When shooting with a wide angle lens, photos can exhibit a fisheye effect called barrel distortion. The distortion is especially noticeable in photos where straight lines are visible, giving these photos a curved look. Depending on the severity of this effect, you may be able to correct it using the GIMP software. GIMP features a lens distortion filter to adjust the effect but, depending on the image, you may have to crop the edges for the best results.
Press "Ctrl-O" in GIMP, and then select the photo with the distortion that you want to correct.
Click "Filters" from the menu bar, select "Distorts," and then click "Lens Distortion" from the drop-down list to open the Lens Distortion filter menu.
Click the "Preview" check box if it is not enabled by default so you can see a preview of the adjustments you are making before applying them to your image.
Click and drag the "Main" slider to the left while checking the results in the preview window. The amount of correction depends on the severity of the distortion, so use the preview window to judge the results.
Click and drag the "Edge" slider to the left or right, depending on the distortion around the edges of your photo. Once again, use the preview window to determine the optimal amount of adjustment.
Click "OK" when you are satisfied with the preview window to apply the correction to your photo.
Select the Crop tool from the Toolbox, and then click and drag your cursor over the part of the image with the least distortion. The aim is to exclude the areas around the outer edges that are too distorted to correct properly. Double-click the images to apply the crop, and then save the results.
Information in this article applies to GIMP 2.8. It may vary slightly or significantly with other versions of the software.
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