Some sketches I did on the iPad using the app Procreate. I think the brush is Pastel. Very cool way to see values. What I do is split the screen of the iPad and on one side I have the grayscale photo and on the other side of the iPad screen I have the Procreate app canvas. (see below image) It's a pretty cool way to work. As for getting the correct values, I just use my eyes. I'm not using the "eyedropper tool" to get the correct value. To me, that's kind of cheating. Also, I like to keep both images (The photo and the sketch) the same size.Anyway it's a fun way to work and you don't get any charcoal on your fingers!