Lesson Of The Month
Correcting Perspective With Architectural Shots

Lesson Of The Month

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Photos © 2002, Ben Clay, All Rights Reserved

One of the biggest challenges in taking architectural photographs is being able to control the perspective of the shot. Rendering an image so that the vertical lines of a building are parallel to the crop of the image usually requires the photographer to shoot with a view camera or Perspective Control (PC) lens.

Lacking that, it is possible to make adjustments for perspective after you've already taken the shot with your camera. This lesson illustrates some simple perspective techniques using an Olympus E-20N digital camera and Adobe Photoshop.

On a recent trip to Las Vegas for a photo trade show, I happened to be staying in a hotel room that looked out at the back of the Paris hotel. As with most hotels and casinos in Vegas, the Paris is spectacularly illuminated at night. From my hotel window, the building seemed to glow from within and since I had my gear with me, I decided to photograph it.

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I set my Olympus E-20N digital camera on a tripod, opened the balcony window and framed up a horizontal shot through the viewfinder. I guessed that the outside lights were probably high pressure sodium vapor flood lights (which run somewhere between 1900-2100ÞKelvin), so I set the White Balance to the closest preset (Tungsten, 3000ÞK) and figured I could adjust the color shift later in Photoshop. (For a closer look at how White Balance works, check out the lesson entitled, "Using Digital White Balance Outdoors" in the Digital section of www.webphotoschool.com.)

I set the aperture to f/5.6 and took several shots at various shutter speeds until I arrived at a good exposure. This last shot was captured at 1 second. To prevent any camera shake from such a slow shutter speed, I used an Olympus Remote Cable to trigger the shutter.

Since I had my laptop with me, I decided to view the result on a larger screen. I removed the Olympus SmartMedia card from the camera and uploaded the images to the laptop via an Olympus SmartMedia card reader. Once the images were written to the hard drive, I opened up the last shot I took in Photoshop (#1).

While the exposure of the image was good, I noticed a couple of things I wanted to "fix" digitally. I noticed that the color was considerably different, but I decided to fix that last. Instead, I went to work on fixing the angled perspective of the shot.

Had I been shooting with a view camera (4x5, 8x10), I would have been able to adjust the front and rear standards of the camera to make the vertical lines of the building parallel with the crop of the image. But even though my digital SLR was incapable of performing such a feat, Photoshop would allow me to modify the image to make it look like it was shot with perspective control. Here are the digital steps to straighten out the building and correct the color:

First, duplicate the image layer by pressing Command-J (PC: Control-J). Next, select the bottom layer and press the D key to set the foreground and background swatches to their default colors (black over white). Then fill the bottom layer with white by pressing Command-Delete (PC: Control-Delete). You will notice that the bottom layer is now pure white (#2).

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Next, give yourself some adjustment room by expanding your canvas. Choose Image>Canvas Size..., increase the canvas by 500 pixels in both the height and the width and press OK. Next select the top layer and disable the positioning lock by clicking on the Lock icon above the layer. This will allow you to move and distort the top layer.

Next, you're going to want to widen the top end of the image so that the vertical lines of the building will ultimately be parallel to the crop of the image. First, choose Edit>Transform>Perspective to activate the sides and corners of the layer (#3).

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You may need to position a couple of Guides to help you see when the lines are straight. To do this, press Command-R (PC: Control-R) to produce Rulers on the top and left sides of the image (if they aren't already there). Then click inside the ruler and drag a Guide out onto the image. You can drag as many Guides onto, or off of, the image as you want. Just remember that after you've released the mouse, you can only adjust the position of a Guide with the Move tool.

Next, click either of the top corners of the layer and drag it outward until the vertical lines of the building line up with the Guides. You may need to slide the top center point slightly to one side to have it line up perfectly (#4).

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Depending on the lens you're using or the position you happen to be in, you may notice a slight bubble distortion in your photographs. In other words, it may look as though your subject (building) was shot with a somewhat wide angle lens because the lines of the building are somewhat bowed. In this particular case, my position in relation to the building was pretty much fixed, as I couldn't move either forward or backward. Subsequently, I had shot this image at the wide end of my zoom lens to capture the entire building, but at the expense of distortion.

And yet, Photoshop once again came to the rescue on this issue as well. To reverse the bowed effect of a wide angle lens, select Filter>Distort>Pinch... to pull up a preview window. To get an idea of how much pinching you need to apply, minimize the preview window until the entire layer is visible. Then slide the Amount until the preview looks straight. In this case, I applied 4 percent (#5).

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This subtle Pinch function can really help to straighten the bowed sides of your image, but keep in mind that it works best with an increased canvas size, like we have here. Otherwise, your results may look contrived.

Once you have your perspective set, clear your Guides by choosing View>Clear Guides and then choose the Crop tool from the Toolbox. Make a crop selection by clicking and dragging diagonally across the image. You can adjust the positioning of the crop by dragging the points in the sides of the selection. To activate to crop, press Enter (#6).

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Finally, I wanted to adjust the color of the image. There are many ways you can modify color in Photoshop, and here's a simple one. Since the yellow lights on the building had a somewhat blue-green cast, I pulled up the Color Balance window (Mac: Command-B, PC: Control-B), moved the top slider 30 points toward Red, and the middle slider 30 points toward Magenta. By adding Red and Magenta to an image, you automatically reduce the levels of Cyan and Green. This quick adjustment made the greenish yellow a more warmer gold (#7).

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Sometimes when you create a drastic shift in color, the colors may appear to be overly saturated. To tone down the saturation, press Command-U (PC: Control-U) to pull up the Hue/Saturation window, reduce the saturation by 15 points and press OK (#8).

Finally, flatten the image by choosing Layer>Flatten. By flattening the image, you will be able to save the image as a TIFF for printing or as a JPEG for e-mailing or posting to a web site.

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If you plan to e-mail or post your image to a web site, make sure to first re-size it appropriately (600 pixels or less) and then apply a little sharpening to give the image some crispness. For a more detailed look at how to prepare images for print, e-mail, and web sites, check out the lesson entitled, "Preparing Digital Images For E-mail And The Web," located in the Digital section of www.webphotoschool.com.

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Here we can see the improvements we've made over the original image. The angles of the hotel are straight, there is no wide angle lens distortion, and the colors are much more true to life (#9 and #10).

Technical Information
Camera/Media: Olympus E-20N digital camera; Olympus RM-CB1 Remote Cable; Olympus USB dual slot media reader; Olympus 128MB SmartMedia card; Manfrotto carbon-fiber tripod with a G1276 head; Apple iBook laptop computer; Adobe Photoshop 7.0

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