Moving backward in cameras?

Thanks for that reply.  It's getting harder and harder to find people who see it like I do.

Around 1980, I spent most of a day on my back porch with a pad and pencil, scientific calculator, my 25' steel tape, and my SLR with a bag full of lenses.  By around 4 PM, I had it all worked out, and then it dawned on me - I can't have been the first person to work this out.  So, off the the public library.  Most books were of no help; but in the Kodak Photographic Encyclopedia I found it.  The same relationships I had spent most of the day on.

Why that isn't more widely known is sort of amazing.  That Kodak Encyclopedia was hardly new, even in 1980.  In any case, the results and the reasons behind it are as follow:

First, we're talking about our human binocular vision and our ability to converge our eyes. Second, it's in terms of 35mm cameras with a 3:2 aspect ratio.  With our convergeance muscles relaxed, the sharpest part of our horizontal field of view corresponds to a 35-40 mm lens.  If we close one eye, we see about like a 55 mm lens.

If we use both eyes and tighten up our convergeance muscles to focus on details in the scene, it corresponds to an 85 mm lens or longer.  It turns out that there's a minimum amount of muscle tension that's stable.  Less tension and our eyes flicker.  So the telephoto equivalent range starts at 80-85 mm and then continues on to very long focal lengths.  it also means that we really don't have a mode of vision from 40-55 mm and from 55-85 mm.  Back when I had the popular 35-70 zooms I found myself using tham at 35 or 50 mm, and never liked the results at 70 mm. That dissatisfaction with 70 mm is what caused me to spend that whole day figuring it out.  Now I know why.

Anything wider than 35-40 mm requires that you turn your head or move your eyes to see it most clearly.

For years, I've used this knowledge to select lenses.  For general purpose, you really want to cover 35-40, 55, and at least 85 mm.  Most "kit" lenses do that, and cover 28 mm as well.  The problem is that they just cover the very beginning of the telephoto range, and never seem long enough.  Back in the film days, I opted for a 28-105, 35-105, or 35-135 mm lens.  When using a 35-105 or 35-135, I usually carried a 28 mm; because on travel, I often found I couldn't move back anymore, and needed the 28 just to fit everything in.  In digital, I prefer an 18-135 (29-216 mm equivalent on my APS-C Canons) as it minimizes lens swapping, covers all the key points, and gives quite a good tele range for focusing on details.