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|The IRIS is usually located inside the lens at the point where the focus crosses over.
It is used to regulate the light intensity hitting the film.
Every lens has it's own unique f-stop based on the previously mentioned formula.
The IRIS then provides additional light intensities based on the f-stop scale.
1 stop = 1/2 light
|The iris is usually located at the focal cross over point in a complex (compound) lens. This point is shown in the first part of the animation. In actual fact, not all focused light crosses at this point, there are actually an infinite number of cross points with some in front and some behind the theoretical point. The sum total of all of these points makes up the total light intensity reaching the film, but at the cost of multiple focal points.
Note: the human eye has a range of equivalent f-stops from about f-2 in low light to about f-8 in higher light. The eye has the overall advantage of focusing the resultant light on a spherical collection surface and not a flat plane. Thus it better fits the multitude of focal distances resultant from the thickness of the lens.
Film and ccds are flat and thus we must focus what we can on a fixed flat surface. A wide open lens (lowest f-stop) lets more light through but at more focal points and thus with diminished sharpness of image. A closed down lens (higher f-stop) reduces the total light but more of it is near the same focal point, and hence a sharper image. This provides what we call depth of field (or depth of focus). A curved lens focuses the world at several points along its axis, but the film or ccd exists at only one. Remember in most images various objects originate at different distances, and we are trying to compress them onto a flat surface.