The Photoshop Ruler tool used to be called the Measure tool in previous versions of Photoshop, which was probably a more sensible name, given that the name is already used for a different feature in Photoshop.
The Photoshop Ruler tool enables you to measure the distance between two points, and even the angle of lines in an image.
Before you use the Photoshop Ruler tool, it makes sense to decide how you want to measure units in an image. Go to View>Rulers. Horizontal and Vertical rulers will appear at the side and top of the workspace. By default, these rulers use pixels as units of measurement.
The Photoshop Ruler tool will use pixels, too. If you’re planning to edit photographs then inches might be a more useful unit of measurement. Go to Photoshop’s Preferences menu and choose Units & Rulers. Set the Rulers drop-down menu to Inches and click OK.
The Rulers along the top and sides will now display measurements in inches. If you click and drag with the Ruler tool, the Options bar’s Width (W) and Height (H) fields will display values in Inches, too.
Open your start image. Grab the Photoshop Ruler tool. Click to place a point at the bottom left of the foreground rock and drag to measure the diagonal line where the rock meets the water.
In the Options bar you’ll see that this line has an angle (A) of approximately 31 degrees and a length (L1) of around 5.5 inches. If you place the mouse on the end point of the Ruler’s line and hold down Alt, an angle icon will appear.
You can now draw a second measurement line from this point and see its angle (A) and length (L2) in the Options bar.
How to straighten horizons using the Photoshop Ruler tool
01 Open the image
When shooting without a tripod your photo’s horizon might be tilted, which can be distracting. You could use the Crop tool to select the image and then drag outside the Crop window to rotate it. The Ruler tool provides a faster way to straighten a wonky horizon. Open your start image.
02 Draw a line
Grab the Photoshop Ruler tool from the Tools Panel. Click to place a point on the shoreline at the left of the image. Drag the Ruler right to draw a diagonal line that follows the tilted shoreline. In the Options bar, the Angle of tilt is 7.5 degrees. You can drag the start and end points to fine-tune the line if necessary.
03 Straighten the image
To make the tilted ruler run level, tick the Straighten Layer icon in the Options bar. This counteracts the 7.5-degree tilt so that it runs horizontally, straightening the shot’s horizon in the process. You’ll need to use the Crop tool to remove any transparent edges created by rotating the shot.
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