Architectural Photography, Part 3: Fixing Converging Verticals

In this third post about architectural photography, we’ll look at how to fix converging verticals in your shots of buildings.

Fixing Converging Verticals

If we can’t avoid unintended converging verticals in camera, we must fix them in post.

Tools for this task are included in many photo software packages, for example, operating system apps such as Apple Photos, well known titles such as Adobe Lightroom, and in dedicated tools such as Hugin and DxO ViewPoint.

Transform Tools in Lightroom Classic

Let’s attempt to fix the 14-30mm lens image using Lightroom Classic. Go into Develop Mode and look for the Transform Panel on the right hand side. We’ll start with the image from Part 2, with the highly pronounced converging verticals:

There are a number of automated options in the button group at the top of the panel - we’ll focus our attention on these (you can use the manual sliders below instead or in addition if you prefer).

First to say: on this particular image Auto really just doesn’t work. It stops far short of actually rectifying the verticals. This may be due to how extreme the convergence is in this case. Generally speaking, there are better options than Auto if you want to be certain that things are actually correct.

Let’s try Vertical next - after all, that’s what we’re trying to fix:

This will detect and attempt to correct converging verticals in the image. You can see that it does a pretty good job, but you might note that it seems to have overcooked things - it looks as if the verticals converge to the bottom of the image now.

How about the Full option?

This gives the same result as the Vertical option, but it also corrects for any unintentional camera yaw relative to the direct on-axis view. But again, the verticals are over-corrected.

The best option, I find, is Guided. This requires you to add guidelines to define the converging verticals and horizontal(s) if needed. Here, I’ve placed two guidelines that define lines on the building that should appear strictly parallel in the image. Lightroom shows a high precision loupe that helps you to place them accurately:

Once applied, the verticals are now corrected - at a price:

In order to achieve parallel verticals, the lower half of the image has been compressed horizontally to a significant degree. The white areas indicate the extent.

Correcting Horizontals

We can make an additional correction too. You would want to do this if you hadn’t quite achieved a perfect alignment in a composition that was intended to be on axis. Here’s an exaggerated example of what an “ off-axis ” alignment would look like: note how the alignment of the camera to the east transept under the red pin is not straight.

If you have any off-axis alignment, you can add two guides to indicate which horizontals are intended to be on-axis and the tool will correct the image accordingly:

Let’s compare the Guided result with the Full result - I think it’s clear that the Guided approach gives a much better, more natural looking result:

It’s not always possible for the algorithm to select the correct verticals or horizontals to align, particularly in more complex architecture. In this image there are multiple receding verticals in close proximity - that’s likely what causes the overcorrection observed when using the Vertical or Full options.

Comparing the Tilt/Shift 19mm and 14-30mm Zoom Images

This is an interesting exercise and might tell you something about whether you wish to invest in a tilt/shift lens. With the same Lightroom adjustments and the widest possible crop allowing for the guided transform correction losses (the white areas you see in the images above), here are the results - the tilt/shift image is on the left and the 14-30mm zoom is on the right.

It’s interesting to see how the tilt/shift image (right) provides more horizontal pixels and the zoom lens more vertical pixels, even though the camera position is unchanged between the two shots and the focal length is the same. Arguably the 14-30mm image is the better crop, even though it doesn’t need to be as tall as shown. A couple of conclusions stand out:

• The tilt/shift gives more pixels that cover the actual subject, the cathedral
• Due to the more extreme camera pitch angle, the pixels from the zoom lens have been “ pushed around ” a lot more in order to correct the perspective

Here’s a comparison of the images at 100% zoom (14-30mm left, 19mm tilt/shift right):

The pixel pushing in the 14-30mm image isn’t particularly apparent to my eye, even though this area of the image has undergone significant horizontal compression during the perspective correction process.

In this particular case, the best possible result would likely have come from doing a tilt/shift stitch, i.e. taking images with the tilt shift at multiple orientations of shift and rotation around the shift axis. Because each image is already perspective corrected, and the picture plane is not adjusted between each frame, stitching these together is a simple operation that gives very high resolution results with an extremely wide field of view.

A wide angle panorama can be achieved with much greater precision and control using the tilt/shift lens than with the zoom. This is a strong argument in favour of having one in your kit bag.

For high resolution print display, the tilt/shift is clearly going to give you more usable pixels on the same camera than a non-perspective correcting zoom or prime.

So, now you know how to fix both verticals and horizontals using the Guided tool. These tools work similarly in other software, so the principles remain the same.

Always be careful to place the guides precisely - a small error can result in strange looking images. Even more important, carefully select where you place the guides - are you certain that the verticals should be vertical (hopefully yes)? Are you trying to apply a horizontal correction to something that was intentionally photographed off-axis (hopefully no)?

With these fixes in place, our image is looking much better. However, there’s still one slightly disconcerting thing - the top of the building at left and right seems stretched out and distorted. It doesn’t look natural.

We’ll cover how to address these wide angle distortions in the next part of this series.