Iris needs one more definition:
5. A variable opening for a camera lens to alter the amount of light being admitted as well as to adjust the depth of field available for the image.

good catch! the iris diaphragm, i think it is exactly

Thanks Lloyd,

I was hoping some photography buff would add this definition. You did a much better job then I would have! It always fascinated me that the iris in the camera affected both the amount of light and therefore the required exposure time AND the depth of field.

I had to look at a lot of diagrams to understand that concept and it has been a long time, so I do not understand it anymore. I just know it is true.

Frank

humans can do that too--squinting, changes the amount of light, and the size of the opening (pupil)--this changes the focal lenght of the image (and sometimes makes it appear clearer/sharper.

Hey, we do, don't we? Thanks, Helen. And welcome to all you new folks!

DON'T DO IT! It causes wrinkles. <- see?