Correcting Lens Distortion
When we take pictures via a SLR camera, no matter what kind of lens that we are using or how good we are, unavoidably, the final pictures would contain some sort of distortion. At the middle of last year, Adobe launched its flagship product – Photoshop CS 2, which includes one handy tool: Lens Correction. The Lens Correction tool tries to address the issue of distortion.
Before we go further, let’s have a look at various types of distortions that we would encouter:
- Barrel distortion is a divergence from the rectilinear projection in geometric optics where image magnification decreases with increasing distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed outwards, towards the edge of the image.
- Pincushion distortion is a divergence from the rectilinear projection in geometric optics where image magnification increases with increasing distance from the optical axis. The visible effect is that lines that do not go through the centre of the image are bowed inwards, towards the centre of the image.
- Chromatic aberration is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. The term purple fringing is also commonly used.
- Vignetting is a loss in clarity towards the corners and sides of an image (for example brightening, darkening, or blurring).
- Perspective projection distortion occurs in photographs when the film plane is not parallel to lines that are desired to be parallel in the photo. A common case is when a photo is taken of a tall building from ground level by tilting the camera backwards; the building appears to fall away from the camera.
The Lens Correction tool is Adobe’s try of killing all the birds by one stone. Now, let’s start the tool by choosing Distort > Lens Correction from the Filter menu. At the top left, there are cursor tools. Then, there is the picture that we want to adjust. At the middle bottom, there are options for preview and grid. All the correction controls are listed at the right panel: from top to bottom, we have Remove Distortion tool for both barrel and pincushion distortion, Chromatic Aberration tool, Vignette tool, Set Lens Default button, Tranform tool, including perspective correction and angle adjustment, and options for Edge and Scale.
The normal workflow is as the following:
- Configure the grid to have the right size and color.
- Reposition the grid along important lines in the subject.
- Adjust the angle of the picture to make the subject’s important lines vertical or horizontal.
- Use the Transform tool to correct the vertical and horizontal perspective.
- Use the Remove Distortion tool set to remove barrel and pincushion perspective.
- Correct vigentting if any.
- Eliminate chromatic aberration if any.
References:
- Dpreview’s Photoshop CS2 Review.
- Tips for straightening things out with the Lens Correction tool – American Photo, March/April 2006
- Change Your Perspective – Popular Photography, March 2006
- Lens Correction filter in Photoshop CS2 – Photoshop CS2 Tutorial @ photoshopcafe
