Dragon Boats...Again!

Tech Tuesday: Architectural Restoration

Why is it that when I take a picture of a tall building, it often looks distorted? Gee, those walls sure looked perpendicular to the ground and parallel to each other when I was standing there! But in the image I come home with, the walls are at all sorts of angles, and the building looks like it's falling over backwards.

Take, for example, the photo here of 160 W. 73rd Street, a lovely Art Deco co-op typical of many of the elegant older apartment buildings on the Upper West Side of Manhattan.

I framed the image carefully in my viewfinder, trying to line up the setback on the right edge so that it looked veritcal. Here's what the image looked like right out of my camear in Photoshop Elements 4. Notice how the left side of the building "leans" like a rickety shore shack, rather than the quite solid limestone-and-brick edifice it really is.

So why does this happen? I'll tell you why. When you point the camera straight at a building, the film plane (or for a digital camera, the plane of the sensor) is parallel to the building. This is great, because parallel lines in the building will come out parallel in the photo, and there will be no distortion.

The problem is, if I did that with my photo of 160 W 73rd Street, I would have gotten a lovely picture of the lobby and the awning. Not quite what I wanted. So of course, I tilted my camera upward to include the top of the building.

But now, the film/sensor plane is no longer parallel to the building, and I get perspective projection distortion. How do I correct this? In the realm of 35mm and DSLRs, there are special, usually expensive perspective control lenses that allow you to tilt and shift the lens relative to the film/sensor plane. Here's a good, well-illustrated article by Ken Rockwell that explains this very well.

View cameras (the kind where the photographer focuses on a screen under a dark cloth) usually have camera movements to control perspective distortion and other problems. But we can accomplish our goal of un-distorting our tall buildings using some simple tricks in even the older versions of Photoshop and Photoshop Elements.

Start with the Image menu, and select Transformation, and then Skew. You'll see a dialog like this one. Just click the OK button.


This will bring up the New Layer dialog. While it's good practice to name layers with a meaningful name, we'll be throwing this layer away soon anyway, so just click the OK button and continue to the next step. (Layers are a subject for a whole other post, or really several posts.)

At this point, you'll see a thin box around the entire image. The box also has tiny circles at each of the corners and at the midpoint along each side. You can grab the box at any one of the circles (think of them as "handles") and tug or push your image.

You can see here how I tugged the top left corner of my photo to "un-tilt" the building. I used my calibrated eyeballs to tell me just how much to tug.

There will be times when you'll take a photo of a tall building straight-on, but still tilted up. In that case, both sides of the building may appear to fall away. So can you do this same maneuver with the other side? Sure! As I said, you can push or pull any of the "handles" until your photo looks right to you.

Here is the final photo, after I gave it my 30-second Photoshop 80-20 Rule treatment to color-correct and contrast-sharpen the image.

By the way, in the latest versions of Photoshop, there are other, sometimes easier or better ways to do perspective distortion correction. But if you're like me, you just don't have the big bucks to buy Photoshop CS3. That's why I'm showing these techniques that you can do even with just an old copy of Photoshop Elements 2 (if you search carefully online, you should be able to by a perfectly good, perfectly legal OEM copy of PSE 2 for about twenty bucks.)

Yellow Crane

A simple composition, bright yellow diagonal against a black background.

This is Version 1.0 of this scene - I need to try it again. The subject matter is about six blocks west of our office building, and I shot this through a window. Just now, it occurs to me that I can step through the fire exit door just next to the window and shoot almost the same scene with no glass in the way (our 1930's building has an outside fire stairway.)

Shot with my Canon XSi and Sigma 70-300mm lens, zoomed out to just under 300mm.

Canon Digital Rebel XSi

After almost five years of using my Canon Digital Rebel (the original model,) I just replaced it with a new Digital Rebel XSi (also called 450D.) Ben now has my hand-me-down Rebel, which still works fine.

I didn't get my hands on my new camera until arriving home from work today, so I charged the battery while I mowed the lawn, had dinner, and then took a few test shots.

Here's one of my first test shots. I wanted to see how well the Image Stabilization (IS) works - and it seems to work fine. This is the whole .jpg, untouched, shot at 1/20 sec and f/7.1 zoomed out to 55mm. This is equivalent to 90mm on 35mm, so normally, you'd want to hand-hold at 1/90 sec or higher, so shooting at 1/20 sec represents about two f-stops. The Auto-ISO function picked ISO 800 for this one.

Here's a 100% crop centered on the point of focus. Bear in mind that this looks much larger than the writing is in real life. Note that you can see the half-tone pattern in the background. I'd say the IS eliminated shake completely.

Here's the classic self-portrait-in-mirror that every photographer, from rank novice to professional, seems compelled to take.

I set the camera for ISO 1600, the camera's highest setting, to see if the images would be usable at this level of sensitivity. In this case, I adjusted levels in Photoshop to improve the color, but I did no sharpening. In general, I'd say this image is very usable. Exposure was 1/40 sec at f/5.7, with the lens at 36mm.


Here's a 100% crop of a portion of the ISO 1600 photo. Notice that even with the levels adjustment, the noise isn't all that bad. Don't forget, if this were an 8x10 print, you'd have to look with a very strong magnifier to see this level of detail.


For comparison, here is a 100% crop where Auto ISO set the ISO at 800. Exposure was 1/16 sec at f/5.7, also at 36mm. Here, I adjusted levels and used unsharp mask to first do contrast sharpening and then general sharpening.

So far, so good! I'll take some real photos over the next few days and report back.