Under Dark Skies? Put on your "Night Glasses"

7mm Exit Pupil Binoculars Compared from 6x42 to 11x80

One of the factors to consider in choosing a binocular is the “exit pupil” size. The exit pupil is the circle of light that seems to ‘float’ above the eyepiece and contains the illumination transmitted through the binocular. Exit pupil size in millimeters is the ratio of objective diameter in millimeters to magnification. For example, a 7x50 or 10x70 binocular has a 7mm exit pupil; an 8x32 or 20x80 has a 4mm exit pupil; and a 10x50 or 7x35 has a 5mm exit pupil.

A binocular with a large exit pupil, traditionally 7mm or even a bit larger, intended to fill the fully-dilated dark-adapted eye of the observer, may be called a “night glass”. Such a large, 7mm exit pupil is provided by the four binoculars described here:

1) World War Two vintage “GI Glass”, a 6x42 coated binocular intended for Navy air and sea search use. These have a wide field of view, almost 12 degrees, or 70-72 degrees apparent field of view. The model is SARD 6x42.

These are the only individual focus (FI) model among the 4 compared. These have an overall ‘feel’ I’ve only felt matched (among the few higher-end binos that I’ve tried) by Fuji F/MT marine IF binos: heavy, precise and sturdy. The view is comfortable, with eye position close to winged eyeguards. Faint ghosts don’t detract from a lunar image that is perceived as very sharp and high contrast (maybe the lower 6x is a factor that might affect perceived sharpness/contrast vs. other binos at higher power). There is a fall-off away from center sharpness but the expanse of sky in the field of view is impressive. In daylight use, these seemed to show distant details equal to a pair of Minolta ‘MK’ standard 10x50 that I owned, with more detail than any 7x35, supporting some real basis for the perceived sharpness/contrast. These take in a view that just includes Orion’s sword and belt, Rigel and Saiph. They offer a bright and wide view of the Milky Way.

2) Older Japanese 7x50, fully coated, with an 8 degree stated field of view, or about 55 degrees apparent field of view. This model is by Scope. The extra ring of view is noticeable, and appreciated, but not drastically different compared to a 7 degree model.

These, for a reason I can’t explain, present a pleasing view of star colors. I think that a slightly warm tone may not affect blue-white stars but is just enough to enhance other colors. Perhaps a critical mounted resolution test would reveal that stars are tiny blobs that make color more apparent. They show some ghosts around the moon and bright planets, but appear sharp at center with swollen stars at the field edge. For a 7x50 these have a relatively close focus of 15 feet or less, a little closer than my other 7x50, and are fine for backyard birding. A separate tripod adapter is required to clamp the hinge of these and the 6x42 described above. The main gripe I have with the Scope is slow focus in winter weather due to 30-40-year old grease.

3) New moderate cost 7x50, with multicoated optics and 7.1 degree field of view (about 50 degrees apparent field of view). These are the current Orion Scenix.

They are just noticeably brighter than the Scope 7x50, and largely free from ghosts even in lunar viewing. These differences attributable to the coatings and stray light control distinguish the Scenix from the older Scope 7x50. These are the only ones among the 4 compared that show a slight horizontal miss-lap of the field circles at infinity despite apparent collimation (I bought them as a ‘second’). It is sometimes noticeable in daylight and doesn’t seem to matter at night. These are the only of the 4 with a rubberized exterior and it’s fairly well done, reminiscent of but better attached than the Minolta MK series. Elementary school kids managed sky exploration with these on a parallelogram mount at outreach, supervised by my 10-year old. Phil Harrington gave high marks to the Scenix 10x50 in an Astronomy review of $100-range binoculars.

4) Older 11x80 with a 4.5 degree field of view (about 50 degrees apparent field of view); this one can be handheld but at 4-5 pounds is much better used on an Orion Paragon parallelogram mount. These are a multi-coated pair of Comet Kings.

These open up star clusters compared to the 6x and 7x models. Globular clusters take on a bright-cored fuzz ball appearance and brighter Messier object pop out. M27 or the Helix show more shape than with smaller binoculars. Even with both mounted for the comparison, these give the impression of a larger image scale vs. 10x50 than I’d expect.

The Sky Kings are the main binos we use for outreach now unless turning school kids loose with 7x50 as described earlier. These offer nice views of M6 and M7, or Antares and M4, the double cluster, Orion, etc. They are great for showing several M objects in the 4.5 degree field for comparison with adjacent telescope’s closer views. Kids like the view of the earth-lit crescent moon if we hit my favorite outreach dates of a few-day-old setting moon. I’ve seen the serpentine ridge and straight wall, and just made out the smaller crater inset within Atlas or Hercules with these at the right terminator lighting.

I consider that the 6x and 7x night glasses show texture in the sky, with small details glimpsed of brighter or larger objects. The 11x80 zoom in from that kind of sky texture, to start showing sky details within context, bridging toward low-power telescopic views.

