It’s hard to see the new iMac with Retina 5K display as anything other than a breakthrough in desktop computing.

We’ve only had Retina-class displays since the iPhone 4 was launched in June 2010. Defined as pixel density so high the human eye cannot see any single pixel at the normal operating distance, this innovation later transferred to the iPad in March 2012, and then Apple’s notebooks – aptly if long-windedly named the MacBook Pro with Retina display.

[There's a new Retina iMac that's even cheaper, read our preview here: 3.3GHz Retina iMac review]

A 13-inch or 15-inch display may be a perfect size compromise for many laptop users that need to carry their computing with them. But if you need to build a more permanent desktop workstation it makes sense to give yourself a large and comfortable canvas. And with this new iMac model, that Retina canvas now stretches to 27 inches along the diagonal.

The style and design of the new iMac is identical to that introduced two years ago, when Apple slimmed the screen edge to a vanishing 5 mm; and more importantly removed the obstructing sheet of front glass to radically reduce reflected glare from the glossy panel. The top surface of the bonded TFT/glass assemble is also treated to a thick anti-reflective coating, also used on the MacBook Pro with Retina display; and now also the iPad Air 2.

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iMac versus Mac mini

This new Retina-class all-in-one PC shifts resolution from an already high 2560 x 1440 to a staggering 5120 x 2880 pixels. Alongside the upshift in the number of pixels packed into the panel comes improvements to the construction of the oxide thin-film transistor (TFT), and the compensation film above, to regulate how light exits the panel. This is to ensure that when viewed from an angle the image remains consistent, as we’ve now come to expect from IPS displays.

A new Intel processor powers the iMac with Retina display, a quad-core Core i5, and this can be configured if required with an Intel Core i7 running at 4.0 GHz. As standard you get 8 GB of system memory, which can be user replaced up to a maximum of 32 GB.

As standard you get a Fusion Drive of 1 TB capacity comprising PCIe flash drive with 3.5-inch SATA disk. Or you can opt for pure flash memory, either 256 GB, 512 GB or 1 TB.

The graphics processor that powers the new screen – and additionally up to one more connected 4K display – must be capable of delivering enormous amounts of graphics data bandwidth. In contrast to last year’s nVidia selection, Apple has this time chosen the AMD Radeon R9 M290X graphics processor, coupled with 2 GB of video memory.

The R9 series represents AMD’s top consumer graphics cards for desktop PCs, but look carefully for the ‘M’ prefix here which denotes that this is a mobile processor, integrated on to the logic board and originally designed for high-end laptops.

Other specifications for the iMac Retina 5K remain the same as last October’s refresh, although the two Thunderbolt ports have also received a welcome upgrade to Thunderbolt 2 standard.

Thunderbolt 2 doesn’t deliver any more net speed than the original Thunderbolt port. But it does conveniently bond two existing 10 Gb/s full-duplex channels within each cable into a single high-speed 20 Gb/s line.

Price of the iMac with Retina 5K display starts at £1999 in the UK. Substituting the 4.0 GHz processor brings the price to £2199, and the higher-spec AMD graphics another £200 again. If you want to max out the spec with best CPU, GPU, RAM and storage, you’ll need to get ready to hand over £3519 for what promises to be the most irresistible all-in-one available anywhere. [A new iMac may be set to launch this summer. Read about the 2015 iMac release date here.]

Apple iMac with Retina 5K display review: Lab Test

As we’ve seen with other recent iMacs with quad-core processors, the entry-level iMac with Retina 5K uses a straightforward 4-core/4-thread Intel Core i5 processor. Contrast that with the 15-inch MacBook Pro range, and the previous generation of Mac mini, which use Intel chips with Hyper Threading Technology. With Hyper Threading, twice as many threads as physical core can be processed together, turning a quad-core into a virtual eight-core computer, for example.

This preamble is to explain why, in the Geekbench 3 test of raw processor and memory speed, the multi-core results of this Retina iMac are slightly behind those of the current best MacBook Pro. And that’s despite this iMac using a desktop-class chip that’s clocked a whole 1.0 GHz higher than the 2.5 GHz Retina-display MacBook Pro notebook.

