html, body {
  width: 100%;
  height: 100%;
  display: flex;
  background-image: url('img/main_image_small.jpg');
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}

* {
  margin: 0;
  padding: 0;
}

.parent {
  margin: auto;
  background-color: #000000;
  opacity: 0.8;
  display: flex;
  height: 100%;
  width: 100%;
}

.child {
  margin: auto;  /* Magic! */
  max-height: 50%;
  width: auto;
}