Diaphragm (optics) photographic lens is called an iris diaphragm. In the early years of photography, a lens could be fitted with one of a set of interchangeable diaphragms , often as brass strips known as Waterhouse stops or Waterhouse diaphragms. The iris diaphragm in most modern still and video cameras is adjusted by movable blades, simulating the iris of the eye. The diaphragm has two to twenty blades (with most lenses today featuring between five and ten blades), depending on price and quality of the device in which it is used. Straight blades result in polygon shape of the diaphragm opening, while
Thoracic diaphragm every time inspiration occurs. However, there has been argument that the caval opening actually constricts during inspiration. Since thoracic pressure decreases upon inspiration and draws the caval blood upwards toward the right atrium, increasing the size of the opening allows more blood to return to the heart, maximizing the efficacy of lowered thoracic pressure returning blood to the heart. The aorta does not pierce the diaphragm but rather passes behind it in between the left and right crus. The thoracic spinal levels at which the three major structures pass through the diaphragm can be remembered by the number of letters