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I have what is known as a "view camera". I bought this camera so I could use the "shift" and "tilt" abilities of the lens. I could use "shift" when I am photographing buildings to keep the uprights "upright". Also, when I am photographing a building with a reflective surface head on without seeing my reflection. I could use "tilt" when I wanted the plane of focus to be in a different place for special shots. For example I might want mountain flowers near my feet to be in focus as well as the mountain peak behind in the distance. I also might be photographing a fence or the wall of a building that I am not perpendicular to and I want the surface of the wall or the fence to be in good focus all along its length.
There is a very clear wiki article about the view camera and its movements on this page http://en.wikipedia.org/wiki/View_camera . You won't find a clearer explanation of it anywhere else. You will find that some people categorise "rise", "fall" and "shift" together under the category "shift" and "tilt" and "swing" under the category "tilt" and I will do the same. They are logically connected. For "shift" you are decentralising the lens but still keeping the plane of the lens and the plane of the film parallel to each other whether that be "rise", "fall" or "shift". For "tilt" you are changing the plane of the lens relative to that of the film such that they are no longer parallel to each other whether that be "swing" or "tilt".
If you are wondering how a view camera can achieve these things then you need to understand that the lens used in a view camera creates an image circle that is much bigger than the film plate and so there is spare room to tilt and shift the lens and still have the photographic plate covered - though there may be some vignetting at the corners and edges.
At this point you might be wondering if this is all worthwhile. Yes, you would like your uprights to be upright when you are photographing a building. But this can be fixed with image editing software. And as for tilt, then instead you could use a very small aperture so that both near and far things are in focus. But these workarounds have negative effects on your image.
You can expand out the top of your image and stretch it using an image editor. Even free image editors can allow you to do this (sometimes you need to download free add-ins to do this). It works well but then the top of your building might be stretched twice as wide and the detail inbetween has to be extrapolated by the software. It might not do a very good job of this and your image may not be good enough to be published. If the adjustment needed is only slight then this is perfectly viable, but in extreme cases it will probably degrade your image too much.
If you have a wide enough angle lens then you could take a photograph where your line of sight is perpendicular to the building and still have the top of the building in the frame. You would aim your shot at a point on the wall that is equivalent to your camera height. Then you would crop the bottom of the photo and just use the top. This would work but bear in mind that you have probably cropped 45% of your image. The megapixels of your sensor would have to be 14 megapixels or more to leave you with the 8 megapixels you need to get a publishable image. But most full frame sensors are 20 megapixels these days so that should not be a problem. Another problem is that wider angle lenses tend to have a lower resolution than their less wide cousins and that could impact on your image quality. Another is having on you the right lens to do that job. If it is not wide enough then you have not solved your problem and will have to combine it with image editing to give you the complete solution. If it is too wide an angle then you will be cropping more than 50% and again it might leave you wi