The Sony RX10 III was announced in March this year, less than a year after theRX10 II which, according to Sony, the new camera doesn't replace. The most noticeable difference is the inclusion of a variable aperture (f/2.4-4) mega zoom lens, with a reach of 600mm (25x optical and 100x digital zoom), and with this comes a substantial increase in size and weight.

Like the Mk II, the RX10 III is aimed at the serious enthusiast end of the market – photographers looking for ultimate image quality and telephoto reach in a bridge-style package.

The RX10 III has the same 20.1 million pixel 1-inch stacked Exmor CMOS sensor, and offers the same excellent 4K video functionality, as the Mk II. In fact most features and functions are identical, and the main talking point is the large and impressive Zeiss 24-600mm lens. Only the Canon G3 X has the same reach, if we compare other bridge cameras with 1-inch sensors, with the much olderPanasonic FZ1000 only going to 400mm.

The variable aperture of f/2.4-4 still makes this a pretty fast lens, certainly compared to the competition, and it boasts hugely impressive minimum focusing distances of 3cm at the wide end and 72cm at the long end.

As with other Sony compact and bridge cameras, a large range of shooting options and photo modes are included, with 3:2, 4:3, 16:9 and 1:1 formats for both raw and JPEG images. Maximum resolution is 5472 x 3648 pixels in its native 3:2 format. Sensitivity ranges from ISO100-12,800, expandable to ISO64-25,600.

Single shot autofocus (AFS), continuous (AFC), direct manual focus (DMF) and full manual focus (MF) are available, with Sony's very effective focus magnification and focus peaking options making the latter easy to use.

The EVF and tilting rear LCD are both high quality; the latter isn't a touchscreen, but unless you're using the camera on a tripod a touchscreen wouldn't be particularly practical given the size and weight of the camera.

There's no getting around the bulk of the Sony RX10 III. It's very well made and comfortable in the hand, but I suspect many more casual users will be deterred by its size (and price) and, if they want such long telephoto reach, may opt instead for the Canon G3 X or possibly the Panasonic FZ330/300, which has a smaller sensor but also offers 4K shooting, and impressed us last year. The Canon suffers from a slower lens and no included EVF by comparison, but the FZ330/300 does benefit from a constant f/2.8 aperture, albeit with a much smaller sensor.

However, if you want the ultimate image quality in a bridge camera, it's hard to look beyond the class-leading performance of the RX10 III's Zeiss Vario-Sonnar T* lens, and I'm sure enthusiasts looking for more reach and a highly specified camera will be attracted to this. Whether they'll be prepared to pay the £1,250 ($1,500, AU£2,150) price tag is another matter. That price is likely to fall post-launch, but at the moment it's around double the cost of both the G3 X and the FZ1000 (the FZ330/300 is cheaper again). That said the RX10 III is in a league of its own in some respects, so it's likely to find a market, albeit a niche one.

The first thing I noticed when I took the Sony RX10 III out of its box was just how large and weighty it is compared to other bridge cameras I've used. At a little over a kilo in weight (1050g or 2.3lbs with battery and SD card) it feels more DSLR like in-hand. It measures 132.5 x 94.0 x 127.4mm (5.25 x 3.75 x 5.12 inches), and feels substantially larger and heavier than the likes of the Canon G3X. Much of this is down to the sizeable glass of the 24-600mm equivalent, 72mm diameter lens.

Although plastic-feeling, the build is reassuringly solid and its generous, protruding-front grip and rounded body style means you're able to grip the camera securely and comfortably. Those with small hands may find the grip a little too large, but it feels in proportion to the size of the camera. I found the ergonomics very good, with buttons and dials falling in the right place for my hands.

The on/off switch is toggle-style, and is incorporated into the shutter button on top of the front grip; it's easy to access and clicks reassuringly. A lever to the front of this allows you to zoom the lens. The shutter button is also threaded, meaning you can use an old-style cable release. The RX10 III powers up pretty quickly, although there's inevitably a short delay while the lens extends to its start-up position of 24mm.

The mode dial (left) and exposure compensation dial (right) both have a textured side and just the right amount of resistance, making them easy to adjust precisely. Two customizable function buttons, an LCD light and the pop-up flash button are also included on the right, and behind these is a small LCD panel that displays key settings. The small but solidly built pop-up flash and hotshoe complete the top plate layout.

An aperture ring is included to the rear of the lens, and can be de-clicked for video use. There are also two textured rings for zoom and focusing. I found it easier to zoom in small increments using the lens ring rather than the lever on the front grip. I found the focusing ring responsive and easy to use, with a conveniently located focus hold button to the left of the lens. A focus mode switch is included on the bottom left of the camera body; I found it slightly fiddly, but quick access to this feature is welcome.

Much of the rear of the RX10 III is taken up by the impressive 1,228,800 dot, tilting 3-inch LCD screen. The XGA OLED electronic viewfinder is excellent and boasts a 2,359,296 dot resolution. It's responsive, clear and bright, and works well with glasses. It has 0.7x magnification, an eye sensor to allow automatic switching between EVF and LCD, and a generous diopter adjustment. A lens hood is included, as well as a pinch-style lens cap.

Menu and movie buttons sit either side of the EVF. The rear control dial and control wheel operate smoothly and are easily accessed, enabling you to select and adjust various menu items and settings. The control wheel and center button also enable you to change the placement and size of the focus area when using flexible spot AF. AE lock, quick function, playback and delete buttons complete the line-up; the latter can also be used as a third custom function button.