Iris/diaphragm collimators are typically used for:

Iris or diaphragm collimators are specifically designed to create fixed fields of illumination during fluoroscopic procedures. They function by allowing the operator to easily change the shape and size of the radiation beam, providing a predetermined, controlled field of view. This results in a consistent and reliable area of exposure while maintaining image quality.

The term "increased penumbra" refers to the gradual transition area at the edge of the collimated field, which can be a characteristic of certain types of collimators. However, it's essential to note that although iris collimators produce a fixed field, the design can indeed create some degree of penumbra, especially when the edges of the beam are defined sharply.

While other options might seem plausible, iris and diaphragm collimators are mostly employed to create stable fields rather than allowing variable exposure times or dynamically changing the field size extensively during an examination. This design focus aligns with their predominant use in maintaining consistent imaging conditions, hence the association with fixed fields.