How do longer exposures affect saturation?

I read an entry on the Singh-Ray blog about a photographer, Kah Kit Yoong, who sandwiched GND filters to achieve the color in the sky according to what he visualized. With only a single filter, the sky was a little washed out.

I frequently run into this problem when I am photographing sunsets, specifically with magenta/purple skies (just like in his situation). I follow the expose-to-the-right mantra so I expect my skies to look washed out on the LCD but even in post, I have a hard time trying to get the colors to appear as I saw them.

Does anyone know why Kah's technique in general works like that? Why does sandwiching filters bring out more color in the sky? I get the nagging feeling that it's simple but I'm not making the connection.


The part of his entry that I'm referring to is towards the bottom. Search for "Tasmania is well known" and read until the end.

http://singhray.blogspot.com/2008/05/photographing-australias-premier.html

Thanks

Drew, I am sure that if Kah sees this post, he is liable to answer direct as he is VERY active on this site!

Oh, I didn't know that. That's great to hear! I posted a comment awhile back in the blog entry above and didn't see any responses so I decided to venture over here to the nature experts.

Well technically it's not the sandwiching of filters that deepens the color. It's just that in that instance it underexposed the sky further than the typical exposure hence more color.

According to something I read from Darwin Wigget (and if I'm worng hopefully he'll correct me), longer exposure let colors "build up" in areas where color is present but very faint. This is especially noticeable in skies during sunrise and sunset. The building of colors isn't dramatic like increasing saturation in Photoshop, its just reveals them a little more than may be noticeable to the unaided eye. I have used multiple GNDs to achieve this phenomena myself in my avatar and recently in this photo here.

http://farm4.static.flickr.com/3193/2606707518_82b23893d2_o.jpg

Doug, that's been my experience, too. It's why those 30 second images of aurora borealis look more colorful than when we're just looking with the naked eye.

You've quoted him correctly Doug. I saw his comment also on the singh-ray blog:

http://singhray.blogspot.com/2008/03/terrific-triple-threat.html

....and the 5-stop ND filter creates long exposure times which has two benefits; first it shows painterly blur in any object moving in the frame (swirling tides, running water, blowing clouds) and second it builds up colour intensity especially at sunrise and sunset by allowing colour to wash over the land and sky over time.
...
The addition of the 5-stop solid ND filter adds more richness to the colour as the sunrise light moved across the clouds.
...
The 5-stop ND does record changing light over time giving more colour in the scene.

Hi Drew, I didn't see your comment on the blog. The answer is that adding the 2 soft to the 3 hard darkened the sky a bit more - that's a total of 5 stops across the top of the sky. It had nothing to do with exposure time although if you increase it enough coloured clouds streaking across the sky creates a nice effect.