The focal length of the lens determines the size of the image on the film that the light will be spread over. The area of the image will be proportional to the square of the focal length for distant sources. The power per unit film area in the image is the light collected from the source divided by the size of the image of the source. You can compensate for low light power by exposing the film to the light for a longer period of time. Film requires a certain amount of light energy per unit area to be exposed, which is expressed in terms of the speed of the film.
Fast film requires less light energy than slow film.
The amount of energy per unit area of film will be proportional to product of the light power per unit area and the exposure time. This may be acceptable for a stationary object, but results in blurring if the object moves appreciably during the exposure time. The case was designed to highlight this compromise by making simultaneous demands for both large depth of field and short exposure time. This allows members of the group to take different stands on the solution to the problem and realize that there is no one correct answer. They receive the case story and assignment several days before the class meeting when the assignment is completed.