The human eye has a focal length of about 1.7cm. In bright light, your pupils close to a diameter of 2.0mm. What is the "$f$-number" for your eye in bright light?

In many cameras, the sensor is a rectangle with dimensions 15mm × 22mm. You point your camera to take a photo of your friend, who is 1.8m tall. If your camera lens has a 35mm focal length, how far from the camera should your friend stand in order to produce a 22mm-tall image on the sensor?

A camera lens has a focal length of 85mm. At an $f$-number of $f/4$, what is the aperture diameter of this lens?