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Old May 25, 2009, 8:35 AM   #1
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Default Why do straight lines look bent?

This may seem like a dumb question. My daughter's been trying to teach me how to use a dSLR, and something happened while she was teaching me that's got me curious. I've had my fill of asking my daughter questions, so I thought I'd ask you.

We were taking a picture of a building and were standing between some columns in the building opposite (she reckoned they might make the main subject more interesting). When we got home and looked at the pictures, the bottom of the columns looked straight, but as you looked up them they curved inwards. What's the reason for this? I think I understand the physics (I was a physics teacher at one stage), but I'm still surprised by how marked the curvature was.

We were using a Nikon 18-200 lens if that's of any use.
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Old May 25, 2009, 8:53 AM   #2
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Originally Posted by JimBowen1 View Post
What's the reason for this? I think I understand the physics (I was a physics teacher at one stage), but I'm still surprised by how marked the curvature was...
Image-modifying software needs to catch up with the unavoidable defects of optical physics. Lens manufacturers do their best, but it's not quite enough, yet. Cost constraints are rather important! Thye've done it for Hubble's defects!

Last edited by Alan T; May 25, 2009 at 8:56 AM.
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Old May 25, 2009, 9:16 AM   #3
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It sounds like you were using a wide angle when you took the photo. The base of the columns was closer to you, and the top of the columns was further away. Things that are further away take up less of the perspective, and so look smaller. This isn't as noticeable at longer focal lengths, but at short focal lengths (wider angles of view) it happens a lot when one object (or portion of an object) is closer to the camera than another. This is especially important in portrait photography, because if you're too close to the subject, you can make the subject's nose look a lot bigger than it should.

You can easily try that out, just so you are aware of it and know how to avoid it.
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Old May 25, 2009, 9:24 PM   #4
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Several things are probably happening:

1. Perspective alteration (not actually "distortion") cause by the lower portions of the columns being closer than the upper portions by a larger percentage than your brain thinks is "normal". This happens more when using a wide angle lens, but depending on print size and viewing distance happens when any lens to a degree.

2. When the image plane (sensor) is not parallel to the vertical face of the building, you will get a type of "distortion" where the columns appear to lean in. This doesn't cause them to curve; they remain straight. This is why view cameras have tilting movements to allow you to keep the film plane parallel to the building. Some software can correct this.

3. Barrel distortion resulting from optical compromises in the lens design. Today's expectation that zoom lenses have an exceptionally wide zoom range and be extremely small has led manufacturers to put less "effort" into correcting rectilinear (barrel or pincushion) distortion in exchange for getting the lens smaller with a wider zoom range. This can also be corrected to a great degree in software.

Last edited by dwig; May 27, 2009 at 9:32 PM.
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Old May 26, 2009, 6:05 PM   #5
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With the camera tilted upwards, the perspective will render tall objects as leaning in at the top, as has been previously mentioned.

However, with an almost perfect lens(with exceptional design, for example the Nikkor AF-S 16-85mm f/3-5-5.6G ED VR DX has almost no barrel or pincusion distortion at 18mm), the straight lines on the columns would still be rendered as straight lines. In this case dwig's point 3 is correct, that your lens is exibiting barrel distortion.

In fact, the Nikon 18-200mm lens exhibits LARGE amounts of barrel distortion (4.1%) at 18mm.
This is one of the disadvantage of a "do it all" lens with such a large range of focal lengths.
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Old May 27, 2009, 8:19 AM   #6
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JimB, could you post a copy of the photo?

I agree that there should be no curving if the column are straight unless there is a problem with the lens - and then there should be curving in horizontal lines near the edge as well. Some columns taper which could look curved.
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