I just answered a question on another forum regarding WA rectilinear lenses and distortion. If you've ever shot a group with a lens 24mm or wider (18mm or wider on an APS-C camera) then you've probably noticed that people away from the center of the frame are progressively more distorted. At the frame edges, they are noticeably stretched. Most shots where this happens are taken in landscape orientation, and so people at the left and right extremes of the frame are stretched horizontally. it's *very* unflattering to the subjects, making them look unnaturally... well... stocky.
Many folks don't realize that rectilinear lenses are *not* distortion-free. They just trade one form of distortion for another. Specifically, they keep straight lines straight at the cost of appearing to stretch objects in a direction radial to the center of the frame (distortion is worst in corners).
My reason for posting this today is to offer a post-process solution :-)
The following shots were taken with the Canon 5D at 24mm focal length. The left shot is the original, and the right shot is corrected.
The correction was performed by "remapping" the shot from rectilinear to equirectangular projection using panorama stitching software (PTGUI). I' essentially creating a 1-frame panorama. The output "pano" needs to be re-cropped since it's not rectangular anymore. Notice I've lost some image at top & bottom, but nothing at the sides. Look carefully at the people near the edges of the frame. There's quite a dramatic difference in appearance, isn't there? In the remapped image, people across the entire frame have exactly their correct proportions. No more "stocky edge-dwellers."
The first thing you probably noticed is that now the lines of the ceiling are noticeably curved. Notice, though, that the effect is not at all unpleasant, and things within the room still have a natural appearance.
When creating a very wide multi-shot panorama, it's also wise to select a mapping other than rectilinear. For landscape-oriented panoramas (wide, but not very tall) try cylindrical mapping.
That particular ceiling looks fine distorted but I wonder if that would be the case normally. Now you could have painted back the original ceiling and have the best of both worlds but that's a little more work but shouldn't be that difficult.
Never heard of PTGUI, is it a plugin for PhotoShop?
Good point! PTGUI is a panorama stitching program that got its start as a front end for Dr. Helmut Dersch's PanoTools. It's now a stand-alone program, and has it's own stitching engine, but can still use the PanoTools engine if desired. It's one of the most powerful panorama creators on the market, and is not free but is very inexpensive. It has a somewhat less elegant interface, and therefore a steeper learning curve than some other stitchers. It's output is second to none, though, and it has capabilities that far outstrip most basic pano stitchers.
That particular ceiling looks fine distorted but I wonder if that would be the case normally. Now you could have painted back the original ceiling and have the best of both worlds but that's a little more work but shouldn't be that difficult.
Though the ceiling looks fine distorted it is unlikely you could easily merge that with the rectilinear version. They would not match up properly. It could be done, but would require extra work.
Thank you for posting this. I don't shoot wide angle very often but I have been playing around with it more, just to stretch my legs a bit. This was also shot with a 5D @24mm. I played around with it to keep the walls and wainscoating level and square, much to the detriment of my daughter's hiney. Don't get me wrong she is quite wonderfully and squeezably chubby, but this shot does make her look alot wider than she is in real life. And someday when she's 16 she'll hate me for posting this on the internet.
It's just a fun shot, imperfect in many ways, but it did get me thinking more about distortion issues.
Thanks again. I will have to investigate this further.
That particular ceiling looks fine distorted but I wonder if that would be the case normally. Now you could have painted back the original ceiling and have the best of both worlds but that's a little more work but shouldn't be that difficult.
Though the ceiling looks fine distorted it is unlikely you could easily merge that with the rectilinear version. They would not match up properly. It could be done, but would require extra work.
Yep. Merging them can be done, but the room takes on a strange, jarring look. Then you notice why; things like the door frame at far right, compared to the line of the ceiling. Yikes. Things get out of whack. Actually, this room, with the strong crossing lines in the ceiling,m is one of the worst examples of this kind of correction. Where these kinds of "grids" don't exist in the image, you'd never notice the distortion.
But I paid a lot of money for photoshop already. Is there a way to do it with CS2?
I'm assuming you can just warp with Edit/Transform/Warp and choose FishEye.
