Dealing with geometric distortion

I admire Mark Power’s photography. He uses a large format camera to capture human-less urban landscapes where still subjects; buildings, industrial sites, houses, trees appear perfectly perpendicular, and the ground occupies less than 20% of height. With a large format camera, pictures turn up how we see them in real life, with minimal distortion.

It is important to understand that not all distortions should be or can be eliminated. Vanishing lines originating from perspective vision give us a useful sense of depth.

However wide angle cameras exaggerates perspective distortion when vertical axis is even slightly tilted. This typically happens when you tilt the camera along its horizontal axis in order to include the top of a building or reduce the ground.

In this article I will explain an editing method to get around geometric distortion inherent in full frame cameras with wide angle lenses.

For the record I use DxO PhotoLab raw image editing tool, but I am sure there are similar tools available for Lightroom users.

I use the geometric correction feature to fix distortion. After the correction the image frame will look like a Trapezoid. The correction algorithm rotates the image in the opposite direction, straightening the distorted image at the expense of losing some real estate from the bottom of it.

This is fine as I sometimes need to shift the crop rectangle vertically upwards anyway, to cut off much of the ground.

The image size reduction occurs during correction is affordable. In this example my original image resolution went down from 24MP to 21MP since the corrected image is slightly smaller. The resultant image was still quite large and as sharp as the original one.

Note, the images on this post are not full resolution.

I have two additional tips regarding geometric correction:

• When you use the correction reference guides, do not make them perfectly aligned with the edges that you want to straighten. Allow a marginal perspective distortion offset. Otherwise you may end up with an isometric image with no perspective depth, which will look odd.  
• Apply correction first before applying any other processing, such as presets, brightness, contrast and shade filters. It is best to apply the correction algorithm on a raw image straight from the camera to eliminate the possibility of aliasing.