PhotoShop: Straighten the Horizon

One of the most common fixes required in landscape photography is to straighten the horizon.

Sloping horizons are caused simply by not having the base or side (for portrait shots) of the camera parallel with the horizon and it's easy to do because we get caught up in capturing the scene rather than taking the time to be somewhat clinical in making sure that all the elements (like having a flat horizon) have been checked over.

In some cases, that may not be possible as a play of light may only give you a second or two to capture the scene and the time just isn't there to be more technical in setting up the exposure.

As with all aspects of photography, getting as much right as possible at the time of exposure will save time in post-processing the image. This is especially true if you're shooting with slide film.

Nevertheless, despite our best efforts, there will be times when we need to straighten the horizon. The technique below can be used with Adobe Photoshop and, once you know what do do, can be applied to any image in less than a minute. The images below are screenshots of Photoshop with an image in various stages of preparation.

To start with, here's an image I took from a ferry as we approached the coast of Santorini, off Crete, in the Mediterranean. The sea was a little choppy that day and the ferry pitched just as I took the photo, resulting in a fairly badly slanted horizon:

Photoshop V5 and above has a tool called the Measure Tool which is tucked away under the Eyedropper. Click and hold down the left mouse button (on PCs) until a little sub-menu of tools appears. The last one on the sub-menu is the Measure Tool, so select this:

The Eyedopper tool in Photoshop's tool palette will change to a ruler to indicate that tool is now in use. When you move the mouse cursor over your image, the cursor will be a diagonal ruler with a "+". Move the cursor onto your horizon (zoom into the image if you need to) and click and hold down your mouse button. Then drag the cursor to another point on the horizon (sufficiently far away from the the first one) and release the mouse button. As you drag the cursor, a line, anchored to the first point you clicked will appear. The line is always between that first anchor point and the mouse cursor (so long as you hold the mouse button down) and provides a visual aid for seeing how well you've selected points on your horizon. When you're satisfied with the position of your second point, release the mouse button and the line, terminated at each end with a "+", will sit on the image:

Now open the Image menu, select Rotate Canvas and then select Arbitrary from the sub-menu:

Photoshop will then calculate the angle of the line you drew using the Measure Tool and tell you what that angle is in a small dialog box:

All you have to do to strighten the horizon is click OK. Neat, huh? You don't have to try rotating the image manually, hoping to get a flat horizon. Photoshop does it all for you in a single click:

Now the image is crooked, but the horizon is flat, so it needs to be cropped to get rid of the white areas. Select the Crop Tool (it's third down on the left on Photoshop's tool palette), then click and drag a rectangle across your image. Little handles appear on the corners and sides of the rectangle which you can click and drag to resize it. Everything outside the rectangle is shaded (the white areas will turn grey) so you can see exactly what the selected area will look like. Move the handles so that the corners are just inside the grey areas:

When you're happy with the area you've selected, press the ENTER key to tell Photoshop to perform the crop. And there's your corrected image:

Here are the before and after images (slightly smaller):

Before	After

To Summarize:

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Tags: Tool Palette, Ruler, Flat Horizon