Some lens systems are designed with an internal area (aperture plane) where an iris may be inserted, and manipulated, without cutting into the field of view.

This iris serves two useful purposes:

First, it permits the reduction of image intensity at the image plane, which reduces "blooming" and other damaging artifacts. Some lens/camera systems will automatically adjust the iris to a pre-determined signal level at the image plane. The lenses themselves are referred to as "auto-iris lenses".

Second, closing down the iris, essentially "stops down" the lens. Stopping down refers to a reduction in the Numerical Aperture (NA) of the lens. This narrowing of the light gathering cone produces a significant increase in the depth of field (the thickness of the envelope at the object, where various object heights are still in reasonable focus). However, nothing is "free", and the reduction is NA also relates to a reduction in the resolving power discussed above.