In single-threaded operation, the iMac’s score of 3877 points was superior to the 3658 point score of the 15in MacBook Pro 2.5GHz.

But in multi-thread mode, the iMac scored an average of 12,418 points, while the MacBook Pro could stretch as high as 14,360 points.

Cinebench results were good, if again trailing the MacBook Pro with all its hyper-threaded cores blazing. Version 11.5 of Maxon’s graphics rendering test returned results of 1.64 (single-core) and 6.15 points (multi-core). The MacBook scored 1.55 and 6.48 points here respectively.

Using version 15 of Cinebench the iMac scored 143 and 544 points (MacBook Pro 134/599 points).

It’s swings and roundabouts in overall processor performance, depending whether you’re focused on a mono- or multi-threaded operation. All told and balancing the single-core and multi-core modes’ relative speeds, we’d say that in overall CPU and memory speed this iMac compares very closely with the Core i7-powered flagship MacBook Pro.

iMac or Mac mini - Mac desktops compared

iMac versus MacBook Air

Apple iMac with Retina 5K display review: Graphics performance

With 14,745,600 pixels to coordinate – in short, around 14.7 megapixels – the graphics subsystem of the iMac Retina 5K has got its work cut out.

To power this new panel Apple developed a new timing controller (TCON) with LCD silicon specialist Parade Technologies. The DP665 TCON employed here is said to have four times the bandwidth of the previous model, which itself was channelling four lanes of embedded DisplayPort 1.3, in order to handle 40 Gbit/s of data flow.

As with all other Apple products with Retina-class displays, the iMac display is used in HiDPI mode, where the native horizontal and vertical resolutions are exactly halved as far as the user is concerned. So here the 5120 x 2880 pixel counts are rendered on-screen like a 2560 x 1440-pixel display.

That 2560 x 1440 resolution is the same size as the previous 27-inch iMac models, and so the true 218 ppi pixel density appears to the user like the 109 ppi desktop of the non-Retina iMac – but with all screen fonts and graphics effectively rendered much sharper.

We started with the pro workstation test of graphical horsepower, Cinebench 11.5, which returned a result of 45.1 fps in its hardware-accelerated OpenGL test. That’s fractionally behind the 48.2 fps result of our benchmark MacBook Pro.

Stepping up to the current version 15 of Cinebench, the iMac’s 91.7 fps put clear space between it and every other Mac we’ve tested in the last few years. And that’s including the Mac Pro (Late 2013) which played out at 87 fps in the same test.

Modern Mac action games presented no problems to the iMac Retina 5K. With the Batman: Arkham City game from Feral, we often have to creep up slowly to a MacBook’s native resolution, with framerates dropping down into the unplayable level as we do.

Not so here, where the iMac could play at full-HD 1920 x 1080 resolution and High details settings, with an very capable average framerate of 89 fps. We pushed the screen resolution up to its intrinsic HiDPI figure of 2560 x 1440, and the framerate barely sagged to 85 fps.

Turning to Tomb Raider (2013) and its OpenGL port for the Mac, we found that with full-HD and High detail settings the game could play at close to the limit set by the 60 Hz V-sync function (switchable but on by default in this game). The average framerate was 59.4 fps.

So we again ratcheted up the resolution to 2560 x 1440, sticking to the High detail preset, and saw a result of 46.1 fps. Unlike the Windows version of this game, which has Ultra and Ultimate settings above High, there’s nowhere to go in increased detail with the Mac game.

Spurred on by these great results, it seemed the iMac was up to the challenging Unigine Heaven synthetic graphics benchmark. This often brings Apple Mac graphics processors to sub-20 fps framerates at anything other than very low resolutions and details; but at a Retina-mode resolution (2560 x 1440 pixel) it averaged 29.4 fps at Medium detail, and 29.3 fps in High detail mode.

If you want to try with better detailing, you could try Ultra mode as we did, and engaged V-sync to remove any visual tearing. Tried thus, it played through at a smooth 50 fps.

Apple iMac with Retina 5K display review: Display quality

Impressive to the naked eye, the display of the iMac Retina 5K also turned in some of the best numbers our humble Spyder4Elite colorimeter has ever measured.

