Just like the eye, the camera has a mechanism for regulating the amount of light falling on the film or image sensor. Aside from the physical lens, and lens assembly also houses the iris diaphragm. They are designed to admit only light rays parallel to the axis of the lens tube.
There are two types: (1) fixed and (2) movable. The fixed type is composed of a rotating disk with fixed perforated holes of varying diameters. The movable type is a mechanism composed of two to eight movable blades that twist to control the diameter of the hole.
Most SLRs use the movable type. To determine the number of blades, try taking a shot of a bright object: the number of “rays” or “light spokes” emanating from the light source will give you the number of blades.
The example above was taken with an 18-55mm Pentax lens that came with my Pentax K-x. It shows six (6) “light spokes” emanating from the sun (the light source). That means that the iris diaphragm of the lens assemby has six (6) movable blades.
Incidentally, the movable type is the same mechanism used in leaf shutters.
Next, we’ll examine the effect of the iris diaphragm diameter on aperture and its effect on images captured.