C&C Is this perspective distortion?

nandystam

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This is more for 'comments' rather than critique per se, but based on the image below, you can see the buildings, particularly Taipei 101 is at angle rather than being straight.

This was shot with the 12-40 Pro @ 12mm. I'm pretty sure my tripod was level on the horizontal axis, I don't believe it was level on the vertical axis.

Is this an example perspective distortion? Is it a characteristic of the focal length, the lens, camera not level on both axis, or a combination of those elements?

Does focal length affect this kind of distortion?

Thanks 🙂

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It would be difficult to point the camera up much with a long focal length so it is not a simple question. The problem will be better explained by someone else, I am sure, but most lenses are designed to translate a flat, parallel field (eg a wall) into a flat image on the sensor. Once your subject is not a flat, parallel one it creates distortion. There may be another name for it. Simple look at a square object with the camera pointing directly at it and it will look square. Start to point up a little or to one side and it will start to distort.
 

Joris

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This is the result of pointing the camera towards the top of a building. Correcting this at a certain point, will result in buildings that seem wider at the top than at the base, which is much more disturbing to me. It is the very same "distortion" that shows when you point your camera to the end of a road : convergence at the horizon. The human eye however doesn't see that as distorted.
 
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archaeopteryx

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Is this an example perspective distortion? Is it a characteristic of the focal length, the lens, camera not level on both axis, or a combination of those elements? Does focal length affect this kind of distortion?
Yes. Yes, kind of, depends on the camera, and yes. Yes.

The usual physical tool for managing what you're asking about is front rise, most commonly implemented on mirrorless cameras by using a tilt-shift adapter that rotates 90 degrees to change the shift to rise and fall. This is a subset of the movements offered by view cameras and many, many, many descriptions of what view cameras do are online. The best way to answer your questions is to do the trigonometry yourself until it's familiar. I think that should help with what you're trying to disentangle, which seems mainly that the amount of perspective distortion in a rectilinear projection follows the arctangent of the lens's angle of view. Keith has some pieces of one corollary, which is a wider angle of view is more likely to be used in ways where distortion is obviously visible.

The usual software tool for managing projections is Hugin. There are numerous threads about Hugin here and, whilst they focus on stitching and defishing, single image rectilinear is fine too. What Joris is referring seems likely to be residual error following the keystone warp transforms most image editors support. In general, the most complete discussions of projection distortions are found in mapping. These are also abundantly available and can be pursued as you wish but the nutshell version is choosing a projection allows selection among a choice of distortions. However, it is (rather by definition) not possible to reduce three dimensions to two without incurring some amount of distortion somewhere.
 

bassman

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As Joris says, what you see is the result of tilting the camera up and has nothing to do with the Len FL or otherwise. The simple solution is to stictly level the camera when taking such pictures, which may require a wider FL than you would otherwise use to capture then entire scene.

If you can see a real horizon - the sea, or flat land in the distance - then it should be in the middle of your frame vertically when the camera is level, and of course it should not be tilted side-to-side either. If you get this close to correct, then perspective correction processing such as LR offers will make it appear perfect and small anomalies will usually not be noticeable.
 

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