Last year I had a chance to see an exhibition of photographs prints that had been taken in and around the City of Perth. We might not necessarily think of our town as an exotic place for photos...but the vision of the photographers was acute enough to find dozens of unique and memorable sights. Some of these were historical but a complete mystery to me - I'm glad there were captions.
One point of amazement was how many of Perth's buildings are built with walls that slope inwards from the bottom. I think that the designers must have been trained on Mayan ziggurats...or maybe they just ran out of materials as they went higher and had to make the buildings narrower.
Of course, it may have been to counteract the forces of nature, as I noted in about 20% of the entries that the horizon sloped down from right to left or vice versa. Perhaps the buildings were only reacting to the shifting gravity. Amazing how well the ocean stayed on some prints, when it should have dripped off to the side...
Well, snide comments aside, it did point out the basic optical fact that when you point your camera up or down get in all of the top or bottom of a building, the structure will seem to taper away from you. Your eye sees it as well, but when your brain processes the information it lies to you and tells you the building walls are straight up and down, because it knows they probably are. The camera is a much more literal observer.
Pro shooters have known for years that they need to keep the camera level get the building walls vertical. Even if it is just the back of the camera, it has to be straight up and down to avoid distortion of the image. This means that the person taking the picture has to be far enough away or use a wide enough angle on the lens to get the part of the building they want on the sensor...and in some cases it means that they have to half-way up a building nearby their target to satisfy all the criteria.
Old timers like me who used view and field cameras with rising and falling fronts and lenses with a large coverage could sometimes fiddle this to perfection from ground level . Modern digital shooters with tilt/shift lenses (Nikon, Canon, Samyang) can also do it.
If pressed to it we can also go into many of the digital editing programs like Photoshop and Photoshop Elements and pick a portion of the filters that will correct this distortion. It is amazing to see how far a bad situation can be recovered, though not getting that far away from the correct shooting makes the repair a lot easier.
At this point I will admit that there are any number of photographs that have wildly converging verticals and have benefited from the look - they have been made with a deliberate use of the forced perspective to make an artistic statement. It is only the ones that have tried to be realistic and failed that we address with the special lenses or programs.
As for the tilting horizon? Well, Cullmann, Manfrotto, and Promaster all make extremely neat little bubble levels that will allow you yo shoot with the camera level to the horizon. Lots of camera s have artificial horizon indicators built into the EVF or LCD display. If you use them they work.
If your image passes through a computer between your camera and the printer...then take the time to correct that horizon in the editing program. Even if you have to measure the screen with a wooden school ruler you can still make your vision conform to the way gravity actually works.