One of the most distinctive features of the papilio are the converging objectives, for good image merging at closer focus, the video demonstrate how this works:
At close focusing distance and seeing with just one eye it cover 50mm of ruler, with the two eyes it cover about 7cm field of view, at closer focus. Infinity focused the 6.5x magnification give about the same image of a 200mm tele in a APS-C sensor camera.
Have seen citations in internet that when at closest focus the magnification in fact is more than 6.5x, but never found the complete information. As a test, I measured the exit pupil as 2.85mm when at closest focus and about 3.30mm when focused at the end of range. At middle of focusing found 3.25mm exit pupil so I believe it decrease in size more as the focus start to became close. (The advertised in the papilio booklet is 3.2 which is in line with the 21/6.5 = 3.23) By a simple rule 21/2.85 = 7.36 (magnification at the close range). This calculations are subject to errors, one is that I measured the exit pupil with a micrometer seen by eye, another is the possibility of aperture change when the binocular focuses close that could result in a smaller exit pupil.
As you get closer the subject you will run depth of field problems, I have found at the closer focusing distance when the binocular is at a 45º viewing angle from the subject (one of the most used angles for practical purposes). The usable depth of field is about 8mm, but the real depth of field parallel at the subject should be about 2mm, this is under low light conditions. For simulation I have found the depth of field to be the same of the canon 100mm macro lens when at 1:2.2 and f/3.5 (this is by ruler observation not by magnification calculations). Another interesting point is that at full sunlight looks there is a increase in the perceived depth of field, from to the point of f/7.1 or f/8 in canon 100mm macro.
It's possible this increase in depth of field is more related to observer eye physiology than for the optics as the binocular always work wide open. One possibility is that in full sunlight the pupil is so small that act as a diaphragm increasing the depth of field. Another possibility is the fact that the human eye have worse resolution at low light conditions and possibly there is more selective attention to the area in focus (low ability to resolve detail is compensated by selective observing of the more detailed areas, a theory to be confirmed) resulting in a false perception of short DOF, while in full sunlight conditions there is more capacity to observe detail.
Uses and observation at different distances:
The papilio truly shine for observation of bushes in low light scenarios, the relatively wide field vision and bright of the 6.5x version is perfect for this scan and the optics of this binocular works better at close distances, at this observing arena is possible to see the fine movements of a jumping spider at each jump, the detail of mating beetles and interesting feeding activities of the insects, it make me slow down and observe interesting behaviour to be registered in shoots in future. The resolution is enough to resolve a medium size fly o compound eyes, when the fly is steady, the eye pattern is clearly seen, or the compound eyes of a medium size beetle, is possible to observe some detail in a jumper spider with the movement of legs in 3D at each jump a very interesting observation.
The middle of focusing range is very good for inspecting bushes and scanning trees at distance of 1,5 to 4 meters (this is already covered by some roof prism binoculars but the papilio allow to to get close when something really interesting pop). For focusing objects at infinite or distant the papilio give a good image but not stellar as the long range binoculars, at night observing stars the papilio give a gain of magnitude of (I believe about one half magnitude), some star clusters not visible at naked eye are possible to spot with the papilio but the resolution is not on par with astronomy binoculars, of course I wouldn't expect too much for the 21mm aperture. The papilio truly shine at close observation and the images taken at long distance should be seen as a bonus of the binocular.
One unexpected but appreciated excellence of this binocular is for flower photography, with his images is possible to observe the best angles to capture flowers and a fast survey of the available backgrounds for faster setup of the camera, is much faster than with camera alone and the binocular view prevents a lot of the tiredness involved in scene study.
It's plastic :) and you should take care of it to avoid high temperatures and other abuses, have read consumer reviews of people who desire a metal build and better resolved optics binocular as the papilios are the only one that allow this close observation (something like the most expensive well build roof prism birding binoculars). I do not share these opinions and I personally prefer the binocular the way it is, the resolution is good enough to observe detail and considering the most subjects will be moving (even the flowers will have movement by wind) more resolved optics would be a expensive overkill and for observing dead insects or flowers inside home there is the better resolved stereomicroscope, for this binocular my opinion is that the plastic is preferable than metal, as my macro gear is already too heavy.
Use and constancy
Have the binocular for tree months at the time off this review, used it a lot in the first week test period, and left behind just after, as the weeks passed I'm using it more and more as I discover more usefulness of his features.