Keeping Your Perspective (more post-processing)

Started Feb 18, 2004 | Discussions thread
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Gordon W
Gordon W Veteran Member • Posts: 4,062
Keeping Your Perspective (more post-processing)

Hmm...yes, another in what is becoming a series of post-processing tutorials. If anyone is curious why I keep posting them here, these are at the request of a couple fellow STFers who'd rather learn these things here in the STF, their forum home, than in the Retouching forum.

Anyway, the subject of this posting could have been Removing Vertical Perspective Distortion, but as I understand it, perspective distortion is an entirely different beast and what I'm showing here is actually called...

Removing Vertical Convergence

What's that, you ask? Unless you're pointing your camera perfectly level (or using a studio camera with 'swings & tilts'), lenses with fixed orientation parallel to the imaging surface produce an image with vertical objects that are no longer truly vertical (i.e. parallel to the sides of the image) but lean toward the center of the image. In case, you don't have a clue what I'm talking about, here's an example...

Notice how the Canadian Parliament's Peace Tower looks like it should be called the Leaning Tower of Peace-a.

Also notice vertical lines on the other side of the image are leaning in the opposite direction. They are converging. Here's how this image looks with the convergence being removed with Photoshop's Perspective command...

And here's the image all cleaned up (cropped and adjusted with Shadow/Highlight, Levels, Curves, etc)...

Notice the difference? This is how your mind saw it because it has an amazing way of ‘auto-correcting’ for such visual anomalies, and how a professional image would look taken with a studio camera which can remove vertical convergence right in the camera, just as can be done with a professional enlarger in the darkroom. Fortunately we digital photographers have Photoshop.

The problem arises in our digital cameras when we aren't shooting perfectly level, which is most of the time so most of our shots will have it, and therefore most of our shots will look less professional because of it.

So, to deconverge our convergence...

1. Open your image of choice in Photoshop (most of the better image editors also have a way to do this)
2. Double-click on the ‘Background’ layer icon
3. Hit Enter to close the renaming dialog window that opened
4. This layer should now be named Layer 0 (by default) and unlocked (important)
5. Zoom the image out a bit until you have some gray area around it (important)
6. Select ‘Perspective’ at Edit > Transform > Perspective

7. You should now have little square ‘handles’ at each corner and mid-points of each side

8. If the camera was pointing up from level when the shot was taken, grab either of the upper corner handles and pull outward into the gray area
9. almost magical, isn’t it

10. to maintain the original aspect ratio, you also need to pull in one of the bottom corner handles until the midpoint handles are centered on the side of the frame

11. repeat steps 8 & 10 until your verticals are again truly vertical and hit Enter
12. and that’s it. From there, crop, edit, and adjust to suit.

If the camera was pointing down from level, the handles get pulled in the opposite directions.

Usually our images don’t have that much convergence, but that’s really the problem. When something is off just a little, it looks like a mistake. When something is off a lot, it looks intentional, as in this image...

More information on vertical convergence can be found here...

The process I described above is very basic. If you want to do all kinds of wild and wonderful convergence distortions, give Photoshop’s ‘Distort’ function a whirl.

 Gordon W's gear list:Gordon W's gear list
Canon PowerShot S100 Panasonic FZ1000 Nikon Coolpix B700 Panasonic FZ80/FZ82 Nikon Coolpix P1000 +5 more
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