Although there's obviously a wide range of opinions on the subject, I tend to follow the old-school train of thought that zoom lenses are necessary evils. They're much more convenient and accessible than fixed focal lengths, especially on reportage style jobs where timing is everything, but they can't match the bite, separation, or just the sheer tactility of a prime lens. Still, they're essential parts of every pro photographer's toolkit.
Over the first few years of the Micro Four-Thirds system, one of its defining characteristics was an abundance of great prime lenses in the normal to short-telephoto range, such as my much loved Olympus 45mm f1.8, or Panasonic's excellent 20mm f1.7 pancake, which became a staple walkabout lens. These primes fitted the early days of the system down to a tee, making good on the 'smaller, lighter' ethos which was the main draw of M4/3 and had never really been achieved by the legacy Four-Thirds DSLR system. Even today one of the advantages M4/3 has over other mirrorless system is the prevalence of these high quality primes, whereas other systems tend to be loaded with slow consumer zooms (Canon EOS M, we're looking at you).
As M4/3 matured though, the high end of the system grew aspirations to move into the pro space, with cameras like Olympus's OM-D range and Panasonic's GH models offering top-end features and build quality to match the best SLRs. if you're going to play in the pro leagues, you need pro-level glass to match your bodies, and Olympus to their credit realised that. Released alongside the OM-D E-M1 in 2013, the 12-40mm f2.8 was the first of Olympus's PRO series of lenses, designed as fast aperture zooms for the working professional, built to offer the quality and withstand the beatings such users expected. The 12-40 (equivalent to a full-frame 24-80mm) has since been joined by a 40-150mm f2.8, a 7-14mm f2.8 and 300mm f4, creating a full suite of wide to telephoto options.
The 12-40 was designed and released alongside the OM-D E-M1, so that's what I opted to test it on. First impressions were incredibly positive - the lens has a reassuring weight to it, probably due to the fact that almost every surface you touch is proper metal, a beautifully machined black aluminium finish. The fat zoom ring and slimmer focusing ring are both nicely smooth and well damped. Olympus are notoriously stingy on their lower end lenses, never including hoods and sticking to old-fashioned edge pinch caps, but that trend is defied here, as you'd hope for a lens which hits an RRP of £849. The cap is a lovely metal number while the hood is a solid, bayonet locking affair. Overall, the lens is one of the most well put together bits of photographic equipment I've ever used, oozing solidity and quality. Part of this might be because, as a PRO lens, Olympus advertises the 12-40 as extensively environmentally sealed - it's meant to be shock, dust, splash, and freeze-proofed to the same level as the E-M1 body. I haven't dared test it myself, but reports from braver photographers seem to indicate it's the real deal. Olympus has a pretty good history in this field too - the E5 famously bordered on being indestructible - so I'm confident that if I ever do have to take the lens into a hostile environment, it'll hold up OK.
One super neat feature that instantly makes the lens stand out from a lot of the more consumer level option available is what Olympus refers to as the 'manual Focus Clutch'. Like all M4/3 lenses the focusing ring is fly-by-wire (there's no actual connection to move the elements) and manual focus can be enabled my toggling to appropriate setting on the camera body, something I normally bind to a custom button. On the 12-40 however, you can pull back on the focus ring and it'll click into a new position. This automatically enables manual focus and reveals a distance scale, as well as giving the focus ring hard stops at either end (20cm and infinity). Pushing the ring back forward instantly returns to autofocus mode, but pulling the ring back again will remember your last point of focus, making it incredibly useful for pulling focus between too points, something videographers are sure to appreciate. The clutch also works in conjunction with the body so that functions like zoom and focus peaking are automatically enabled in the same way as selecting manual focus on the body would do. It's a really neat mechanism and a nice way to add some feel' back into the manual focusing process.
If you're thinking all this ergonomic goodness sounds too good to be true, there is a flipside, and it's size and weight. The 12-40 weighs 382g, which is nothing by normal standards but makes it something of a brick for M4/3. It's also pretty thick and long by the standards of the system, and when mounted to an E-M1 the entire thing approaches the size of an entry level DSLR. Of course, it's important to bear in mind that there's still a huge advantage when it comes to like-for-like comparisons. For example, the 24-70 2.8G I use on my full frame Nikons weighs more than twice as much (900g) and is noticeably larger in every dimension. The 12-40 balances really nicely on the E-M1, although on the smaller OM-Ds and PENs I'd recommend an additional grip piece to counter the front heaviness. Still, the point remains that you'll definitely feel this thing if you plan to lug it around everywhere.
Mechanically the 12-40 is a pretty complicated design, utilising 14 elements in 9 groups. There's a bunch of ED glass in there, and nearly every surface has some sort of exotic treatment, including Olympus's top end ZERO coating tech. I don't claim to be an expert or the technology, but whatever they're doing is clearly working - the 12-40 is one of the most optically stunning lenses I've used in quite some time. Even wide open at f2.8 it's razor sharp across the field, with only the very tiniest hint of softness at the extreme edges of the image. At f4 and f5.6 it's basically flawless, and only the beginning of diffraction prevents f8 being just as good.
Colour is richly saturated, without any major shift towards warm or cool, and the lens feels very natural and transparent in its rendering. Flare is very well controlled, and although you'll still need to stop down to avoid excess coma that's hardly a fault unique to this design. Being a 2.8 lens on a smaller sensor the 12-40 is not a bokeh bravo but it can still produce nice smooth backgrounds at the longer end, with very little evidence of the 'onion' shapes which often characterise aspherical elements, and none of the nasty jitter that can sometime be seen on the Panasonic 12-35 f2.8. Focusing is lightning fast in S-AF and continuous focusing on the E-M1 feels better than it does with some of the primes, though still not up to the standards of the mid-to-high end Nikons.
One unexpected bonus of the complex design is that the 12-40 has a near focus limit of just 20cm, even at the telephoto end. This makes it a surprisingly viable macro option, with a 1:3 reproduction ratio and all the optical goodness of regular shooting concentrated down. It's never going to replace a dedicated lens but again, the purpose of a zoom, especially in this range, is flexibility, and the macro aspect adds yet facet to lens's multi-talented nature.
In fact, I'd probably go as far as to say that the 12-40 represents the very best one lens solution currently available for the system. As a Swiss Army Knife for photography it's pretty much unparalleled. Pretty much the only things you can ding it for are size and price, and fast zooms of all shapes and sizes tend to suffer from both of these issues - it's just the nature of designing such an optic. Considering I use my M4/3 cameras mostly for travel and street work the 12-40 seems an excellent option to replace a couple of primes, and paired with a lightweight telephoto option (I've got the cheap 'n' cheerful 40-150mm on order) it could be the perfect portable pairing. Highly recommended.