An ultra-high resolution Canon DSLR had been rumoured for some time, but with the company adamant that its cameras wouldn't exceed the 22-megapixel count, it's taken a considerable about-turn with the announcement of the EOS 5DS: a 50.6-megapixel full-frame DSLR.

Perhaps it was inevitable, what with Nikon making serious headway in the market with its 36.3-megapixel D800 and D810 models, for Canon to muscle in and attack not only that major rival, but medium format manufacturers too. At a secret product preview session ahead of the 5DS announcement Canon's reasoning was simple: "customers are asking for it".

Whatever the reasons, it doesn't really matter - because from what we've seen the EOS 5DS ought to be brilliant for certain photographers. We got to sample its mega-megapixel delights from a London rooftop overlooking Trafalgar Square to see what it's all about.

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First and foremost the 5DS is, essentially, a tweaked 5D MkIII: there's a more pronounced grip to the left-hand side for better two-handed hold (you'll need all the steady-handedness possible at this resolution), plus the inevitable "5DS" name and, if you squint and look really close, the "Canon" logo emblazoned across the front is now too.

That's what can be seen, but there's more going on beneath the surface. A new base plate and mirror box construction with mirror vibration control system have been added, again essential given the significant resolution. In terms of dimensions the 5DS and 5D III are otherwise one and the same, each weather-proofed body measuring 152.0 x 116.4 x 76.4mm.

The same 61-point autofocus system in 5D III is employed in the 5DS too, and even if the likelihood is that you won't be out and about shooting fast-moving sports images, up the available shutter speed and there's no reason that you couldn't. As we found with this autofocus system in Canon's other model, it's hard to find a bad word to say about it in the 5DS.

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In this new body, however, the ability to utilise 1.3x (30MP) or 1.6x (19MP) crop modes - which show marked-out crop lines in the 100 per cent field-of-view viewfinder - sees the positioning of those autofocus points fill the given frame yet more. At 1.6x the AF points fill almost the entirety of the frame.

With premium glass popped onto the front - we got to test the brand new non-fisheye 11-24mm, which is wildly wide-angle and ideal for landscapes - the camera shows its worth. We snapped a number of images that looked great having zoomed in on screen, but that given the camera's pre-production state we weren't allowed to take away to take a closer look at. Sob.

A closer look is exactly what the images from a camera of this type demands. As with the Nikon D810 or any medium format system, having such a significant resolution is great - but brings its own raft of issues. Getting a pin-sharp shot requires more precision in use, as any movement from such a highly concentrated resolution is amplified in the results, often meaning a faster shutter speed pays its due. And let's not forget: the 5DS has a 60 per cent higher resolution than the Nikon D810. Wowzers.

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That will leave some of you gleaming, others pondering the impact on higher ISO results. The 5DS caters for an ISO 100-6400 range as standard, expandable to ISO 50-12,800, and we suspect it's the lower end of the spectrum that will best match the target audience.

Impressively at full resolution it's possible to shoot at five frames per second (5fps), showing off the capacity of the dual Digic 6 processors, but the lack of 4K video capture therefore feels like an oddity. The camera could handle it, for sure, but we suspect the presence of a certain EOS Cinema range is what's keeping Canon held back in that department. Furthermore, with its stills-focused remit, the 3.5mm headphone hack has been removed in place of a USB 3.0 terminal for fast file transfer.

In our initial use of the camera, another stand-out feature is the newly customisable quick control screen. No longer do you have to have it Canon's way, but can select, size adjust, reposition and edit the various options across the given gridded structure.

This custom quick control option is buried within the menus, and many of the available scales are limited to "small/large" only, but it's a logical progression we'd like to see on other Canon DSLRs. Here, you may choose to move the autofocus arrangement to one corner, or even shrink its size to one grid square rather than four, but lay it over any other present option on the screen and it will delete what lies beneath, rather than nudging the arrangement around. Given most users will work out a suitable screen and leave it be this shouldn't matter, although what we would like to see would be multiple custom screen options to suit different shooting situations.

So there we have it, the daddy of the 5D range is the first DSLR to not only tip the 40-megapixel bracket, but dive headstrong into 50-megapixel territory. This is Canon carving out its spot in the high-end and, for some, will be an irresistible proposition. An expensive one too, given the £3,000 body only price tag. Lump in the 11-24mm and you'll need to add an extra £2,800.