Getting the right tone
Make a tonal strip
A tonal strip is a simple yet very useful gadget to use in painting as well as sketching. It is simply just a strip of paper shaded from black to white. You can print one off from the link below, or use a black and white paint strip from a decorators shop as shown here, or even just paint your own.
Then use the strip to compare the tones you have drawn with the ones on your subject. Hold the strip up to your subject, close one eye, and see which tone on your strip is closest to the tone on your subject. Then see how closely you have matched this tone in your drawing. I think you will be surprised at how much lighter your drawing often is!
Use white paper 1.
Sometimes it is useful to check your tones (and colours) directly against your subject. Try these two methods:
a. Paint your colour, or shade with a pencil, against the edge of a piece of paper, and hold this against your subject to see how close they are (as shown here).
b. Cut or tear a hole in a piece of white paper and then use this to isolate a small area of your subject. Paint or shade along one edge of the hole until it matches the subject.
Use white paper 2.
You can also get a good comparison of the shades you need by just laying a piece of white paper on ground in front of you. This will give you a guide as to how light or dark you need to shade things in your sketch to match the scene. Note here how dark the gravel path is in comparison to the white of the paper, even though the gravel is a light grey.