I use viewfinders to isolate sections of 2-D images, to enlarge
sections of 2-D images and to isolate a view when hand holding it at
arm's length. I usually make one example of a VF with 1/4 and 1/2
markings on the outside form and inside window & draw the diagonal,
horizontal and vertical lines that connect with these points, and
then make photocopies of this for students to cut out. If having
students make their own, mis-measurement causes more problems with
the window opening than can easily be corrected, and as I do not
spend a great deal of time using them as a drawing aid, prefer to
skip the student instructions.
Now, I have a beef with using them for teaching drawing - I can say
this because I have worked as a professional artist for 20 + years
before turning recently to teaching art. They drive me and my
students crazy! How do you really draw what you see inside the VF?
that means precise realignment of eye squint, arm position and
holding the pencil and drawing board just so. YUCCH. I think they
are ok for isolating a view but for teaching angles and exact
measurement, I prefer sighting with a pencil.