I prefer in a sunset to get a full range of tones. A pure oange sky doesn't really fill my boots. To do this, you'll need a short focal length and at least two stops of filter over the sky. I don't like a totally clear sky, but not a completely clouded one, 25%-75% cloud cover usually works. I tend to like to shoot them out to sea, so I am guaranteed a low sun onto the horizon. Clouds cannot be low, you want them high so the sun dips below them ideally.
St Lunaire - Brittany - long exp - Lee Little stopper, Lee 0.75HE and Lee 0.9SE grad. Blue and orange tones. Sun nice and low
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SFTPhotography, on Flickr
Fairlie, Ayrshire. Lee 0.9HE grad, full range of tones and a great reflections of the tones too
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SFTPhotography, on Flickr
Helensburgh - interesting pink sky that night - not a full range of tones. Lee 0.9SE and Lee 0.6SE
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SFTPhotography, on Flickr
The issue of low cloud manifests itself here. The sun burns the clouds, but doesn't come out as a defined round shape, you get the tones but not the actually sunset the above images have. Low clouds = bad/ Hight clouds = good.
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SFTPhotography, on Flickr
To be honest, its what happens before sunset (shooting to the side or away from the sun) or after sunset (shooting into the light source) that interests me more as the colours often get more interesting. Its perfect cityscape conditions just after sunset
10 mins after sunset - notice the gorgeous range of tones here. Lee 0.9SE and Lee 0.75HE
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SFTPhotography, on Flickr
Rough time of day shooting away from the light source. Lee 0.9SE and Lee 0.6SE
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SFTPhotography, on Flickr
No Grads here, but a lot of tweaking in RAW. Just after sunset in the city is really great. Sunset itself would have wrecked havoc IMHO. Had I been at sunset, these clouds would have blocked the suns passing. You want them higher than the level the sun is at when ever you shoot it.
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SFTPhotography, on Flickr