The exif says the shot is taken at 28 mm, so not super wide.
Secondly I think the horizon looking out over the sea is always straight, even with the widest angle lens, the points at which we see the horizon are all at exactly the same distance from where we stand, and therefore by definition perfectly horizontal and straight.
Robert, I think pegelli's explanation makes sense - possibly, what you always thought it's the Earth curvature in other photographs was, in fact, some lens' barrel distortion?
Thanks both for your comments.
1. pegelli: I'm not sure about this. I'm sure that if you have a wide enough view you can see the curvature of the earth, though it is very slight. You do often need to be high up to experience properly.
2. Vieri: I wasn't thinking about other photos, rather (as above) what I'd experienced previously.
I'd not noticed this absolutely level and straight horizon in other photos of yours; I suspect that may be because you generally have something at the edge blocking one end of the horizon.
3. I was also thinking of
entasis. This is a feature of Greek architecture, where vertical columns bulge very slightly in the middle. This isn't apparent to the naked eye, but makes their proportions appear "more natural"; it's a sort of visual illusion. The Parthenon uses entasis in its columns.
There is a similar architectural "trick" on the horizontal. The
Library of Celsus at Ephesus was built on a very constricted site, and to make it look bigger it is slightly bowed in the middle, and the outer columns are slightly shorter than the central ones. The Parthenon also has this, as apparently does the radiator grille of older Rolls-Royces; alas, I'm not in a position to confirm this.
I'm not criticising your image, Vieri; as usual, it is technically perfect. But because of this, in relation to the horizon, it
appears to me to be not quite right. I'm rather suggesting that there should be a very slight curve to the horizon, concave downwards, to make it
appear straight.