Apple iMac (Retina 5K, 27-inch, Late 2015) review

27in iMac 27in Late 2015 - front

As satisfied users of Apple products with Retina displays will already know, screen image quality is stunning. Ultra-high resolution panels here mean fluid typography at familiar and readable sizes — plus of course the vaunted razor-sharp photography — leaving these devices with an art print-quality interface.

Expectations changed forever the day the iPhone 4 and its 326 pixels-per-inch (ppi) screen landed in 2010. The iPad, then MacBook Pro, received the HiDPI treatment in 2012. And the big 27-inch iMac went Retina just late last year. So now we have ‘pixelless’ Retina displays across almost the full range of Apple phones, tablets, watches, laptops and desktop PCs.

(The notable exceptions are the MacBook Air series; and Apple’s only remaining separate PC monitor, the unloved Thunderbolt Display.)

But when it comes to the user experience there’s always room for improvement, and now Apple is expanding image quality in another direction — by increasing the display colour gamut. This simply means that a wider range of colours can be reproduced, bringing us closer to the reality we see around us with our open eyes. Relatively few people probably realise just how short-changed we are by most LCD devices’ displays.

27in iMac 27in Late 2015 - side

The new Apple iMac (Retina 5K, 27-inch, Late 2015) — along with its newly Retinafied 21.5-inch junior — now benefits from a wider-gamut LCD panel. One which has talent outside the limited sRGB colour space.

With luck, following most other Apple innovations, the rest of the tech industry will scramble to catch up with this welcome progress. It should mean that some of the rich colour reproduction that was taken for granted in the CRT age, sadly but universally bleached out with the arrival of flat-panel LCDs, may finally be restored to the computer interface.

Before focusing on the new display though: a recap of the computer to which it’s been wedded.

What is this iMac of which you speak?

The iMac is the leading all-in-one personal desktop computer, Apple reinventing the category in 1997 with its unmistakeable Bondi Blue one-box wonder; you could say it was the premiere launch product for the ‘new Apple’, led once again by Steve Jobs with his return to the company.

The aim then as now was to make desktop computing simple. In the mid-1990s, most people’s idea of a PC was a random collection of beige boxes knotted together with arcane cables. Accompanied by a tower of floppy disks to load new software or to exchange data between PCs.

That kind of modular PC approach still exists if you need it. Hard-core PC users, like hi-fi enthusiasts before them, swear by the flexibility and performance available by building a system out of a selection of separate units, sometimes assembling the complete PC from the motherboard upwards.

Normal people on the other hand run away from such complexity. They want a plug-and-play solution that gets the job done. And better yet if it’s actually styled to blend into their living room, rather than turn it into a geek’s basement.

The cute ‘n’ fruity iMac of ’97, just like the latest knife-edged iMac of 2015, did just that. They both represent computers you’re unlikely to resent sharing personal living space with. They both require the bare minimum of setting-up when removed from the box.

The original iMac required you to plug in a mains cable, telephone line, keyboard and mouse. Today, through the miracle of short-range microwave radio — Wi-Fi and Bluetooth — we only need to find a power socket.

The iMac’s role has changed somewhat over that time. Where once the desktop PC was the everyman gateway to the world-wide web, it’s now been sidelined for many by, ironically, products that Apple subsequently pioneered, such as the multi-touch smartphone and the tablet.

Early iMacs with their titchy 13.8-inch CRT screens were never powerhouses of computing; instead they were used as a home workstation, for basic office productivity and light creative duties like photo editing, perhaps even a little DTP work.

Over time the iMac evolved physically, first gaining an angle-poised flat-screen display, then with all its electronics shoehorned just into the screen; all the time benefitting from advances in mobile computing technology to allow increased performance while, for the most part, containing heat issues.

Today’s iMac can stand alongside tower PCs in raw computing terms, making it a popular choice in test labs, publishing houses and design studios that also demand quick computers. That’s never more true than with the new incarnation, launched 13th October 2015, thanks in part to the supporting role of new components from the likes of Intel, AMD and Samsung.

Back to your (i)Mac

Focusing on the 27-inch Apple iMac, there are three off-the-peg versions available, priced in the UK at £1449, £1599 and £1849. (Note that the top model here is still £150 cheaper than the first iMac with Retina 5K display that launched October 2014.) Each of these iMacs is based on exactly the same chassis and screen, and differ only in the choice of processor, graphics, storage and memory.

