How to Use the Magnetic Lasso Tool in Adobe Photoshop

The Magnetic Lasso tool in Photoshop is one of those tools regularly overlooked in the process of making a selection. However, that is a mistake because you can use it to do amazing things once you understand how it works.

Magnetic Lasso Snaps to Object Edges Automatically

Unlike the regular Lasso, which you use to make freehand selections of an area of a picture, the Magnetic Lasso makes selections based on edges and delivers a relatively accurate — 80 to 90 percent accurate — selection. The tool selects the edges of an object as you move the mouse by finding the changes in brightness and color values between the object and its background. As it finds those edges, it displays an outline on the edge and, like a magnet, snaps to it.

How to Use the Adobe Photoshop Magnetic Lasso Tool

If the selection you want to make has edges that strongly contrast with the pixels around it, the Magnetic Lasso Tool will come in handy. Here's how to use it.

Pull up the image you want to modify in Photoshop.
Select the Magnetic Lasso Tool from the Tools menu. It's in a menu with the standard and Polygonal Lassos.
Press Caps Lock to change from the default lasso cursor to a precision cursor, which is a circle with a +-sign in the middle.
Once you have selected the Magnetic Lasso, the Tool Options will change. They are:
- Feather: The value is the distance the vignette or blurred edge of the selection will extend from the edge of the selection. This is how one softens the edge of a selection. If you are new to this try and keep the value between 0 and 5.
- Width: This is the width of the circle when the Caps Lock key is pressed. You can make it larger or smaller by pressing the [ or ] keys. Keep in mind this is not a brush. All you are doing is enlarging the edge detection area.
- Contrast: The width of the circle determines where Photoshop finds edges. This setting determines how much of a difference there needs to be in the color and contrast values between the object and its background. To change the contrast value on the fly press the period key (.) to increase the contrast and the comma key (,) to decrease the contrast.
- Frequency: As you drag along the edges the Lasso will drop anchor points. This value determines the distance between them.
Once you have determined your options find an edge to drag along and make your selection. Click to turn on the Magnetic Lasso Tool, and then move it along the edge of the object you want to select. As you move your mouse, Photoshop will automatically drop anchor points (shaped like squares) along the path you follow.
Keep following the path until you get back to where you started tracing the edge. When you reach the point you originally clicked, the cursor will gain a small circle in the lower-right corner to show you the loop is complete.
Click to finish the selection, and the image will get a dashed line across the path you followed.
Now, you can treat the selection like you would any other. Some options are moving it, filling it in, adding a stroke around the selected edge, or copying it.

Correct Selections Made by the Adobe Photoshop Magnetics Lasso Tool

With the Magnetic Lasso, there are a few ways of correcting errors. They include:

Add an Anchor Point: Click the mouse to add another point if the Magnetic Lasso isn't including a spot you want.
Remove an Anchor Point: Press the Delete or Backspace key to clear the last anchor that Photoshop put down.
Switch Between Lasso Tools: Press the Option/Alt key and click on the edge. If you continue to drag you will automatically switch. If you release the mouse after clicking on the edge, you will switch to the Polygon Lasso tool. Releasing the Option/Alt key after switching tools returns back to the Magnetic Lasso.
Subtracting Areas: You have selected the edge of a doughnut but you need to remove the doughnut hole from the selection. You have a couple of choices around accomplishing this task. The first is to hold down the Option/Alt key and drag around the hole. This switches to the Subtract from Selection mode. You will know you are in this mode when a minus sign (-) appears in the cursor. The second method is to select the Mode in the Tool Options and then click the mouse around the edge of the area to be deleted. Be sure to close the selection.
Adding to Selections: Switch to the Add to Selection mode by clicking it in the Options toolbar. Click around the edge to be added and be sure to close the selection.