Lens Distortion

The Lens Distortion transformation lets you correct barrel or pincushion distortion caused by certain camera lenses. For many lenses, this kind of distortion is a minor annoyance that causes slightly curved horizons, but certain wide angle lenses such as fisheye lenses exhibit extreme barrel distortion. You can also use this transformation to introduce barrel or pincushion distortion deliberately as a special effect.

Lens Distortion should only be used on uncropped images since cropping can change the scale factor of the image and center of the image differently.

When you initiate the Lens Distortion transformation, a broken line connecting five control points is displayed as an overlay near the top edge of the input image window. A marker is also displayed as an overlay at the center of the input image window. Distortion is corrected by dragging the five control points into alignment with a curved element in the original image that should be a straight line (such as the horizon or part of a building). When you then click the Compute button, Picture Window automatically determines the best method and distortion coefficients to make the control points you have selected lie on a straight line.

Once you compute and save the correction coefficients for a given lens, you can re-apply the same settings to other uncropped photographs taken with the same lens. In this case, do not click Compute again as this may alter the saved settings.

For the most accurate results, the selected control points should lie on a line near one of the longer edges of the frame, since this is the part of the image that exhibits the most pronounced curvature. Having determined the distortion coefficients for a given lens you can then apply them to other images taken with the same lens as long as the images are uncropped. For zoom lenses, note that distortion usually varies with focal length. In most cases, the center marker should be left at the exact center of the image -- if you move it, the resulting distortion will be asymmetrical and will not work on rotated images.

Method

This control lets you choose between two distortion correction methods: Curvature and Polynomial.

The Curvature method assumes that distortion is caused by the focal plane being distorted into a spherical shape and corrects the distortion based on a single distortion coefficient that depends on the radius of the sphere. When you select this method, a slider is displayed below to show the current value of or let you modify the distortion coefficient.

The Polynomial method fits the radial distortion to a polynomial with variable third and fifth order coefficients. Depending on which method you select, different sliders are displayed below. When you select this method, sliders are displayed below to show the current value of or let you modify the third and fifth order distortion coefficients.

Scale Factor

This slider lets you magnify or reduce the result image. After adjusting the Distortion Coefficient slider to remove distortion from the input image, you will be left with an image that is no longer rectangular. You can use the Scale slider to trim black space around the border of the image as necessary.

Optical Center

A crosshair marking the optical center of the input image is displayed as an overlay over the input image window. Initially, this marker is placed in the center of the image window. For the most accurate possible correction of fisheye lens distortion, it is important that this marker be positioned at the point in the image in line with the optical axis of the lens. This will generally be the default center location, but you can adjust the position of the marker by clicking and dragging it in the input image window.

Tips

Start with an uncropped image that covers the entire frame so you can identify its optical center.

When correcting extreme barrel distortion (as from fisheye lenses), the result image will be quite blurry in the corners as the image data in the corners of the frame are highly compressed in the input image.

You may find it worthwhile to sharpen the result image to compensate for the slight blurring caused by resampling the input image.

This transformation simply removes radial distortion – unless the camera was perfectly level when you took the picture, you will often need to do a subsequent perspective correction as well (see the Warp transformation).