Wow, this could be a fun tool.
No, not that I am aware of, at least not with any built-in functionality of PS. It's a complex transform, and it's *not* a fisheye transform. It's completely different, neither rectilinear nor fisheye.
How does PTGUI compare with PTLens for this type of distortion correction. I've used PTLens for years (currently running the latest licensed version) and it does an admirable job of distortion correction, particularly for WA shots of 12-18mm (APS-C; 16-24 35mm equiv).
How does PTGUI compare with PTLens for this type of distortion correction. I've used PTLens for years (currently running the latest licensed version) and it does an admirable job of distortion correction, particularly for WA shots of 12-18mm (APS-C; 16-24 35mm equiv).
I also have PTLens, and it does an outstanding job of correcting deviations from rectilinear performance. This type of correction it will not do. I'm actually intentionally making distortion of linear elements much worse, to correct the stretching inherent in rectilinear lenses.
How does PTGUI compare with PTLens for this type of distortion correction. I've used PTLens for years (currently running the latest licensed version) and it does an admirable job of distortion correction, particularly for WA shots of 12-18mm (APS-C; 16-24 35mm equiv).
I also have PTLens, and it does an outstanding job of correcting deviations from rectilinear performance. This type of correction it will not do. I'm actually intentionally making distortion of linear elements much worse, to correct the stretching inherent in rectilinear lenses.
Gotcha. I re-read the oringal post and see now you mention altering the image from rectilinear to equirectangular. Makes sense in that regard, since PTLens isn't a pano tool (no pun intended). ;-)
Never heard of PTGUI, is it a plugin for PhotoShop?
Good point! PTGUI is a panorama stitching program that got its start as a front end for Dr. Helmut Dersch's PanoTools. It's now a stand-alone program, and has it's own stitching engine, but can still use the PanoTools engine if desired. It's one of the most powerful panorama creators on the market, and is not free but is very inexpensive. It has a somewhat less elegant interface, and therefore a steeper learning curve than some other stitchers. It's output is second to none, though, and it has capabilities that far outstrip most basic pano stitchers.
Hugin is a good, free panorama stitching tool. Hugin requires a bit of learning. I prefer AutoPano Pro, but it costs a little bit of money. APP is fairly good at automating the stitching, and is easy to use.
how does this compare to "lens correction" in PS CS2?
or is it working completely differently?
"Lens Correction" in CS2 allows you to correct for (or exaggerate, for that matter) optical aberrations in a lens; barrel distortion and pincushioning. The "distortions" we are discussing here are not really optical distortions, they are "real" depictions of what happens when you map 3 dimensional objects to a flat surface (a "plane").
Any map of a city, for example, is highly accurate as far as scaling of edge-to-center goes, but when you try to map the entire world to a plane the scale of polar regions is vastly exaggerated; thus the various different sorts of "projections" cartographers have come up with. The mapping of the city is equivalent to a telephoto, the mapping of the earth is equivalent to a fisheye, more or less :-)
So it's not a matter of "lens correction"; there's nothing "wrong" with how an ultra wide angle lens sees.
Robert's used some good analogies. Just like there are multiple "right" ways to make maps, there are also multiple "right" ways to build a lens. Which way you use depends on the purpose of the lens. Rectilinear lenses dominate the market because they produce images that "look correct" to us. Wide angle lenses start to violate this "correct look" especially for some subjects. Correcting for this isn't really lens correction, it's "changing the mapping" and it's just like choosing a different type of map projection. In the original example, my final result is neither rectilinear nor fisheye, it's something different from either one.
FWIW, I'm sure there are other software packages out there that can remap a single image. I've only used PTGUI for this, though, and I'm not aware of other applications that can pull it off.
Great thread, Kirbic. My solution would have been to clone out the fat people on the edges and replace them with like, Kate Moss, or other skinny people - OR - replace the people in the middle with fatter people to make it less noticable and if necessary, photoshop a sign like "Weightwatchers Convention" in the pic.
...but I guess everyone has their own techniques and yours may be preferred by most. :)
