Correcting ultra-wide angle images

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Digital Nigel Forum Pro • Posts: 13,579
Correcting ultra-wide angle images
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There has been a recent discussion about correcting the apparent distortion in ultra-wide angle lens images in another thread . That discussion was well off topic for that thread, and I think it deserves a thread of its own. This is it.

My understanding is that the human eye is roughly equivalent to about a 43mm FF lens. But our cameras can take much wider angle images than that, and that's suddenly become much more common, now that many smartphones have UWA cameras. So, we are seeing much wider images on screen or paper than our eyes can see directly. The objects at the edges are viewed obliquely by the wide angle lens, whereas we would would have to turn our heads and eyes to see those objects directly, and we would not be seeing them obliquely.

It turns out that with lenses wider than about 28mm, this oblique view causes an apparent distortion at the sides of landscape images that cause objects to be stretched horizontally. With group photos, it makes the people at the edges look fat. We've all seen unflattering pictures like that, and some of us are guilty of taking them. Here's an uncorrected example of mine, from the Louvre:

The people at the edges look wider (ie, fatter) than they actually are. This is what an OOC JPEG would look like, and it's what most post-processing software would produce.

Most people think this is a camera fault or lens distortion, but it is not: the camera is accurately recording what the lens saw. So, perversely, we need to distort the accurate, but deformed-looking, image to look more pleasing to the human eye.

It would be nice if there was a single, reliable, magic bullet solution to this problem, but of course there is not: if there were, our cameras and post-processing software would all already be applying it. So, we have a choice of solutions, each its own pros and cons.

Here's the above picture with one possible correction applied, which compresses the stretched wide edges.

The people at the edges now have the right shape.

I used DxO ViewPoint to do this, using one of its two available methods. In this case, the compression was applied along the diagonal axes, which produces a pleasing image, but at the cost of bending straight lines (look at the floor slabs). I think this is the right method for this image, but in other cases, the bent lines give the impression of uncorrected barrel distortion.

So there's another method, with the compression applied only along the long axis:

The straight lines are now straight, but the corners are stretched

This method generally works better with buildings with lots of straight lines. I therefore use it by default, but occasionally switch to the diagonal method, particularly if a subject of interest is in the corners.

Here's an example of that, from an old railway depot in Bulgaria. First, no correction:

Volume deformation not corrected. This is what OOC JPEGs would look like, and the front wheel looks badly squashed.

Horizontal correction applied. It works fairly well, but the wheel in the corner is still slightly stretched and distorted. All straight lines remain straight.

Diagonal correction applied, at the default level (150). The wheels now look better (if still slightly distorted), but straight lines are bent.

I then pushed this correction slider up to the maximum:

The wheels are now undistorted, but the loco boiler appears to be bulging, and the rails are bent.

I think, in this case, I'd prefer the middle version, with the diagonal correction at its default setting.

But by default, I apply the long-axis correction automatically to every image I process in DxO PhotoLab. It has no effect on longer lens pictures, but makes wider angle pictures look much better.

Here's the raw if you'd like to play with this yourself, perhaps by making manual transformations or using other automatic correction software:

https://www.dropbox.com/s/ls2s2wn6z788zds/Asenovo%20steam%20reserve%2C%20Bulgaria%20A7R01311%20.ARW?dl=0

The Louvre image is https://www.dropbox.com/s/ssmmppgypzxvn7b/Louvre%20Museum%2C%20Paris%20A7R03426%20.ARW?dl=0

Finally, here's another, older, example of mine, shot in a restored Ottoman house in Gjirokastra, Bulgaria (in Ottoman times, only men would sit in the room, with women hidden behind the screen behind me):

No volume correction — the people at the sides are distorted. This is what OOC JPEGs and most post-processing software would produce

Horizontal correction applied

Diagonal correction applied. I think this is the best result.

___

How do other people handle their ultra-wide angle shots?  I wonder if some smartphones apply this correction automatically?

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