Distortion and Perspective – tilting buildings

Especially when using a wide-angle lens, or wide-angle setting, photographing tall buildings, trees, or other structures, causes the resulting photos to show the structures “falling in on themselves”. What causes that distortion, can it be avoided, and can it be fixed?
The situation is illustrated here in an exaggerated manner to show the problem. This is not distortion, although we may commonly call it “perspective distortion”. This is really how the world – optics, physics, and geometry – really work. When we look at the scene we seem not to see this distortion. Our brains take care of what comes through our eyes, instantly process the view, and present us with an “internal model of the world”. We know the buildings are vertical, and that is what we think we see.
Here is a view looking up at a building. Again there is much perspective distortion, but in this view it does not seem to bother us. When we look up, this is what we see also, and we accept this perspective as natural.
Why the difference? In the wide-angle view we think we are farther away then was actually the case. The camera covered a wider field of view than we would see. From an actual distance where we could take in that view with our eyes, we would not see the same perspective. So the image does not jive with our normal experience. Can that problem be avoided or corrected?
Yes, the perspective distortion can be avoided: Do not tilt the camera upwards. It is as simple as that. When the camera is horizontal, more specifically when the sensor plane is parallel with the vertical surfaces, the resulting photo will show all the vertical lines indeed parallel and vertical. That comes from the optics and geometry, but let’s not get into the science.
But, you say, I can’t get all of the building into the picture, and besides, that gets a lot of uninteresting foreground into the photo. So my first suggestion is rotate the camera to take vertical pictures. Now you can’t get in all that you want to show. So take several pictures. Here are a series of photos that were taken with the camera pretty much horizontal and rotated for vertical pictures.

Note that I took a series of photos that overlap the scene. The reason is so that I can let Windows Live Photo Gallery combine the photos into a panorama. Here it is. Now I have a wide-angle photo of this streetscape in Buckhead, Atlanta, Georgia, that looks pretty good. With some cropping and less of the monotonous foreground I get a nice photo.

Can Photo Gallery do such a nice stitching job with photos if I tilt the camera up a bit as I take the shots so that more of the buildings shows?
You may have seen claims that you can do that (see my post “Distortion and Perspective”). Unfortunately when you ask Photo Gallery to stich such a set it works out differently.
Here again is a set of five shots, this time with the camera tilted up to eliminate the foreground pavement.

Now hand them to the Panorama tool in Photo Gallery. Does it correct the perspective distortion? Take a look. The simple Panorama tool in Windows Live Photo Gallery does a fine job when you hold the camera to shoot horizontally. With photos that were taken aiming up, or down, the resulting stich will be more like part of a circular fish-eye image.
But there is a solution even for this set of photos. Photo Gallery, under the Create > More Tools command offers Create Image Composite… – if you have Microsoft Image Composite Editor installed. (It is a free download, you can get it here).
ICE, as I like to call it, is a very powerful and versatile stitching and perspective control program. It permits control over the projection of the resulting image – in this case I set it to Cylinder (Horizontal) because that matches the situation. The resulting image can be controlled in real time – you see exactly what you get as you drag the image with pointer. You can also crop the image right in ICE.
Here is a resulting picture from ICE with a horizontal view of over 100 degrees.

Notice that I have not completely eliminated the “tilting” – note the tall building at the left – it looks more natural this way.
So you see, perspective distortion is readily overcome. A bit of planning ahead and you can get the pictures you envision.
Good shooting to you!
For another post on perspective correction using ICE, see Perspective Correction using Image Composite Editor.

Please also visit my blog on computer tips: This ‘n That

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