What does it mean to "set the system aperture" in an optical design program?

In lens or optical design, you have to "set the system aperture". What does this really mean? Here, I explain and highlight the significance of "setting the system aperture", illustrating examples of why it matters so much in lens design. This is not a video about how to design optical systems using Zemax OpticStudio®. To learn more about designing using OpticStudio, visit Mark's excellent site, "Design Optics Fast":    / @designopticsfast   Other channels to learn optical design using OpticStudio­®: Zemax LLC channel:    / @zemaxllc   OpticsRealm by Scott Sparrold:    / @opticsrealm