Aperture blades moving when in Ae mode on x100

I am a new owner of the X100, so am just getting used to this beautiful camera.

One thing I notice is that in Ae mode (say f4) and having only the OVF operational, when I move the camera around, I notice that the aperture blades are opening and closing in response to the brightness.

Why is this? I assume setting the aperture to f4 should have the blades remain constant...

(I await a response that will show me up for being stupid)
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Kind Regards,
Rich
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