Apple loaned us a sample of the latter ‘best’ model at £1849, although it and any other model can be pushed further in performance (and cost) through a Configure To Order (CTO) option. This is only available if you order from the online Apple Store.

The model under test here features a 3.3 GHz Intel Core i5 quad-core processor, 8 GB of memory, a 2 TB Fusion Drive, and AMD Radeon R9 graphics with 2 GB video memory.

See Pea You

Besides the tone-expanding new display, the stand-out upgrade for Late 2015 iMac is the choice of central processor. It has adopted a 6th-generation Intel Core i5 chip from this season’s series. Codename: Skylake. This one’s hot off the fabrication line.

The processor should usher in more efficient operation again — doing more work using less energy and creating less heat — bettering the 4th-generation ‘Haswell’ CPU found in the previous Retina 5K iMac.

Intel shrank the silicon process size in its subsequent ‘Broadwell’ series of early 2015, from an already impressive 22 nm down to 14 nm, in what what it calls a Tick upgrade from its biennial clockwork schedule. That shrink to 14 millionths of a millimetre proved to be a quite a challenge for Intel, if Broadwell’s year-late arrival is anything to go by.

As it turned out, Apple skipped Broadwell — in this iMac model at least — and seized upon the Tock upgrade of Skylake. This sees an optimisation of the 14 nm process through a rejig of the microarchitecture. The result? A processor that should be able to do more than the last generation at the same gigahertz rating; or complete as much work as before while operating at a lower clock speed.

The specific processor chip employed here is the Intel Core i5-6600, a straightforward four-core chip. That is, it does not receive the benefit of Intel’s Hyper Threading Technology that we find on every other Apple Mac, and indeed the Core i7 CTO versions of the iMac.

It has a baseline frequency of 3.3 GHz, and because it’s ratcheted relatively high already, there’s less headroom for any additional Turbo boost. (For reference, most Macs now run at a baseline clock between 1.4 and 2.9 GHz.) But when required this Core i5 can dynamically overclock for short bursts up to 3.9 GHz.

Memory Four

After years of cruising at 1600 MHz in most PCs, random-access memory has finally been given a dust over with the introduction of DDR4 RAM, seeing a memory clock increase here in Skylake to 1866 MHz. Processor and memory work together intimately in just about every operation on a computer, so any uplift in RAM speed should be measurable across the whole system.

27in iMac 27in Late 2015 - vent

Behind the support pillar, a grille for ventilation and hatch below for memory upgrades

We can see the compounded effect of new processor and faster memory clearly enough by referring to results from Geekbench 3, which quantifies CPU integer and floating-point maths solving as well as memory stream performance.

Handy for these comparisons, the baseline CPU clock frequency of the prior Haswell-generation iMac — Apple iMac (Retina 5K, 27-inch, Mid 2015) — was also 3.3 GHz, albeit with a slightly reduced Turbo capability to 3.7 GHz.

The headline difference in Geekbench results between that 4th-generation Haswell and new 6th-generation Skylake iMac is a circa-11 % higher overall score.

Specifically, the multi-core average moved from 11,792 to 13,096 points (10 % increase) while the single-core movement was slightly greater, 3699 to 4223 points (12 % increase); perhaps helped here by that extra Turbo headroom which typically provides more assistance to single-threaded operations.

Focusing on individual areas in the Geekbench 3 tests, multi-core integer and floating-point performance rose by around 9-11 %, while memory saw a 13 % score increase.

For multi-core operation, differences were slightly more marked: 11-12 % in integer and FP tests and again 13 % in memory score.

The Cinebench 15 benchmark revealed larger differences again between old and new, which may be of particular interest to creative professionals that rely on this benchmark, based as it is on real-world scripted rendering tasks.

The previous iMac scored 134 points; or 515 points with four cores on the case. Today’s new iMac returned figures of 164 and 596 points respectively. That translates to around 18 and 14 % score increases, which are all nice nudges in the right direction.

27in iMac 27in Late 2015 - mics

Gee Pea You

The M in Radeon R9 M395 denotes that once again Apple has plumped for one of AMD’s mobile graphics processors. The reasons are not hard to find — GPUs destined for laptops are physically smaller in size, draw less power and emit less heat than their full-size counterparts designed for traditional tower PC boxes. The downside is that you’ll never see quite the performance as you will from an old-school slot-in graphics box.

