Noticeable perspectival distortion occurs now and then in my photos and I recently decided I should do something about it. This prompted me to think a little about when distortion matters and when it doesn't.
Here's a simple example of where it matters. This is a shot of my wife and daughter, standing outside our front door.
Notice that the edges of the door frame are not parallel to the edges of the image frame. It's especially noticeable on the right side of the photo.
In theory, some amount of perspectival "distortion" is inevitable. It is in fact perspective plain and simple. Wherever I put my camera, I'm closer to certain parts of the scene than to others. In this photo, the camera is closer to the top of the door than to the bottom.
The problem isn't perspective in itself, but the fact that it's noticeable. I think it's noticeable in part because there are actually two "perspectives" here. There is the perspective of the camera inside the photo, and there is the perspective of the viewer of the photo. The viewer of the photo sees a nice rectangular/parallelogram picture frame, and inside that frame, he sees a door that he knows is rectangular, too. But the presence of the photo frame certainly makes it appear that the door frame is distorted.
There's just a little more to it than that. It's not just that there's a kind of perspectival conflict between the frame of the photo and the frame of the door. It's also that our brains aren't prepared to accommodate this conflict. The distortion visible in the door frame is actually affecting the way Joan and Catherine look, too, but we don't notice it. We naturally make adjustments for faces and bodies. We notice perspective in shots of people mainly when the photographer gets too close to the subject's face, causing the nose to appear unnaturally large (a bad thing), or when the photographer shoots looking up at the subject from a perspective near the floor, which might be done to make a person appear taller, or to make a pretty woman's legs to appear longer (both of which can be good things). I would point out also that, if this were a photo of a straight highway in the desert, although the sides of the road would appear to get closer to one another as the road receded into the distance, it would not trouble us, because that is in fact what our minds see, too, when we look ahead at roads. But when look up at a tall building or even when we look at our own front door from eight feet away, our brains do not register distortion. Our eyes see it, but our brains adjust. The photo however shows the truth, and since our brains are used to fixing things for us, we find the distortion in a photo like this objectionable.
Anyway, the problem is most commonly noticed when the photo includes architectural elements that clearly involve parallel lines, as in this photo. I also had the problem in spades last year when I photographed paintings by a local artist.
What to do about it? Well, you could try changing your perspective. If you can get farther away from the subject, the effect of your perspective will be less noticeable. Unfortunately, when I was taking this photo, my back was up against the wall of our kitchen and I could not go back any further.
So my only option was to correct the distortion in post-processing. Unfortunately, neither of the programs I do most of my post-processing in - Adobe Photoshop Lightroom or Light Crafts LightZone - has a perspective correction feature. So I did it using the great (and free) open-source program, The Gimp. I used the transform perspective tool, which is a bit crude but very easy to use. The improvement is subtle, but real. The frame of the door is now parallel to the frame of the photo, which is the way our mind wants to "see" it.