Edge Contrast

While the width of the circle determines the size of the area that Photoshop looks at for edges, a second and equally important option when using the Magnetic Lasso Tool is Edge Contrast, which determines how much of a difference there must be in color or brightness value between the object and its background for Photoshop to consider something an edge.

You'll find the Edge Contrast option in the Options Bar to the right of the Width option. For areas with high contrast between the subject and its background, you can use a higher Edge Contrast value, along with a larger Width value (larger circle). Use lower Edge Contrast and Width values for areas with poor contrast between the object and background:

Like the Width option, the Edge Contrast option in the Options Bar can only be set before you click to begin your selection, which doesn't make it very useful. To change it on the fly as you're working, press the period key ( . ) on your keyboard to increase the contrast value or the comma ( , ) to decrease it. You'll see the value changing in the Options Bar.

Frequency

As you make your way around the object, Photoshop automatically places anchor points (little squares) along the edge to "anchor", or fasten, the line in place. If you find that there's too much of a gap between anchor points, making it difficult to keep the line clinging to the edge, you can adjust how often Photoshop adds anchor points with the Frequency option in the Options Bar, although again, you'll need to set this option before you click to begin the selection. The higher the value, the more anchor points will be added, but generally, the default value of 57 tends to work well:

Rather than changing the Frequency value, an easier way to work is to simply add an anchor point manually whenever you need one. If Photoshop seems to be having trouble keeping the line in place at a certain spot, just click on the edge of the object to add an anchor point manually, then release your mouse button and continue on.

Fixing Mistakes

If an anchor point gets added in the wrong spot, either by you or by Photoshop, press the Backspace (Win) / Delete (Mac) key on your keyboard to remove the last anchor point that was added. If you continue pressing Backspace / Delete, you'll remove additional points in the reverse order they were added, which is helpful for times when the selection outline starts acting a little crazy and unpredictable, as it sometimes does. Here, I've completely missed the hair on side of the sculpture's face, so I'll need to press Backspace / Delete a few times to remove the unwanted anchor points, then try again:

This time, using a much smaller circle width, I have better luck. Adding some anchor points manually also helps:

If you've completely messed up with the Magnetic Lasso Tool and need to start over, press the Esc key to clear away everything you've done.

Switching Between Lasso Tools

The Magnetic Lasso Tool can often do an amazing job of selecting an object on its own, but it also gives us easy access to Photoshop's other two lasso tools if needed. To temporarily switch to either the standard Lasso Tool or the Polygonal Lasso Tool, hold down your Alt (Win) / Option (Mac) key and click on the edge of the object. What you do next determines which of the two lasso tools you switch to.

If you continue to hold your mouse button down and begin dragging, you'll switch to the standard Lasso Tool so you can draw a freeform selection outline around areas where the Magnetic Lasso Tool is having trouble. When you're done, release your Alt / Option key, then release your mouse button to switch back to the Magnetic Lasso Tool.

If you release your mouse button after you click with the Alt / Option key held down and move your mouse cursor away from the point you clicked on, you'll switch to the Polygonal Lasso Tool which is handy for selecting areas where the edge of the object becomes straight. Keep Alt / Option held down as you click from point to point to add straight line segments. To switch back to the Magnetic Lasso Tool when you're done, release your Alt / Option key, then click on the edge of the object to add a point and release your mouse button.

I want to include the platform the sculpture is sitting on in my selection, and since the edge of the platform is straight, I'll temporarily switch to the Polygonal Lasso Tool:

Closing The Selection

Once you've made your way around the entire object, click back on your initial starting point to complete the selection. When you're close enough to the starting point, you'll see a small circle appear in the bottom right of the cursor icon, letting you know that you can now click to close the selection:

And with that, the sculpture is selected:

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