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I used the Panasonic Lumix G Vario 14-42mm f/3.5 -5.6 Asph. / MEGA O.I.S. Lens to take a photo from my balcony.

Here's the original RAW (ORF).

The symptom I am experiencing is that different applications render this image differently, and I am not sure "who is right" and how to fix it.

Here's how Windows 10 Photos renders it: Windows 10 Photos

Looks good. But here's how it looks in Corel AfterShot 2:

Corel AfterShot 2

You can see some barrel distortion.

I opened this photo in PTGui, which allows me to control the lens parameters.

By default, PTGui considers my lens, 14mm, to be rectilinear, and renders the barrel distortion:

PTGui - rectilinear lens setting

Finally, when I adjust the lens parameter from rectilinear to Full Frame Fisheye, it again looks correct:

PTGui - full-frame fisheye lens setting

It appears that my original image has a significant amount of barrel distortion, why is this lens categorized as rectilinear in PTGui?

More generally, why is there even a drop-down for me to choose between rectilinear and fisheye, given that it's purely a function of focal length, which is already an input? I must be missing something.

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"...why is there even a drop-down for me to choose between rectilinear and fisheye, given that it's purely a function of focal length..." This is a false assumption. You can have a fisheye or rectilinear lens at many overlapping focal lengths. Canon, for example makes an 8-15mm fisheye lens that uses fisheye projection at all focal lengths. Canon also makes a rectilinear 11-24mm lens that is rectilinear at all focal lengths. Both those lenses project a large enough image circle for full frame cameras. They also make rectilinear 10-22mm and 10-18mm lenses for APS-C sized sensors. – Michael Clark Mar 30 at 4:21
up vote 12 down vote accepted

Unlike most other digital cameras, Panasonic micro four-thirds cameras record lens correction information (distortion and CA, iirc) into the EXIF information of their RAW files (and will bake in the correction if you shoot JPEG). Some applications recognize and apply this correction information, some don't. That's why you're seeing different results, depending on which application you use. This is distinct and separate from the typical "lens profile" schemes that some programs use.

It appears that my original image has a significant amount of barrel distortion, why is this lens categorized as rectilinear in PTGui?

Because PTGui has a list of lenses that it can identify as either a rectilinear or fisheye lens, and barrel distortion alone isn't enough to make a lens a fisheye. Rectilinear basically refers to the projection of the lens. Fisheye lenses are designed to give an equisolid (or in rare cases, stereographic) mapping, rather than a rectilinear one.

The 14-42 lens is recognized as rectilinear, because it is rectilinear. And the amount of distortion changes as you change the focal length. And when the correction is applied, you may not see any barrel distortion at all.

More generally, why is there even a drop-down for me to choose between rectilinear and fisheye, given that it's purely a function of focal length, which is already an input?

Whether a lens is rectilinear or fisheye is not purely a function of focal length, but of design. PTGui offers both types in a pulldown because most folks use PTGui to stitch 360x180 spherical panoramas, and fisheye lenses are often used for their much greater scene coverage than rectilinear lenses, so you can get away with shooting fewer images and spend less time stitching. The field of view of the lens, which is important when trying to stitch images, changes drastically between rectilinear and fisheye designs. A rectilinear ultrawide can typically still require 20-30 images to cover a sphere, while a fisheye can cover it in 4-8 images.

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3  
It is also worth adding that most programs display embedded JPEG preview and that one is corrected in camera already. – Euri Pinhollow Mar 30 at 5:14

More generally, why is there even a drop-down for me to choose between rectilinear and fisheye, given that it's purely a function of focal length, which is already an input? I must be missing something.

Your assumption is incorrect. Rectilinear and fisheye (or curvilinear) lenses are constructed differently, with different mixes of distortions. If someone hands you a 15mm lens, you can't tell whether it's rectilinear or fisheye unless you either ask or look at the images it produces.

It appears that my original image has a significant amount of barrel distortion, why is this lens categorized as rectilinear in PTGui?

Possibly just bad information, or it could be that that works better over more of the lens's zoom range. Considering the crop factor, the lens is equivalent to a 28-85mm lens on a full frame camera. It's really not in the range of what anyone would think of as a fisheye lens, which usually implies a very wide field of view and a lot of distortion.

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The image with barrel distorsion is the actual image recorded by the camera in raw format.

The corrected image on top of it is the jpeg that the camera pre-processed (with knowledge that that specific lens has that type of distorsion) and embedded into the raw file.

In short, raw processors which don't have a correction profile for your specific lens with show the distorsion by default and those that do (including the camera itself when generating jpegs) apply a specific correction. Of course, most raw processors will allow you to tweak or disable such corrections.

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A long range zoom lens can have slight fish-eye (barrel distortion ) at the shortest focal length, slight pin cushion at the longest focal length, and be perfectly rectilinear at an intermediate setting. I would expect this effect to be less on more expensive lenses, and anyway SLR lenses have a much smaller range of zoom ( magnification). This is one reason why professional photographers may use fixed focal length lenses, when zoom options are available. – Arif Burhan Mar 29 at 20:56
2  
@ArifBurhan Did you mean to leave this comment on this answer? I don't see the connection. – mattdm Mar 30 at 2:39

The Panasonic lens you use is a rectilinear zoom. So is their 7-14mm lens which is much wider. Even though 14mm is achieved by both lenses, their construction is that of a rectilinear lens. A fisheye lens can be 14mm or wider, as they often are, or longer. They are characterized by a curving of lines which do not pass through the center and are made differently.

Most lenses are not perfect and usually show some kind of distortion. The case of the 14-42mm lens is that it exhibits barrel distortion on the wide end. This makes some line curve but this is not the same distortion which would be produced by a fisheye lens.

It looks like your rendering of Windows 10 takes the distortion into account and removed it. There is no free lunch though as this results in some cropping of the image. Aftershot though seems to show what was actually captured. This may be a case of the camera using correction to produce the JPEG which is stored in the RAW file for preview purposes. You can confirm this simply by looking at the image on the rear LCD of your camera which shows the embedded JPEG.

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