Photographs with a crooked horizon are one of the worst mistakes you can make as a landscape photographer. You need to learn to run through a check list of questions before pushing the shutter and is the horizon straight? should come near the top of the list.

However, we all make mistakes from time to time and other factors - such as attempting to take a photo in a force 9 gale - can also come into play. This article will show you how to quickly and accurately straighten a horizon using Photoshop’s measure tool.

Here is an example photograph in which the horizon looks distinctly wonky.

The first step in fixing this is to select Photoshop’s measure tool. By default, this hides behind the eyedropper tool, so point to that tool with the mouse and keep the left button pressed until the button springs open to reveal three options. The measure tool looks like a small ruler.

With the measure tool selected, we’re going to click and drag a line across our photograph to tell Photoshop what angle on the image would be straight if we’d got things right in the first place. In the example photograph above, the best target would be the far shore of the lake - it gives us a very easy reference.

Clicking and dragging a line across it results in the following:

Finally, click on the image menu item, select rotate canvas and then choose arbitrary. The dialogue box that opens allows you to rotate the canvas by any degree that you choose - but in this case, the initial value it displays is the angle needed to bring the line we’ve just drawn back to the horizontal.

Click OK and Photoshop will go ahead and straighten your image, leaving you with something that looks like this:

Voila! One straight horizon. You now have a choice whether to crop the white areas out of the picture or use a combination of the clone and healing brush tools to extend the picture into those areas.