In the middle of Mark 8, Jesus heals a blind man in two stages--the blind man recovers sight a little bit (and "sees men as trees, walking") and then on Jesus' second touch receives 20/20 vision.
Lloyd-Jones remarks that Jesus 'dealt with the blind man as he did in order to enable the disciples to see themselves as they were' (Spiritual Depression: Its Causes and Cure, 39).
All through the four Gospels Jesus does two things, he teaches and he heals. This is the only time he does both rolled together into one: he teaches by the way he heals. He's showing the disciples that they are the blind man, at partial healing. They understand that Jesus is the king, the Messiah--Mark 1-8 has convinced them of that ('You are the Christ!', 8:29). What they don't yet understand--as indicated in the very next conversation in which Peter rebukes Jesus at the thought of Jesus' death--is that this Messiah is a suffering king. Mark 9-16 will convince them of that. Then they will have 20/20 vision.