Your lens size and fstop are automatically tied to one another. The reason for this all comes down to how much light is getting to the lens. I'm not going to go into a huge lesson about aperture, but just trust me on this (check out more info on Shutter & Aperture).
You only need to do two things to get the fstop under your control. Personally, I like pictures that show me what to do as opposed to just being told, so here ya go…
1. Your camera must be switched to manual mode.
2. Press the Iris button.
Super. Easy.
And just to double check yourself, the "A" (for automatic) should disappear from in front of the f number.
Happy filming!
