If the currently selected layer is an image layer, it is shown in the canvas view at full opacity. By default, the other image layers are semi-transparent, to help you focus on the selected layer. However, if you wish to see all the image layers at full opacity (as they will appear in the final rendering), press the circle above all the layers to turn it black. If on the contrary, you want to concentrate on the selected layer and hide all the other layers, press again the circle to turn it blank.