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Tutorial: fixing distortion in photos. Part 1. Perspective crop

Friday, January 25, 2013

Unless you own a tilt-shift lens (or a large format camera) from time to time you will face a problem of distortion. Lines that in reality should appear as straight seem to bend as they go upwards.

Look at the photo I took in the La Oliva town on Fuerteventura island to see an example of what I'm talking about:

There are a few ways to minimize this effect:
  • shoot with narrower angle. Using wide-angle lens, and especially ultra wide-angle lens, results in more distortion. Using a lens like 50 mm should help significantly. But if you're like me probably it's not an option for you. My favourite focal length is 24 mm and it is already considered a wide-angle. Not ultra but still wide.
  • step away from your subject so it fills the frame instead of tilting your camera - keeping your camera level will reduce this effect.
However, even if you follow the rules above (or simply can't or don't want to limit yourself) you will encounter this issue. So I will show you how to fix it in this new series of articles which will describe following tools:
  • perspective crop tool,
  • perspective/free transform tool,
  • lens correction filter,
  • adaptive wide angle filter.
We'll start with the first one on the above list, i.e. perspective crop tool which recently became one of my favourites. A note of warning. All of these tools are great and make fantastic job but they have one disadvantage. Each of them will result in parts of the image being cropped.

Perspective Crop is a new feature in Photoshop CS 6 and I simply love it. It's similar to a crop tool but what it does apart from cropping is correcting perspective. And believe me, it does some great job with it. Using it is fairly simple:
  1. Open your image in Photoshop CS 6.
  2. Select Perspective Crop Tool from the toolbox as shown below:
  3. Drag a rectangle around your image to select it.:
  4. Drag corners of the crop rectangle so they match horizontal and vertical lines in the image. Make sure Show Grid checkbox is checked as it is a great visual aid. Note that when you're moving left corner you will need to readjust right corner and vice versa because corners influence each other. Adjust them until the grid will match the lines in the image:
  5. When you're happy press Enter key or click on the Tick button in the top part of the window to accept the crop. At this stage you will get something like this:
  6. Much much better. The lines are straight (compare with the image at the beginning of this tutorial). There is still one serious issue though. The image now looks a bit vertically squeezed so we will use Free Transform tool to correct that.
  7. If you're making your adjustments to your background layer, Photoshop won't allow you make any transforms to it. So we need to unlock this layer. Double-click it and rename it to anything you wish.
  8. Now select Edit -> Free Transform (or press CTRL + T) and resize the layer by draggin vertically:
  9. But here is another problem - part of the sky above palm on the right is now cropped. Fixing that is simple. Just select Image -> Reveal All. However, in this case I quite like this crop so here is the final version:

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