice to solve the problem.
It's my guess from some very detailed artworks that even in the dark Early Medieval times they possibly knew something like "liquid lenses" for improved vision. You need a small hole and a drop of oil or water to create such a lense that helps to see smaller structures, but with distorted colours. Using a large mirror (can be metal or a reflecting stone) will help with candela requirements.

Is there research how problems with optical requirements were solved in prior ages or are we just doing guesswork based on our assumptions about the past?