To see to subtle features of the sketches, your monitor must be calibrated. Here is a simple calibration image. The strips consist of 17 shades, ranging from pure white to deep black. When your monitor is calibrated, the 17 distinct shades should be visible. When your monitor image is too dark, the dark grey shades will appear black. When your monitor image is too bright, the light grey shades will appear white. Try to adjust the brightness, contrast and gamma settings of your monitor to get all the shades well represented. Once your monitor is calibrated, the displayed images and sketches will reveal all their subtleties.