OTH, he may have used that along with Photoshop and stitched together a rather large panorama and then edited it all together. But I'm with SW on this. The alternative is a lot of work.
Well, its just angling really, I don't see any special trick, get the right angle on the horizon and you can do it at a standard wide, like 25-35mm. The trick is finding a sky to shoot, I have to travel about 15 miles south before I can get naked land like that...
It's just a wide angle shot that is mostly sky. The wide angle also has a perspective effect on the clouds that adds to the feeling of space in the sky. Looks like it's also a slow shutter speed because the clouds are blurred a bit. But I'm with the others - nothing really special has been done here. It's mostly the angle and the wide lens.
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