When you take photographs of a landscape using a wide-angle lens, the horizon may appear slightly curved. The curvature increases when you use wider lenses and point the camera downward. This issue can be corrected using the "Lens Correction" filter in Photoshop, although you will need to crop the photo afterward to remove distorted edges.
Launch Photoshop and open your image.
Click on the "Filter" menu, then select "Lens Correction."
Remove the check mark from the check box labeled "Auto Scale Image" and select "Transparency" from the Edge drop-down menu underneath the check box.
Press the "A" key to select the "Straighten" tool. Click on the point where the horizon meets the left edge of your image and hold the mouse button down. Drag the mouse to the point where the horizon meets the right edge of the image and let go of the mouse button. This will straighten the image.
Press the "D" key to select the "Remove Distortion" tool. Select the camera make, model and lens you used to take the photo using the drop-down menus on the right. Photoshop should automatically adjust the photo to remove the horizon's curvature.
If the automatic adjustment does not satisfy you, click the "Custom" tab and adjust the value of the "Remove Distortion" slider until you are satisfied.
Click on the "Crop" tool in the tool palette and crop the image to remove the transparent edges. Alternatively, use the "Clone," "Healing Brush" and "Patch" tools to fill in the missing areas.