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Paul
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Hi Richard,
I'm sure Martin and John will be along with more details soon, but here's my take on your questions....
The camera exposure does enhance the effect - the exposures tend to be a few seconds at moderate sensitivity - in fact a lot of mine are shot at ISO 100 - my Dimage A2 bridge camera is noisy at anything higher - but the NLCs are very visible to the naked eye. Estimating magnitudes is difficult with such big objects, but the brightest stars in the area such as Capella, Mirphak and Algol will generally shine through NLCs whereas dimmer stars will generally not be so visible, so you could say that perhaps the clouds are equivalent to 2nd mag stars on average.
NLCs can last all night, indeed it is usually the oncoming dawn that brings displays to the end.
No, it doesn't get completely dark at night. Here, just under 55deg N there is still plenty of light in the sky at 0124 BST which is the time that the Sun is furthest below the horizon. From mid-May to the end of July one can pretty much forget about looking for faint galaxies and the like - the sky is simply too bright. Some, like us here, go for the NLCs, some observe double stars and planets.
Hope that helps!
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