Review: Sky Rover 6x32 SRBC vs. APM 6.5x32 APO

The making of a binocular with a decently wide subjective (apparent) angle of field requires the design of sophisticated eyepieces. If, on the other hand, a wide objective (true) angle of view is wanted, then the objective lenses have to accommodate oblique incident angles and the resulting light beam turns wide, so that large prisms are needed, which make the binocular heavy. The wide intermediate image then has to be captured by a correspondingly large field lens of the eyepiece. However, a simultaneous requirement of both, large objective and subjective angles, is the optical designer's worst nightmare and perhaps the reason why - up to now - there haven't been decent binoculars around with these combined properties. Who is interested to learn more about binocular design, may also have a look into my book.

When APM Telescopes introduced the APM 6.5x32 APO Porro binocular, it caused a bit of a sensation: For the first time, a low-power and wide objective angle (9.3°) binocular was available with an image quality that significantly exceeded its competitors. There have been binoculars with wide objective angles before, but those were hardly usable for astronomy due to their massive field curvature and the image blur in the outer areas of the field. Instead, the APM 6.5x32 APO offered a very bright image of high contrast, combined with an edge-sharpness that made it suitable for astronomical wide field observations.

Now, Sky Rover has gone another step further and introduced the 6x32 SRBC binocular with no less than 12.2° objective angle of field. The SRBC 'Banner Cloud' series has in recent years turned out to be of outstanding performance, with very wide and highly corrected images which almost reach high-end quality, as I have confirmed in separate tests of the 10x50 and 12x50 models of the same series. The 6x32, which expands the range of magnifications towards lower values, is therefore eagerly awaited as being a high performance binocular for very wide field observations, in terrestrial applications, but particularly under the stars. The APM 6.5x32 APO is a worthy competitor, so here comes a comparison between these extraordinary instruments.

Fig. 1: The Sky Rover SRBC 6x32 binocular

Externally, the 6x32 does not exhibit anything special: It just looks like a somewhat shorter version of its larger siblings with 42, 50 or 56 mm lenses. It is somewhat larger and heavier than the typical 32 mm binocular thanks to its oversized prisms, which are part of the wide angle design. The eyecups are now of a narrower layout and seem to be interchangeable with those of the 10x50 version. The objectives have threads which - in combination with an adapter - take standard 2" eyepiece filters for astronomy. The adapter rings can be left in front of the objectives and then accommodate standard 49 mm camera lens caps, which stay firmly in place and thus seem to be a pretty good solution. At the time of writing, the 6x32 is offered for 399 US$, plus shipping and perhaps extra tariffs. I don't know whether the adapter rings are included in the standard shipment or only come in combination with the filters, but I suppose the first option is preferable since some buyers may already own filters.

Fig. 2: The APM 6.5x32 APO

This Porro binocular explicitly displays its wide prism housings and eyepieces, and it is immediately obvious to the experienced eye that it has to contain some sophisticated optics. Despite of that, it is essentially designed to be a budget binocular with individually focusing eyepieces (though, meanwhile a 'CF' version with central focuser is available, too) and simple rubber eyecups. Its optical performance, however, is remarkable, and it has already gained a high reputation among amateur astronomers as an outstanding instrument for wide field observations. It is currently available at APM Telescopes for 229 Euro (the CF version costs 249 Euro).

Fig. 3: The 6x32 SRBC and the 6.5x32 APM

The following table summarizes some of the specifications of the two contenders.

Optical performance

Field of View: Since both binoculars have different magnifications, we compare the subjective angle of field. The 6x32 comes in at 70°, while the APM is just under 60°. With these values, the APM offers pleasantly wide images of star fields in the Milky Way or of mountain ranges during daylight trips. The SRBC is even wider and offers a true 'Wow!'-experience when placed in front of the eyes. This point goes to the SRBC.

Edge Sharpness: The APM shows well-defined star points across roughly 70% of the field, after which sharpness gradually declines. In the outermost 10% of the field, the image blur is considerable. The SRBC maintains sharpness a bit further out, until about 75–80%, and then the softness gradually increases towards the edges, at which the blur isn’t as pronounced as in the APM. The optical layout of this 6x32 thus differs from the SRBC 10x50 or 12x50, in which the images turned just slightly soft, about 80% out, and then became sharp again towards the edges. Anyway, this point the SRBC's.

Color Fidelity: Both binoculars deliver very bright and contrasty images. The SRBC’s image feels just a bit cooler compared to the APM, but both are excellent in this regard. Result: draw.

