6. The Tools in the Toolbox / 6.2 Tools for All Kind of Layers
The magnifier is used for zooming the picture in or out.
By clicking into the picture the zoom factor is doubled, and the click position is centered.
You can create a rubber rectangle, too. The zoom factor will be adjusted, so that the rectangle is completely visible.
In order to decrease the zoom factor, you have to hold Shift while clicking, or you have to click with the right mouse button (not available on macOS).
While pressing and holding the right mouse button, dragging to the left will zoom out and dragging to the right will zoom in.
The zoom factor can be changed via keyboard with "+" and "-" (Windows) or Command+"+" and Command+"-" (macOS).
By double clicking the magnifier icon the zoom factor is set to 100%.