For many artists painting a canvas is an exciting project to do, that is soften sketched in a sketchbook before. Especially when speaking of big canvases every artist wants to avoid mistakes while using the canvas in a perfect way. In doing so, sketching before in a sketchbook is a must to-do. Sketching in sketchbooks also helps with the image composition or if the idea of a picture is still in a development process, as proportions, arrangements and constructions get clearer.
However, a sketchbook is also helpful for portrait drawings: thanks to different formats and sizes, faces and landscapes are easily to draw.