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The focal length of the human eye is generally quoted as being about 22mm, although figures vary from source to source - and of course for particular eyes. The focal length also changes with the distance at which the eye is focused. The distance from the front of the eye to the retina is 24 - 25mm, so parallel rays of light have about that distance to converge to a single point. The eye's focal length must therefore be less than about 24mm. The focal length is also dependent upon the diopter value of the lens, or its refractive power.

Measurements of the typical diameter of the iris generally produce figures in the region of 7mm, although the diameter tends to reduce with age. The light falling on the retina is of course proportional to the square of the pupil diameter, so when the pupil doubles in size the amount of light passing into the eye increases fourfold. The iris can expand to about 8mm in diameter in low light conditions and can contract to a diameter of only 2mm in bright light. This fourfold change in diameter therefore corresponds to a factor of 16 for the change of illumination of the retina - the equivalent of a four-stop range extending from approximately f/2 to f/8. However, using the typical figure of 7mm for the diameter of the iris, the f/stop equivalent could be said to be 22/7 = 3.14.


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