At the Huawei P9 phone launch in London's Battersea on Wednesday 6 April the Chinese company made a lot of noise about its new camera features. And for good reason: it's the only company to brandish Leica co-engineering and a unique dual camera arrangement (other makers have dual cameras, but not with one as a black-and-white-only monochrome sensor).

Sure, there was a Scarlett Johansson selfie as part of the promo, while Henry Cavill took to the stage to talk about something or other - whose autocue couldn't keeping up with his Superman speeds - but despite Huawei throwing its Hollywood stars up in the air the P9 is, first and foremost, aimed at the everyday user.

READ: Huawei P9 phone preview

So is its camera any good? Why would you want two cameras anyway? Does having one as black and white only make practical sense? We explore the ins and outs - plus the highs and lows - of the Huawei P9 camera experience.

Leica is the ultimate high-end camera maker, based in Germany. It's renowned for producing among the best optics in the world, while its cameras are typically hand-made (and usually cost an absolute fortune). Leica has a cult-like following and many classic images have been taken by photographers using its cameras.

The pairing with Huawei is about the new-world Leica; a company branching out, reaching the masses as the world of digital imaging evolves. And pairing its optical know-how with a phone maker certainly makes sense - as does spreading its name to a new, younger (and probably less affluent) audience.

The first obvious question: why the heck are there two cameras? They're here for a number of main reasons.

One, the two lenses (which are identical 27mm f/2.2 equivalent optics) can be used to perceive depth by offsetting one overlapping image to the other, which is useful for not only long-distance focus purposes, but also delivering relevant data for post-shooting depth adjustment via software - more on that later.

Two, each of those lenses has its own sensor: one is full colour, the other is monochrome (black and white only). And here's the Leica connection again: no other mainstream camera maker produces a monochrome sensor (a larger one featured in the Leica M Monochrom), which, thanks to an absence of an RGB colour array - used to calculate colour using a 4x4 red, green and blue grid per pixel - therefore has "pixels" three times larger than a colour sensor of equivalent size, translating as greater possible quality.

So here's a thing, who is ever going to shoot in black and white anyway? It's not 1955 any more, right? Why not just shoot colour and convert it with a filter after?

That's a valid point in many regards. However with the monochrome sensor there's potential for images with less image noise because, as we say, those "pixels" are that much larger.

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A monochrome sensor also has the potential for more dynamic range (ignoring that once data is lost to clipped highlights its irretrievable, colour conversions have more room available in the upper-end) and absolute tonal range, because it's not basing values on surrounding data, but absolutely on what is being seen.

But back to the big question: will it ever be used? Will we swipe across to the right on the P9's screen, select monochrome, then snap an image? It's going to be a personal taste thing. Once you have a monochrome-captured image you can't suddenly make it colour again. Are today's smartphone users really that savvy with B&W photography that this is a feature they'll not be able to live without? We doubt it.

Which brings us to the usability of the P9's camera arrangement. First up, the positioning of the cameras so very close to the top edge of the phone means, when in landscape orientation, fingers are perilously close to the lenses. Unless, that is, you use the physical volume buttons to snap an image, which helps keep your hand out of the way - but if you're right-handed this feels somewhat backwards (lefties will be all over it).

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Software-wise, Huawei was making it loud and clear that the "co-engineered with Leica" label means great things for software. Funnily enough, though, Leica isn't renowned for having the best or most usable menu systems; if anything it's renowned for having the most finicky ones.

And this shows in use with the P9's cameras: you need to swipe right to bring up a full-screen display of shooting options, selecting between Photo or Monochrome here. But it would make more sense to use an LG G5-like solution here and have an on-screen software button to switch between those two critical options (in the G5 it toggles between normal and super-wide cameras).

READ: LG G5 review

In addition, not all modes are available from this screen. To access "Pro" (i.e. manual controls) mode you'll need to swipe up/down on the shooting screen. To take advantage of the pseudo aperture control - which isn't based on a real, mechanical aperture, but applied using software - you'll have to be in the colour camera and not using said Pro mode.

