Tips / Distortion Correction /
2 - Keystoning
If you look down railroad tracks, the tracks converge.
This is called perspective, of course.
When you stand before a tall building, and look up, the sides
of the building converge just like railroad tracks.
However, the convergence is now often called keystoning.
We don't pay much attention to keystoning until it appears in
a photograph.
When a building is seen on a flat surface, with the edges of
the photograph as a visual reference, keystoning can be distracting.
Prevention
You can prevent keystoning by placing the camera parallel to
a building.
By doing so, you may not be able to see the top of the building.
And, the foreground may not be making any contribution to the
photograph.
If so, move back, and use a more telephoto focal length.
You can also use a view camera or perspective-control (PC) lens.
They enable you to keep the plane of the film or sensor parallel
to a building, while moving the lens to get the entire building
into view.
PC lenses can be rented, if you have little need for one.
Correction - Photoshop &
Photoshop Elements
You can use Photoshop and Photoshop Elements to correct keystoning.
Photoshop
Photoshop has a sophisticated tool called the Lens Correction
filter.
With the filter, you can:
• Straighten the photograph
• Make the bottom and top of the subject the same width
• Correct the distortion created by the above correction by using
the Remove Distortion slider or tool.
Here's an Adobe tutorial:
Correct Lens and Camera angle distortions in One Simple Process
(PDF)
Here are before and after photographs from Colin Smith's book,
How to Everything with Photoshop CS2.
Photoshop Elements
With Photoshop Elements, place a grid on the photograph.
Go to View > Grid.
If the keystoning is even on both sides of the photograph, go
to Image > Transform > Perspective.
Drag the image with one of the corner handles, using the grid
to align the subject.
The Perspective tool links the opposite sides of the photograph
together.
If you grab the handle on, say, the upper right corner, the handle
on the upper left corner is dragged along, as well.
So, the Perspective tool works well when you have a subject in
the center of the frame, that has an even distortion bilaterally.
If the keystoning is uneven, such as two building leaning inward
from the left and right sides of a photograph, go to Image > Transform
> Skew.
With Skew, unlike Perspective, each corner handle moves independent
of the other handles.
Other Software
Flo's UnDistort
Filter Corrects keystoning, barrel and pin-cushion distortions,
and chromatic aberrations (color fringing)
