Swarovski 2x Booster Review


Several manufacturers sell power boosters and the basic idea is pretty crude – a low power monocular (quite usable on its own) that somehow attaches to the eyepiece of one barrel. Look through it and you get the magnification of the monocular times that of the binoculars (so a 10x binocular with a 2x monocular becomes a 20x telescope). You could get an idea of the effect by carefully holding one barrel of an opera glass up to the eyepiece of your binoculars. Of course, this being Swarovski, the 2x Booster is bit more refined than that.

Firstly, why would I (or you) want such a thing? Well for a start, only if you own a pair of SLCs (but not the 8x30WB – I checked) or an old-style (pre-Swarovision) EL. It doesn’t fit Swarovski’s newer designs.

For me it comes down to being an accessory for my 15x56 SLCs.

I’ve owned Swarovski 15x56 SLCs for a couple of years now and I have to admit I was wrong. As I wrote in my review, they do indeed suffer from too much chromatic aberration on high-contrast subjects in daytime; they also have too little eye relief to be really comfortable for use with specs’. However, in all but name they are now my reference standard in a high-power astronomy binocular. My others languished and were sold, even the 12x50 Nikon SEs – the big Swaros just show me more. I did a mini Messier marathon with them on a recent dark and transparent night; it was ludicrously easy to find all those faint fuzzies.

One thing the big Swarovskis have in abundance is quality: the optical quality in particular being amongst the highest I have tested in a binocular (scope-like lens and prism quality are a necessity at this power). So I reckoned Swarovski’s booster might work with the 15x56s, in theory turning them into a 30x56 telescope that would extend their “scope” for astronomy (sorry!).


Design and Build

Typically for Swarovski the Booster is a beautifully made thing. Essentially just an 11cm x 4cm fully-waterproof monocular, one end screws into the binoculars and has a tiny (~5mm) objective; the other sports a large and deeply recessed eye lens with a cup that simply pulls in and out. A knurled rubber ring is provided for easy grip.

The tiny objective lens of the Booster – even so, it’s a good 2x monocular on its own.

Putting the Booster up to your eye, the effect is like a high-quality opera glass. The reason is that it offers an almost flat field (distortions creep in for just the last 10% or so) with huge eye relief and fixed focus with depth of field from about a metre to infinity.

Installing the Booster

So how does it attach? You just twist and screw out one of the eyepiece cups, then screw the Booster in its place. To avoid over-tightening, it clicks round when you’ve tightened it fully – thoughtful engineering

Remove an eye cup – it unscrews in the same direction it extends (anti-clockwise).

Carefully thread the Booster on in place of the cup; avoid cross-threading it!

If fitted properly the Booster clicks and ‘freewheels’ once it’s tight enough.


Warning: it is relatively easy to cross-thread the Booster, so take care to offer it up straight. It should screw in easily. Any resistance means cross-threading - take it out and try again.


In Use - Daytime

The first thing to note is that the measured exit pupil is about right - no vignetting, at least with the 15x56s; you get use of the whole objective area, which is good news, especially for low-light terrestrial and astronomy. This really does create a 30x56 telescope.

The booster reduces the exit pupil to the expected ~ 2mm (56mm/30)

The second thing is that the Booster actually fixes the eye relief problem of the 15x56 SLCs, so that it’s easy to use with glasses on. However, apparent field width is well down on the spacious 64 degrees of the original binoculars.

The booster also significantly reduces chromatic aberration. Chromatic aberration on high contrast subjects (plumage against sky, for example) is quite intrusive with the 15x56s, but much less so with the booster in place. The bright green and violet either side of focus on the basic binocular becomes much more muted with the booster in place. I suppose this is because the Booster effectively serves to increase the focal ratio of the objective.

The boosted magnification of 30x really is too much to hand hold and trying to do so just gives such a jiggly view that you’re inclined to think it’s poor optics. It isn’t. Attach the binocs to a tripod and it becomes apparent that the daytime view is very good indeed – sharp, bright, clear and detailed with high resolution; much like a good terrestrial scope in fact. The width of field isn’t great, but at least it’s sharp virtually to the edge and the whole field is now easily visible wearing glasses.

A DLSR snap of the view through the Booster.

