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 (birth control) woman may be fitted with a larger size diaphragm when she is near ovulation. The correct size for a woman is the largest size that she can wear comfortably throughout her cycle. In the United States, diaphragms are available by prescription only. Many other countries do not require prescriptions. The spring in the rim of the diaphragm forms a seal against the vaginal walls. The diaphragm covers the cervix, and physically prevents sperm from entering the uterus through the os. Traditionally, the diaphragm has been used with spermicide, and it is widely believed the spermicide significantly increases the effectiveness of