For this, I took photos of a gray card outside my house, a photo every hour for a full day-light.
The camera was on ISO 100 and the shutter speed on 1/125 and with that I measured how much the F-no will be.
Unfortunately that was a sunny mid-winter day and the light hours were limited. If it was a mid summer day the graph was probably bigger and more ellipse shaped
i gathered the result in a graph:
