First off, Dave's blogging again. HOORAY! Second, he's coming into my RSS aggregator feed. YES. Third, there are so many reasons a horizon can get shifted. I've had the worst time switching from Canon to Nikon. The reference points in the viewfinder are different, and I've had slews of images come with a left-side tilt I've had to correct. Everything looks straight - I put it on a tripod and it's straight. I've come to the conclusion that at certain points of the day, I'm just cockeyed. I have no compunctions about altering the tilt.

Sigma makes a 12-24 'rectilinear' lens. Wonderful for architecture shoots. BUT. Get something too close to a corner, and what is square becomes an odd trapezoid. Architectural features aren't so bad, but if you catch a chair ... it'll turn into something from Alice in Wonderland.

So you're always making compromises to get a 'believable' shot. And, when you pause for a second, a wave of appreciation for what a miracle the human eyeball is.