Assume you are taking pictures of the sun with a digital camera.
You have a 1024 x 1024 pixel sensor that is 12.5mm by 12.5mm overall in size.
What focal length lens do you need to use so that the image of the sun fits onto the sensor?
How big does the exit pupil need to be in order for the lens to operate at f/4?
The angular diameter of the Sun can be found at this page:
It is A = 30' = 0.0087 radian
To maximally fill the ...