Hi there scottishmaximus,
I weld for a living, so I thought I'd offer my 2 cents. (I used to do landscaping, enjoyed it even, that's why I look at this board from time to time)
The last time I went to my eye doctor, he told me that he could see that I'd been 'flashed' a few times. I told him I'm very careful about that and I couldn't even remember the last time it had happened. He then asked what kind of helmet I used, and I told him it was an automatic one. He suggested I check the spec's of my helmet, because a study that he had been involved in with a university where he teaches part time, had found that some of the less expensive helmets had a darkening rate of less than 1/20,000th of a second.
He continued to explain that anything less than that rate allowed the user's eyes to be subjected to dangerous flashes of light from the welding arc before the lens darkened. It can't be detected by the user as it is happening, but over a period of time it will damage the eyes just as badly as if you had 'flashed' yourself.
Probably not as big of a concern to someone who only welds occasionally, but still, you only have one set of eyes!
Hope this helps.