What is geometric distortion and what causes it?

First let’s recap on what geometric distortion is, identify the common types observed in many lenses, explain why they happen, and how they’re corrected.

Most lenses produce some geometric distortion, though in certain models this is negligible to the point of being invisible under regular observation. An ‘ideal’ lens would produce no distortion at all, and would translate any straight lines in the scene into identically straight lines in the image – a situation called ‘perfect perspective projection.’ Any deviation from this is classed as a geometric distortion.

A basic pinhole camera (which has no lens) produces no geometric distortion at all. However, as anyone who’s used one knows, it captures very little light by the nature of its design. Real cameras use lenses to capture more light and obtain sharper images.

Unfortunately, while many lenses do an incredible job of manipulating light, the laws of physics have their limits. By combining several lenses, and by using aspherical elements, engineers can minimize these aberrations, but there are trade-offs between image quality, cost, weight, zoom range, and so on – and in some conditions, it is tough to avoid geometric distortions.

There are three main types of geometric distortion:

• Barrel — typical of all wide-angle lenses,

• Pincushion — typical of zoom lenses at their long end, and

• Mustache — typical for large-range zoom lenses.

Also, it’s not unusual for compact lenses that go from wide to telephoto to demonstrate different types of distortion as you zoom in. So, you might see some barrel distortion at wide focal lengths which then shifts to pincushion distortion at the longer ones.

Modern lenses with their many elements can also exhibit more complex distortion than these three well-known types, and therefore resolving the issues requires higher-order distortion models to be created and applied.

Distortion – like the ‘barrel’ effect shown here – can make straight lines in an image appear curved, especially toward the edges of the frame.

How does DxO correct lens distortions?

With complex distortions requiring equally complex correction, this is not something that can be adjusted manually by the user; for truly accurate results, they must be calibrated using special equipment. That’s where DxO comes in.

Distortion depends firstly on the focal length (which can be extensive if it’s a zoom lens) and secondly on the focus distance. To create each lens’s model, DxO’s experts test both by photographing a reference image that features dots laid out on a highly precise grid that is perfectly flat and parallel to the camera sensor. To achieve this, many calibrations need to be made, particularly for large-range zoom lenses — a lengthy and demanding process.

However, once the extent of distortion is precisely calculated for each position in the field of view, the calibration data can be compiled into a correction file called a DxO Optics Module.

With the hard work done, it takes only seconds for photographers to download the specific DxO Optics Module required and apply corrections to their images. One of these Modules exists for almost every camera and lens combination produced in the last 20 years, and the required module is automatically identified from a photo’s metadata.

The degree of geometric distortion in a lens is evaluated using a reference image containing a dot plot laid out on a very precise grid.

With DxO, correction doesn’t have to come at a cost

However, there is a downside to addressing geometric distortion in software. After the distortion is characterized, the photo is warped to compensate for it, but this means the picture itself will no longer be rectangular. For example, images adjusted for pincushion distortion will have convex edges; and those corrected for barrel distortion will have concave edges.

Before exporting an image, it needs to be cropped in order to make it rectangular again, effectively shaving off the curved sides. This means that the original field of view becomes smaller, which is a significant problem when using wide-angle lenses – after all, maximizing field of view is why a wide-angle lens is chosen in the first place. Zoom lenses at their wide end are particularly affected by distortion, and after improvement, can require the harshest cropping.

At DxO, we can minimize the amount of cropping required after correction because:

• our models are more accurate,

• they correct higher-order distortion, and

• our calibration has very dense coverage of all focal lengths and focus distances, providing excellent corrections up to the image borders.

By contrast, competitors — whose corrections are not as good — need to crop in further.

All this means that, with DxO’s method, many images require less cropping after adjustment and so less of the captured view is lost.

What’s more, if the photographer’s creative intentions require it, DxO’s software allows the full cylindrical projection of the lens to be used without cropping, meaning that all pixels are available for manual trimming or editing. Our method therefore grants the most freedom possible in the process.

Better corrections than any other software — and even the camera itself

As you can see, rather than using generic profiles that may not accurately address geometric distortions at all focal lengths and distances, DxO’s precision measurements are always perfectly suited to the specific lens, focal length, and focus distance.

Therefore, you get higher quality and greater field of view compared to using other software, and often better results than the JPEGs generated by your camera itself.