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Which brings us to the whole aperture situation. It's become a real battle ground in phone cameras these days, with an f/1.7 in the Samsung S7, f/1.8 in the LG G5, f/2.2 in the iPhone 6S all vying for top spot. The smaller that number is the wider the opening and, therefore, the more light that can reach the sensor - which is useful for low-light shooting and, in certain focus situations, blurring the background. All useful things for creativity.

So Huawei's f/2.2 isn't class-leading, but it's pretty good - if it's good enough for Apple, right? What's not so good, however, is the option to apply a pseudo aperture effect - which can be done in the standard mode either pre- or post-shooting. It's something HTC has offered in the past with its M8/M9 handsets and something that, well, has never really worked that well.

Same goes here: Huawei, Leica, whoever, the software solution simply doesn't match-up to a real aperture solution. There are too many glitches. Sure, using two lenses on the P9 to obtain depth information is better than a single lens solution by far, but if the software messes up then the image just looks wrong.

And with an f/0.95 post-shooting option available - that's based on the famed Leica Noctilux 50mm f/0.95 lens - many shots will quickly become a dog's dinner, especially if there's a lot going on in the frame. You can see it in action in real-time pre-shooting, which is kind of fun, but after taking a shot you might notice a whole section that's blurred when it shouldn't be, for example.

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We've even had whole subject edges considerably distorted at "f/0.95", fake bokeh really blown out of proportion in white/highlight areas and, well, it's just best avoided. You won't find this aperture control in the Pro mode, because pros won't likely stand for it.

Sometimes though, just sometimes, it does work a treat. Have something close-up to the lens with nothing else close to the focal plane and it's easier for the two cameras to differentiate depth and blur accordingly. We shot in a bicycle shot and this worked well.

Stick to f/2.2, though, without fiddling around, and you'll be happy with what you shoot in the first instance.

We've mentioned the two lenses operating to assist focus, but that's not all Huawei is calling on in the P9. For closer shooting there's a laser-assisted focus, much like that of the LG G5.

As we concluded in our LG G5 vs Samsung Galaxy S7 vs iPhone 6S camera head-to-head, however, because it's laser doesn't always mean it's the very fastest - the Samsung is typically quicker.

READ: Best smartphone camera - iPhone 6S Plus, Samsung Galaxy S7 or LG G5?

But accuracy, on the other hand, that's where it's a winner. Click anywhere on the screen and focus is swift enough and accurate; the focus area is larger than the G5's, though, which makes it more generalised.

We also like the click-and-drag ability to offset the auto-exposure away from the focus point - a feature we've seen in plenty of Android phones, but that works well.

All in all, whichever focus option the Huawei P9 opts for, critically it's up to scratch against it's main competitors now. Hats off for that.

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Typically with Huawei cameras - and it's a trait of many Chinese brands - there's a big song and dance about so-called Beauty modes. At the launch event this wasn't the focus, but such modes - which blurs the face of blemishes, enlarges the eyes and so forth - do still exist in the Modes panel. As do other modes: from video, to slow-mo, time-lapse, HDR (high dynamic range), panorama, night shot and more.

That such modes have been pushed to the back leaves a greater focus on the camera interface itself. We like that it's easy to switch between manual and point-and-shoot, we like that the speed and quality is up there with the other flagships on the market, we like usability quirks like a horizontal level and grid overlay, and we like the ability to shoot raw files in addition to JPEGs.

But does having two cameras really make it better than one? You're paying for that monochrome sensor in there, the one that you'll probably never use, and the defocusing for pseudo wide-aperture effect is just as broken (while simultaneously being full of potential) as it was in HTC's iteration.

What people really want is a single, fast camera that can take great images, easily. Huawei has that here, but the distraction of the second lens and sensor does begin to partially detract from the goodness that's within too.