The focus snap of the original binoculars is still there (more so if anything) with all but the extreme edge of the field sharply defined. So the extra magnification doesn’t reveal quality problems with the optics (as it might with many binoculars).

This answers my key question – the 15x56 SLCs have optics that are perfectly capable of taking the 30x magnification without introducing too much softness or distortion.


In Use – Night Sky

Turning to a gibbous Moon (or trying to) I realise that the effect of 30x and a narrowish field of view is that a finder is required; fortunately one is to hand – the other barrel of the  binoculars! The two barrels – one boosted the other not – are virtually parfocal, making it easy to switch between views with no refocusing (any slight discrepancy can be fixed using the dioptre adjustment).

Focusing on the Moon is slightly tricky though, not because of soft focus but quite the reverse. Perfect focus is now such a fine point and snap that you almost need a microfocuser – fortunate then that the SLC focuser is so smooth and precise to begin with (arguably better than the EL’s).

Once in focus the Moon is a treat. Absolutely sharp, the view is very similar to a fine small astro-refractor (think TV60 + 11mm Plossl, for example) with only a narrow band of purple around the limb to give away the difference. Otherwise, all the same detail is visible: central peaks of Copernicus and Tycho; Tenerife mountains and Pico; the curving crater chain in Clavius, the jutting headlands of Sinus Iridium. As with the base binoculars, the view is unimpeded by flare or reflections.

Now stars aren’t usually the forte of binoculars because they tend to produce flare and spikes from the prisms. But in focus, a bright star is a clean, dazzling point with the SLC/Booster combo. The brightest stars show just a tiny cross shape that is the hallmark of roof prisms, but the effect is very subtle and does nothing to spoil the view. A quick star test reveals near ideal circular diffraction patterns either side of focus – remarkably impressive for binocular optics.

Jupiter, rising in Taurus (something my wife would doubtless regard as portentous - I just can’t fix her belief in Astrology), is the only planet around. An early look is promising, but I have to wait til late to get a good view. The wait is worth it because again Jupiter is shown with astro-scope clarity. I fancy the Galilean Moons are resolved into tiny disks and you can spot Ganymede because it’s noticeably brighter than the others. The main equatorial cloud belts on Jupiter itself are clearly visible, but the really impressive thing is that I spot a moon just coming off transit, a tiny distance from the planet but perfectly visible because there is no flare from Jupiter’s disc at all (and consider how many binoculars completely fail the Jupiter test at regular magnifications, producing a smeared blur).

Deep sky is similarly enhanced by the booster: I have never seen the Orion Nebula in binoculars show the detail available with the Booster. Again the view is like a small astro-scope, with whorls in the nebulosity, extended ‘wings’ and the four main Trapezium stars clearly and easily resolved.

One slight ‘problem’ of course is that you have to look straight through, so targets near the zenith involve some neck strain. Another is that the relatively small field of view means that things quickly drift out of it. Nonetheless the Booster turns the 15x56s into a small but capable astronomical telescope.



Call me an old romantic, but I imagine a hunter or birder out there, sitting bored in some remote hide, idly screwing in the Booster to take a peek at a dusk Moon and getting hooked – a new astronomer is born.

Swarovski’s Booster works a treat with the 15x56 SLCs in testament to the high quality of the binoculars. Of course it’s designed for hunters and birders as an occasional scope substitute and works well as such, but it really broadens the range of the binoculars for astronomy as well.

Arguably the 15x56s + Booster will do most of the things a small astro refractor will, including revealing atlas-level detail on the Moon and showing you the basic Solar System sights. Meanwhile, smaller DSOs take on a lot more character at 30x, so you can see the shape of the Dumbell Nebula and the Crab, resolve some easy double stars and start to resolve out bright globular clusters like M13.

So, given the modest cost of the booster (about £250 new, £150 used), I could imagine someone tight for budget (or simply storage space!) might buy a pair of SLC 10x50s, a booster, a tripod adapter and a lightweight tripod as a do-everything system for birding, scoping and casual astronomy. What’s more, it’s all waterproof so no stress taking it camping (or leaving it out whilst making a cuppa).

 It’s a shame that the new Swarovisions don’t take it.

Highly recommended for SLC/Swarobright-EL binocular owners looking to do some casual astronomy or terrestrial scoping.