Good things about a large exit pupil:

• Easy eye placement – rapid acquisition of view, tolerant of small ‘interpupillary distance’ (IPD) errors when sharing views at outreach

• Some ability to pan with eyeballs – in general, you can look around the view a little without blackout, the specifics depend on the particular model. Smaller exit pupils tend to make you move the binocular with less ability to move you eye within the view. This is a subtle point.

• Maximum dawn/dusk or dark sky brightness. For some this could mean fully illuminating poor eyesight or an eye pupil that doesn’t open to 7mm, in which case this advantage won’t fully apply. If the eye is stopped down in daytime, the only real downside would seem to be carrying the extra weight vs. a smaller binocular.

• Colors may seem brighter than with smaller exit pupil.

Drawbacks to a large exit pupil:

• Daytime use factors – weight, perhaps less close focus compared to more compact binos.

• Astronomy use – sky brightness shows up under light pollution, at some point an acceptable dark blue sky becomes more washed out with 7mm exit pupil than with smaller exit pupil. The ‘Binocular Performance Index,’ or ability to see faint stars, increases with higher magnification and smaller exit pupil.

I see a very dark grey/dark blue sky from my backyard in all of these ‘night glasses’ and often travel ½ hour + to truly dark skies. These 4 all work well in both of those settings. But - they all show too much sky glow when I go from my home outside the city to more urban park locations in or near Las Vegas proper.

A walk on the wide side

The wide angle 6x42 show more than twice the area of sky seen by the 7x50’s and about six times the area of sky seen by the 11x80. While the edges of a wide view typically show greater fall-off in sharpness, so that the sharp field of view in a wide angle binocular may be no greater than in a standard angle binocular of the same magnification, that extra sky can provide an enjoyable impression of added context and texture in the view. The edges can be used for detection as well, filling up more of the transition from central to peripheral vision. The faintest objects will be lost in the blurry edges, but brighter objects can be spotted more easily while sweeping with the larger field of view. There are big binoculars and wide binoculars, but it’s hard to find big wide binoculars.

Going back 2 generations to World War II vintage “GI glass” there are a few models like the 6x42 I was lucky to inherit and convert from marine to astronomy use, and similar 7x50, or larger ‘collector’ models.

Going back one generation there are ‘classic’ made in Japan models – several 9 to 10 degree 7x50 models from Celestron, Swift, and Sears etc.

More recently, you might find the Miyauchi fully-multicoated waterproof, IF 9+ degree 7x50 and 12-13 degree 5x32.

With a step down in exit pupil, there are hundreds of older Japanese wide 7x35 models that are less bright than 7x50, but much easier to find secondhand. There are a few similar models of Russian manufacture. Keep in mind that all of these older binoculars require some ‘buyers beware’ inspection, but if in good shape they are often robust and will last if not abused. Many have low-index prisms, are not water proof, have short eye relief, single-coatings rather than multi-coatings (that may just mean a few tenths of magnitude light loss or a few more bright-object ghosts), or some other trade-off to consider.

Among old 7x35s I’ve tried, for night use I like the apparently common 11.5-degree lightweight Wards/Tasco model that seems cheaply built with some plastic exterior covers, but holds up well to travel and gives me enjoyable views. A Sears 11 degree was nice too, until my effort to clean internal fog turned it into science class parts. A Scope 10.5 degree is OK for backyard birding in daytime but doesn’t seem as bright at night, it also has the shortest eye relief of these three.

I also use a 9+ degree 8x40 Porro by Bell and Howell that is a good day/night wide angle model. If you look for one of those, be aware a lower quality update looked almost identical to the older version, which had metal prism covers, convex eye lenses, and a sturdier focuser/eyepiece bridge. All of the 7x35 and 8x40 described have low index prisms and are not water proof.

With the Wards or Bell & Howell to supplement the 6x42 at night, and my other experiences with these older models (3 out of 6 proved to be keepers), I’m less keen to be penny-wise and pound-foolish by shopping for more of these older models unless really sure of a secondhand purchase.

There are a few modern multicoated wide 7x35, the Nikon Action Extreme at 9+ degrees may be the best affordable in current production, with waterproofing as an added benefit.

From Sea to Sky – ‘marine’ binoculars for astronomy

The nice views from the SARD and the positive aspects of a large exit pupil seem to make a natural transition from nautical to astronomical use. Splash resistance or waterproofing makes dew less of a concern, and IF is handy for infinity-focus astronomy use.

Among modern marine 7x50 models we’ve read about the FMT Fujinon and Prostar Nikon optical excellence (they should be superb and durable for their price). The “Kunming” IF models sold by Williams and others get good reviews, and the Captain’s Helmsman got high marks from both qualitative and quantitative leaders of the CN binocular forum. There are recommendations for some fully-coated but well made recent classics like the Storm King/Sea King that are close to the concept of the older GI glass. But I haven’t read as much about the current Nikon Sports/Marine or Ocean Pro models, or the similar Fujinon model, as mid-priced alternatives. I’d like to read some user comment about those, and other modern marine ‘night glasses’ in astronomy use.