ND filters - how do they do it?

I'm familiar with the concept and use of ND filters but have one question: How do they improve the exposure balance? From what I understand they simply reduce the level of light entering the lens by a known amount regardless of the subject's brightness.

I realise that you use them to avoid overexposed skies in landscape photos, but how do they do this without rendering the darker foreground landscape underexposed? I could understand it with the graduated ND filters (placing the darkest area over the sky, much as a translucent sun visor along the top of a windscreen) but I've seen advice to use a standard non-graduated ND to get the same effect, which makes no sense at all to me!

A standard ND filter just reduces the amount of light, full stop. I can't see how this would help with anything except allowing longer exposures for effect, and even then you'd need a pretty dense one to help with waterfalls and the like.

So I'm mystified as well...

Have a read here, it pretty much explains it all.
Hope that helps you understanding it.

EDIT: This is another good read about the way the ND Filter works!

I think there's a little confusion here regarding plain ND filters & Graduated ND Filters - a normal ND filter has the same level of coating across the whole glass so just reduces light for the whol scene. An ND GRAD has a different coating top and bottom so helps to balance land & sky in high contrast scenes

http://www.bryanhansel.com/2006/using-a-nd-graduated-filter/

Simon

Yes, the OP was asking about how the non-graduated ND filter could help with the exposure balance, while SmutjeUK's links give information about the normal use of those filters. I believe that Dangermouse was simply given bad advice!

one place you could try an nd filter for exposure balance is when your light levels are too high for your desired aperture setting....

but unless you are shooting HDR and using the nd filter instead of bracketing your exposure, I see no way to use an nd in place of a grad nd.

now maybe the op has gotten thier POLARIZER mixed up with ND filter?
after all a polirizer can be used as an ND filter and will darken the sky and increase saturation (by reducing glare) AND still act as an nd filter.

Doesn't make sense to me either...


There are variable ND filters that can be used to "control" the exposure but they don't balance it.

Definitely sounds like duff advice.

Doesn't stand up to a basic logic test, let alone any scientific scrutiny.


Steve

I'm not going mad then!

I think it was that "How to take good photos" TV series that gave the advice, and it puzzled me hugely for obvious reasons. Given the series was aimed at complete beginners they really should have made it clearer - I wonder how many went out and bought an ND, then wondered why it didn't help?