Due to the low participation level we had for the last challenge, I figured we should ease up a little and take smaller steps — so this week we’re covering keystoning, also known as the tombstone effect or keystone effect, which you can read about in more detail here. It boils down to this; our eyes and brain automatically correct incoming visual data in many ways that camera lenses can never match — among them is perspective control, or making vertical and horizontal edges always look straight and level.
The above image is a good example of this phenomena; upon first glance the image looks okay, but I begin to pick it apart the more that I look at it: The sides of the window aren’t parallel… The level of the horizon through the window in the distance is at an angle… The top and bottom edges of the window are severely skewed… Etc.
For some people, this is a non-issue. However, I’m a very experienced mechanical draftsman and quite skilled in drafting from a two-point and three-point perspective, so I prefer to correct my images as much as I can while keeping them normal looking. There are at least five ways of dealing with the problem of perspective:
- Fix it in production (place the camera differently, use a different lens, or both).
- Fix it in post-production (software or other means).
- A combination of production and post-production fixes.
- Leave the image alone and accept it the way it is, or…
- Don’t take the photograph at all and grump about it to the end of your days (and, yes — I know of some embittered photographers like this. Don’t you be one of them).
Fixing it in production. This can be achieved by choosing a different lens and/or a different shooting position and/or angle. This specific image was taken with a 21mm lens, but the skewed perspective could have been better moderated by using a lens of 50mm or longer (wide-angle lenses are notorious for warping perspective, especially when straight edges are anywhere close to the image plane). There is also the very expensive path of perspective control lenses — also known as tilt-shift lenses (read more about them here) — which would make short work of this particular problem (you can find a variety of tilt-shift lenses here). I could have stayed with the 21mm lens and used a bubble level to make certain the image plane was perpendicular to the floor (bubble levels are cheap, but using them can be a challenge at times; you can find all sorts of models here). Likewise, I could have used a through-the-lens solution to pay closer attention to what the image was doing before clicking the shutter (a change of just a few degrees can produce a dramatic difference with wide-angle lenses).
In this case, I was shooting from my full height; had I bent my knees down and paid attention to the symmetry of the subject (the window), I could have made it a lot more even from a sense of perspective. However, the room was too small for me to use a longer lens; I would have needed to back up through the wall behind me and into the next room to have gotten this shot with a longer lens. I could have bent my legs and gotten a more symmetrical perspective, but then I would have lost the composition that I liked. Compounding the issue was that this building was utterly packed with people (and no tripods were allowed), I was having to wait as much as 3-5 minutes between shots for the frame to clear of unwanted subjects, then I had just a few seconds before someone else would step into the field of view, out-of-control children running into me, etc.
But I made the decision on the fly to go with what I had in hand and deal with any problems in post.
So that leads us to…
Fixing it in post-production. This can be achieved by using two methods within Lightroom; the Crop & Straighten tool (at the very top of the right-hand panel) and the Lens Correction tools (third section from the bottom of the right-hand panel). Other software packages may be able to do this as well, but I’m sticking with Lightroom for this series. First, I go into the Crop & Straighten tool and use the fine grid lines that help guide me in making the far off horizon line appear flat (below). As you can see, the change in this specific case isn’t much; just –1.51.
Then I scroll down to the Lens Corrections tools and play with them. Adobe has created some preset buttons under the Basic tab, Upright section (Off, Auto, Level, Vertical, and Full). Usually I’ll play with these first. Why? Because their algorithms are sometimes right on the mark for the look I want to achieve. For those times when it is not, I then move to the Manual tab and begin playing with the six different settings they have there. In this specific case, I’ll adjust only the vertical settings until the window frame edges appear perfectly parallel to the edge of the frame, which gives me correction of +30 (below).
The result is not quite what I had in mind, as the corrections look forced and unnatural. The image now bothers me more than it did in the beginning, because my eyes keep getting drawn to the lower half of the window, where the perspective lines just don’t look correct. To make matters worse, I’ve lost a lot of the image around the edges — representing a pretty hefty crop over what I had originally.
So I’m scrapping the corrections up to this point and am going back to the drawing board. However, these post-processing steps are important to go through on your own photos, as you can experiment and see for yourself what works and what doesn’t prior to finalizing each image.
In this case, I’ve decided that I’m going to go with just a small correction (fixing the horizon line in the far distance), and I’ll leave the keystoning alone this time. That way at least the horizon line within the image won’t keep bothering me, and I’ll keep the rest of the image that I feel helps to complete the composition I was looking for. In some instances, the overall aesthetics of an image are more important than having other aspects — like perspective — absolutely correct. Only you can decide which direction to take, as it’s your photograph and you are the one that must live with the results in the end.
Here’s your challenge for this week: find an image with keystoning (like the one I started with), and see how far you can correct it before it before it begins to look wonky. Once you’ve gotten to where you understand the process, then you can begin to wield the tools with more moderation, so that the final results look more natural.
Once you’ve finished, be sure to include “#photo101rehab”, “#photorehab”, and “#imagecraftbootcamp” with your normal tags — and if you wish to submit it to the Flickr group (we now have eight people in the team), you’ll need to post it there as well (https://www.flickr.com/groups/imagecraft_bootcamp/).
Good luck, and see you next Wednesday!