(" Bifocals with a small 'D-segment'are prefered to 'execs'because they allow proper distance correction of the runway peripheral vision 'streaming'cues when landing an aircraft")
Mmm.....did rather go into nostalgia mode did I not? It's an age thing...like needing specs really.
Still, it seems that "they" prefer the small D, which makes sense. As I mentioned, when I finally got good specs, I plumped for those, as they gave me the readers that I needed for the Jeps. (At times three, I erred on the side of being too powerful, so that the fine print in poor light would never be a challenge.) With the small D, the bulk of the lens is available for your prime needs.
I chose a large frame because of the panoramic view that they offer and indeed the ease with which the lens can be upgraded or repaired. The rim is only about 1.1mm at the thickest part and did not cause any significant interference to my peripheral vision.
I suppose that with the vary-focals, as you panned towards the edge, because of the hourglass profile, you would be at, or around planar, as your vision passed the edge, so there would be no step of focus. Again, the needs of the individual have to be determined not only on the fundamental prescription, but also on the patient's ability to process round extraordinarily complex lenses like vari-focals. The following is the case against vari-focals.
Personally, I much, much prefer a distinct switching point as I look up or down. The brain's ability to accept a change of magnification is miraculous enough...to ask it to slide up and down on a continuously variable magnification, is to me bewildering. The problem is, that although this miraculous organ seems to be able to cope with such a high workload most of the time, it is in its nature to suddenly throw us a curved ball.
This change of processing can be demonstrated in its simplest terms with the psychology test of looking at a wire-frame box. The average human will have their analysis of that shape assessed by a different set of datums at around 7 secs or was it 15? But whatever, having changed, it will dwell a time, then change back....and keep doing this flip-flopping to and frow trying to make the best of a difficult-to-assess image.
When we are in the late stages of an approach, there is a vast amount of calculation going on at any given time, I just can't accept entering this critical phase and then hitting the brain with variable datums.
But then there are trombone players. I'm not kidding, it seems that users of vary-focals start to ‘play' the lens in the same manner. Using pre-set levels of magnification just as a trombone player would to hit a set note. Learning to play that instrument is (I'm told) is a question of sliding onto a frequency from as near to it as possible. With practice, the fine tuning is reduced. Now, imagine doing this with your eyes.