This is an interesting trade-off that seems like it can actually be used to a photographer's advantage! Even though having a fast shutter speed gives less motion blur and a larger aperture (blurry background), this may be exactly what we want at times as photographers. This is an interesting contrast to most CS tradeoffs that we know such as memory vs time, when it's always better to have both less memory and time.
I'm confused about the 'constant irradiance' here. In the lecture, Prof.Ng said that to make sure the constant irradiance, 'f' should be propotional increased (aperture size decreasing) with the increasing of shutter time. But here, the change seems not propotional. When f become from 4 to 32, which is 8 times larger, the shutter time is changed to about 60 times larger. (But the images brightness is indeed no change... so I'm really confused about that.)