As you might hope, but where so many consumer products fail to deliver, the display was found to have a full 100 percent coverage of the basic sRGB colour gamut. Even IPS display technology often falters here and struggles to achieve even 90 percent coverage. And from the Adobe RGB colour space, the iMac recorded 78 percent cover.

Full brightness from this panel was searingly bright at 445 cd/m^2. We can’t see that high output being healthy to long-term use unless you had to take the Mac outside on a bright sunny day. And at its full output it also gave the highest contrast ratio we’ve measured in the standard chequerboard test, returning a figure of 1160:1.

That may not sound so impressive when many monitor and television brands routinely bandy numbers like 10,000:1 or even millions to 1. But the difference is that our figure is closer to a genuine measure of contrast ratio, rather than a made-up figure that involves ‘dynamic’ trickery.

Down at a more comfortable brightness setting – a nominal 50-percent figure corresponding to 220 cd/m^2 – contrast ratio was still a very impressive 1060:1.

Luminance uniformity across the huge 27-inch panel may seem a little poor, peaking at 20 percent, but in practice this is typical of most displays and barely discernible.

Colour accuracy with a test of 48 spot tones gave an overall average of just 1.66 Delta E, a terrific result.

Peering up close we were unable to see any individual pixels. Stood a little further back, it really is photographic, like the richest, most detailed images you'd find in a photo book. And crucially, we didn’t detect any obvious refresh issues with moving images. We tried some fast-paced native 3840 x 2160 (‘4K’) footage, and this rendered cleanly with no visual smearing.

Apple iMac with Retina 5K display review: Power

Power consumption for this cutting-edge display and the computer that drives it was surprisingly low. The peak figure with screen at maximum brightness and CPU and GPU both under benchmark stress was 215 W. This fell to just 46 W at an idle OS X desktop with no user applications running, and screen set to 200 cd/m^2.

Noise levels became conspicuous at full tilt, particularly in graphics tests, with all hot air seeming to exhaust from a slated hatch just under the screen hinge at the back. In normal use though, even with a 3.5in SATA disk comprising part of the flash/HDD hybrid storage solution, the iMac Retina 5K was suitably near-silent in operation.

Apple iMac with Retina 5K display review: Storage

Our sample was the entry-level model with 1 TB Serial ATA hard disk and 128 GB solid-state drive attached by PCIe. Specifically, these were a 1 TB Seagate Barracuda HDD and 128 GB SanDisk SSD, united by Apple’s Core Storage technology in OS X to form a hybrid 1.1 TB Fusion Drive.

We were unable to measure the speed of the flash component alone, so resorted to testing just the hard disk. Unlike the slow 2.5in notebook drive in the 21.5-inch iMacs, this 3.5in disk is surprisingly speedy. When empty it could reach sequential read/write speeds of 160 MB/s. Smaller files averaged 24 MB/s and 36 MB/s, for random reads and writes respectively and with files sized from 4 kB to 1024 kB.

But of course these figures will not represent actual use as first writes are made to the flash section of the Fusion drive; and commonly used files and apps remain on this high-performance silicon drive for extremely fast reads when required.

Verdict

There is no desktop computer like the iMac. That was true before it gained an ultra-high resolution display, and it’s quite literally doubly true now. While the rest of the PC industry is still issuing low-grade and low-resolution displays with its PCs and laptops, Apple has spotted a key area for improvement and continues to raise screen quality to new heights. Meanwhile the few Windows PCs that have exceeded full-HD resolution displays are hamstrung by an operating system that cannot consistently cater for the tighter pixel pitch. More than just a great display, the iMac Retina 5K has a well-balanced computer driving it, with plenty of performance to power through games and any everyday task. Creative professionals on a budget, whether editing photos or videos, will also find plenty to like here, and at a sub-£2000 price point considerably more affordable than a PC workstation and separate 5K UHD monitor.

Read also: Apple iMac (21.5-inch, Mid 2014) with Fusion Drive review

Wondering which Mac to buy? Read: The best Mac to buy in 2014 (which we'll be updating soon!)

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Karen Haslam's first look review of the Retina iMac follows on the next page.