Perspective distortion is a result of photographing a scene or object with a lens that has a focal length or field of view which varies drastically from our normal vision. It is actually a function of distance from the camera to the subject and causes the subject to appear unnatural. In this photo, the house was photographed with a wide angle lens at close range. Because the lower portion of the house is much closer than the roof, the roof area appears much smaller and this causes the ‘cave-in’ effect shown on the right.
In architectural and interior photography, it is normally desirable to have vertical lines appear actually vertical, instead of angled. While there are specialized lenses to accomplish this, the more common method is to use distortion controls in Photoshop to achieve the desired result. We actually ‘stretch’ the top of the photo to straighten out the verticals. Because this results in some cropping of the image (we lose some off the edge), it is normal to shoot the scene wider than is necessary to allow for this.
Correcting perspective distortion in Photoshop does cause some degradation of the final image, but it is at the pixel level and not visible to the viewer.
The above explanation is ‘extension distortion’. The other type is called ‘compression distortion’, which occurs when using a telephoto lens. A common example of this is watching a baseball game on television; the pitcher and batter look like they are side by side. The distance between them is compressed. This type of perspective distortion is not easily corrected.