Human eye The upper end of the range is given in terms of normal visual performance as 10 cd/m (100,000,000 or one hundred million candelas per square meter). The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it
Diaphragm (optics) by using an automatic ND filter. Unlike a real diaphragm, this has no effect on depth of field. A real diaphragm when more-closed will cause the depth of field to "increase" (i.e., cause the background and the subject to both appear more in-focus at the same time) and if the diaphragm is opened up again the depth of field will "decrease" (i.e., the background and foreground will share less and less of the same focal plane). In his 1567 work "La Pratica della Perspettiva" Venetian nobleman Daniello Barbaro (1513-1570) described using a camera obscura with a biconvex lens as a