I received an email this morning from Alex asking about use of the plastic viewer (page 40 of my book) and I wrote the following reply. Unfortunately the email address he gave me doesn't work and nor do various combinations and permutations of what looks like two email addresses lumped together. Anyway, for Alex and anyone else out there not familiar with using a viewing device:
you are talking about the elbow zoom feature (you straighten your elbow to zoom to a longer focal length) - it also has digital rotate - take your digits and rotate the damn thing.
Back in the days of 4X5 film, if I used a 4X5 inch cutout, then the distance of the rectangle from the eye gave you the focal length of the lens you'd need to capture the same scene - very handy. With the small sensors in digital cameras, I find a hole that size to be too small, too close to the eye and the edges too blurred (especially to my 58 year old eyes) so I prefer a much larger hole, say 2X3 inches or a bit bigger. In theory you could then do the math
---------------- X distance from eye = focal length of lens used.
viewer opening width
In truth, with zoom lenses I don't really worry about it too much, I just guess which lens will be needed - occasionally I get it wrong but it's not much of a deal to change lenses.
I have wondered about creating a viewer which has slides to close off either length or width so that I could 'crop' in the viewer - so far I just use my other hand if need be.
The rectangle doesn't have to be plastic - I just happened to have some around - in the past I have used mount board, but they get beat up relatively quickly - even my plastic has had to be glued. Arborite would be good (counter top material) but is hard to cut without chipping), aluminium would probably be best though would take a little longer to make.