Rectilinear Distortion: Is low with both contenders, although a slight pincushion distortion is present. The APM pans without issue, while the SRBC shows a slight globe effect. This might just be due to the SRBC’s larger angle of field. I traditionally report on panning behavior, but I don’t score it, since it is a highly subjective issue.

Stray Light: In my tests I had no issues so far. Occasionally I noticed very slight brightening in the lower portion of the field in both binoculars, but not on any problematic level. More observation time may be needed with the SRBC to explore critical situations, but it is already obvious that the overall performance here is well above average with both contenders: a draw.

Ghost images: When viewing bright light sources at night, the APM remains absolutely flawless. The beauty of modern Porro prism binoculars is making itself felt in these critical test situations. Contrary, the SRBC shows occasional light-spikes (which are roof prism artifacts) when slowly being panned across distant streetlights. No such artifacts are visible on stars. I don't see any mirror ('ghost') images of street lanterns, an indication for high quality anti-reflective coatings in both binoculars. This point goes to the APM.

Chromatic Aberration (CA): The APM shows almost no color fringing, only a trace near the extreme edge, and here the image is already blurry anyway. The SRBC, on the other hand, exhibits noticeable color fringing in the outer third of the field, and the colors are becoming quite pronounced near the edge. Here again the 6x32 differs from its larger SRBC siblings, and one can only guess how difficult it must be to balance the various aberrations that show up when parts of the light cone enter the eyepiece at very oblique angles due to the wide field of view. Another win for the APM.

Low light performance: The APM has an exit pupil of 4.6 mm, which is less than what is stated in the datasheets, since its magnification is slightly higher than specified (see also Binoculars Today) and perhaps the entrance aperture is reduced by an internal baffle. For the SRBC, I measured 5.1 mm, less than the expected 32 mm / 6 ≈ 5.3 mm, and it was confirmed by Sky Rover that this is due to a slightly higher power of 6.1x and a slightly reduced effective entrance pupil of ~31 mm. In advanced twilight, the image of the SRBC clearly appears brighter than the APM's, so the point goes to the SRBC.

Mechanical construction

Usability and features: The SRBC is definitely more user-friendly, with its central focus wheel and adjustable eyecups. It also has a shorter close focus (around 5 meters for the APM, and 2 meters for the SRBC). Focusing is somewhat faster on the 6x32 than on the SRBC 10x50 which I tested earlier: About 330° from 3 meters to infinity, and surprisingly now in reverse direction, which suggests that the focus mechanism was apparently redesigned (update: Not exactly. The 6x32 uses a positive focusing lens instead of the negative lens used in the other SRBC models. This explains the reversal of the focusing direction!). The focuser turns smoothly and without play. The APM’s eyepiece focusing is a bit on the stiffer side. Both offer enough eye relief for eyeglass wearers. The APM has the advantage of being lighter (700g vs. 820g for the SRBC). The SRBC has the innovative concept of accepting filters on its objective lenses. Overall, point to the SRBC.

Ruggedness: I have no reason to believe either of these instruments is more fragile than the other. Both are waterproof and give the impression of well machined instruments, at least when considering their price ranges. I had some problems with one of the APM's rubber eyecups. It repeatedly fell off until I fixed it with plenty of glue, after which it finally seems to stay put. The SRBC may have a slight advantage due to its rubber armor, while the APM's individual focusing mechanism may be less prone to long-term mechanical issues. Result: draw.

Summary

The 'final score' is the sum of the individual scores and is intended to serve as an orientation only.

Taken together, the SRBC is clearly an improvement over the APM, which is to be expected with its higher price tag. Both are excellent binoculars for low magnification wide field observations and thus closing a gap that was ever present on the binocular market. Here, the SRBC 6x32 is now the new benchmark and I can claim with some confidence that it represents the best 6x wide-angle instrument in binocular history. Nonetheless, it should be noted that the 6x32 SRBC’s image quality doesn’t quite match the stellar performance of its larger siblings. It is extremely challenging to render such an enormous 12.2° field nearly flawlessly, and yes, the 6x32 doesn’t deserve the “APO” label that is printed on its focusing knob. Yet, it is a binocular which offers tremendous enjoyment and which is fully capable of handling demanding observations, especially under night skies. The APM is a worthy low-budget alternative with amazing performance values. Amateur astronomers can regard themselves lucky to have these alternatives in a market niche that none of the high end binocular makers dared to close.

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