Perspective Adjustments

One of the things that separates a professional looking architectural images from an amateur one is the control of perspective. Using a view camera the photographer can adjust the back of the camera to keep it parallel to the building and avoid converging lines. This convergence gives the "falling over backwards" look that afflicts many pictures. Here is a nice church with vertical guides applied. Notice that the sides of the building tilt inward. The image was taken with the equivalent of a 22mm for 35mm film. This is a very wide angle lens and any camera tilt becomes immediately obvious.	Original image with guides
The most common way to fix this is to use the perspective transformation. Here is a screen shot taking in the middle of the process. First we selected the entire image then selected transform->perspective. Using the guides we pulled out the top corner until the sides of the building appeared vertical. As we make the adjustments the guides may no longer be on the edge. In this case we can move the guides by putting the mouse over the guide while pressing the control key. The cursor will change to a double line and allow us to move the guide. We can also shift the outline by moving the control point in the center. Notice that we have shifted it slightly to the left so that the transform trapezoid is bigger on the left. We probably didn't have the camera completely level when the picture was taken so we need an asymmetrical adjustment. When we are happy with the look we press Enter to finish the adjustment.	Adjusting with perspective transformation
Here is the result. The image looks much better. The sides of the building are vertical and it no longer appears to be falling over. Until recently this was about the only thing that you could do to fix perspective. There are now some other options to try as well.	After perspective transformation
The new option is to apply perspective correction while cropping the image. Here is the beginning of the process. We have dragged the sides of the selection over to the edges of the building and pulled in the top so that the crop mark lies along the edge. The area that will be cropped is show in red. Notice that the guides are not any help, we just make the crop marks coincide with the edges we want to be vertical.	Crop with perspective - step 1
Now while holding down the Alt key we select one of the control points on the side crop area and pull outward. Both sides will move out at the same time. This keeps the shape the same while making the crop area bigger. In our case we will stop when the bottom corner reaches the edge of the frame on the right side. The corner on the left will extend outside the frame. This is similar to the look in the first technique. We also will fix the top and bottom so that they are level and even with the sides of the frame. When we have the outline the way we wish we press Enter.	Crop with perspective - step 2
Once again the building is corrected. Because of the way we sized the outline we have a missing corner in the final picture. When we make the final cropping this will be eliminated. A different size in the previous step could also have done the job. As will be seen in a minute, though, the images are not quite the same.	After crop with perspective
Finally we use a third party plugin for this task. There is a very nice free package of image adjustment tools called "Panorama Tools" available on the Web. These are installed as filters in Photoshop and other programs. There has been some problem with the web site lately so just do a search to find a site with a copy. We are going to use the "Perspective" filter. Here is the dialog box to set the options. We need to set the horizontal angle of view for the lens in use (HFOV). In our case this is 75°. You can look this up also if you don't know the values for your specific lens. Press the "As Source" button to set the size to be the same as the original. The difficult part is to determine the angle to tilt the image. The best approach is to try several values on a smaller copy of your image. Use negative values for vertical to compensate for the camera tilting up. We chose -6° after some tests. Note you can also adjust for side tilt and crooked cameras, but we aren't going to use these features. After pressing OK the filter will run and the result will be shown on the original image.	Pano Tools dialog box
Here is the image after the filter has run. Notice that the image is smaller at the bottom, but the building has been straightened. Once again we will crop to get rid of the missing image.	After Pano Tools
Here are the three images. They have been resized so that the distance from the steeple to the curb is the same. Thus differences will show up in the width. They are similar, but not identical. The panorama tools is noticeably thinner than the other two. The perspective crop is next and the original tool gives the widest building. In addition the sides of the steeple appear closer to vertical in the panorama tools version. It would be possible at this point to squeeze the outer images to make them similar to the center one, but the point here was to illustrate the results of the basic settings.	Side by side comparisons
To summarize: The traditional perspective transformation gives visual feedback as you move the control points. The crop tool allows following the edges, but you don't see the results until you commit. The panorama tool seems to give the best adjustment, but the values must be typed into a dialog box. Here is the original image again. The difference in this case is subtle, but sometimes that's what makes for an outstanding picture. Full-size version of the comparisons	Original Image
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Robert D Feinman

Perspective Adjustments

Full-size version of the comparisons