Correcting converging verticals

Original image - Place de la Concorde

Original image - Place de la Concorde

Photoshop plugins are available for perspective correction and transformations.  Using these you can, for example, correct for chromatic aberration and change the perspective of an image to correct converging verticals.  The plugins are part of Panorama Tools (alias Pano Tools) and are quite easy to download and install (from  Download the file [Version: MinGW 2.8.6  Size: 894KB], unzip it and double click on setup.exe.  The plugins should appear in the Filters menu in Photoshop.  A folder will be created on the desktop containing the rest of the Pano Tools – if you don’t need these just delete them.

The images shown here are ‘before and after’ perspective correction.  A wide angle lens was used (18mm on 1.5 crop).  I first applied PTperspective to correct for the pitch angle of the camera (about 15 deg), but the basins in the fountain appeared tilted (as you would expect with rectilinear perspective and horizontal discs near the edge of the frame).  This was remedied by converting the image to cylindrical perspective and whilst this causes a slight curvature of the building in the background the effect is far more acceptable than the tilted basins.

Final image - Produced as a monochrome print

Final image - Produced as a monochrome print

Note that PTperspective performs a mathematically correct transform on the image, and produces exactly the effect of a shift lens.  Free Transform in Photoshop can produce a similar effect but the result must be judged by eye.  So whilst the latter may look right, the former is right.

So don’t bother to save up for a shift lens, just make sure that when you take a picture like this you leave enough space around the subject to allow for the distortion of the frame that occurs when you apply the perspective corrections.

John Widdall  26/03/09


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