To take a series of pictures of the same scene varying the focal length of the lens
As with Exercise 6: Fitting the frame to the subject I used my Canon G10 walkabout camera for this exercise, taking the pictures at Liverpool Street Station as I travelled home from work one day.
The G10 reports focal length between 6.1mm and 30.5mm in EXIF (equivalent to 28-140mm according to Canon). I took three pictures in total: one wide open at 6.1mm (28mm equivalent), one at 12.07mm (55mm equivalent) and one at 30.5mm (140mm equivalent).
Conclusions drawn from this exercise
As the lens is zoomed in so the magnification factor increases and the field of view also narrows.
It is difficult to make out in these images but will be much clearer in those for Exercise 10 that front to rear perspective is also foreshortened as the lens is zoomed in.