Following up on a comment by Paul / Nick on distortion I wanted to add some photos but can't in the comments so hence fresh post.
Both pincushion and barrel distortion are caused by non liner magnification across the image. They are in the same effect but opposite.
Barrel is when the edge of the lens is weaker than the middle. This is the most common and normally seen on wide angle lenses. You often see the tops tall buildings leaning inwards and not vertical with the frame edge, this is barrel distortion.
Pincushion (all one word) is when the edge of the lens is stronger than the middle. this is normally from low end telephoto lenses and not as common these days.
What can you do about it ?
- Keep the image plane (film or sensor) parallel with the subject, however this is difficult if shooting "up" at a building. This evens the effect out rather than correct the problem is though. Paul did a good job this way in Doha.
- Tilt and shift lens, VERY expensive but these you can tilt the lens off the parallel so takes care of the above issue. Both effect are lesser with expensive lenses but not gone.
- Photoshop. You can correct both effect in photoshop but the part of the image is cropped off.
(Images are of Google, same image in multiple sites but think it's thanks to Wiki originally.)
p.s. Sorry for the nerdy post !
Cheers,
Richard B.
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