And once again, confusion reins over what exactly goes into these AMD graphics processors, since many of the available GPU options for the Retina 5K iMac are not available to the wider Windows world; as we saw with the 2013 Mac Pro, Apple is using bespoke graphics processors from AMD, with a dearth of any detailed specifications.

Without getting into speculation about the core architecture and its designation, the number of stream processors, shaders, memory clock speeds and pipe widths, let’s instead look at the results from some graphics benchmark tests.

We’ll compare the original iMac with Retina display, the mid-2015 revision, and this late-2015 model. These run as follows:

AMD Radeon M290X with 2 GB

AMD Radeon M290 with 2 GB

AMD Radeon M395 with 2 GB

The first thing to note before we riffle through the numbers is the subjective experience. When simple fielding a 14.7 megapixel display (5120 x 2880 pixels), each of these iMacs maintained a smooth and slippery graphics interface for OS X; and note that that’s Yosemite for the first two, El Capitan for the latest. Whizzy effects like Mission Control, window minimising and rolling dialogue sheets gave no hint of visible friction or stutter.

Intel’s integrated graphics, the top-notch Iris and Iris Pro editions at least, could probably achieve the same. But the graphics processor has more work to do than paint the user interface on the screen, with two more taxing roles that are of interest to many of today’s domestic and professional users: playing games and accelerating performance in certain software applications.


Playing Batman: Arkham City for Mac, configured for full-HD (1920 x 1080 pixels) and High detail, we recorded fast and fluid rendering: 88 fps, 84 fps and 85 fps respectively for M290X, M290 and M395.

Stepping up to 2560 x 1440 pixels — exactly half the native resolution and the apparent size of the user interface in preferred Retina mode — showed a clearer gap between the original GPU and its newer replacements, favouring the former: 75 fps, then 66 fps and 65 fps.

And what about at native 5K resolution? Apple’s OS X doesn’t make it easy to use a Retina-screen Mac at full resolution, and few people would want to anyway since it makes the interface microscopically small. We did try Batmark on the latest iMac with Retina 5K display anyway, and saw average framerates of just 20 fps. Then again, most full-size graphics cards struggle at 4K, let alone 5K resolution.

Tomb Raider benchmarks told a slightly different story. Running again first at 1920 x 1080 pixels and the High detail preset, we recorded average framerates of 71 fps, 65 fps and 75 fps. Stepping up to 2560 x 1440 pixels, the hierarchy continued: 46 fps, 42 fps and 51 fps.

So in the Batman game, the original M290X was the clear winner with latest M395 up to 10 fps behind, while conversely Tomb Raider showed an advantage of around 5 fps to the newer M395 graphics.

Turning to Cinebench 15’s GPU-based OpenGL rendering test, there was also a small gap between them all, with ultimately the latest M395 just in front. In chronological order again, we measured 89.4 fps, 90.4 fps and 94.9 fps.

The takeaway message from these results has got to be that there’s precious little difference between all these graphics processors overall; and that they’re all quite capable of great gaming results up to Retina-mode (2560 x 1440) resolution.

There’s but one GPU available for the Retina 5 K iMac that may better these results, the CTO option of M395X. At an additional cost of £200 the specs only list double the GDDR5 video memory, but we’ve no idea if there may be any other differences such as raised memory clock or more stream processors. Apple could do better to communicate such specifications rather than let potential buyers trust that, in this example, the added X suffix means your £200 is buying more than just a trifling 2 GB of memory

Solid-State Universe

No other personal computer company has led the way in PC storage technology like Apple over the last two or three years. It put to use the fastest flash technology available, years before the rest of the industry, as well as finding a way to make older hard disks slightly less pedestrian using the foolproof hybrid flash/disk combination it calls the Fusion Drive. The iMac comes with either a single disk (boo), disk-plus-flash Fusion Drive (yay) or pure PCIe-attached flash (way hay!).

The first MacBook Pro with Retina display featured a new style of SSD, resembling an 80-mm M.2 card, only with a modified edge connector proprietary to Apple. Like the mSATA card found in earlier MacBook Air models, this greatly reduced the space required to store the storage. Its SATA Revision 3.0 connection was good for around 500 MB/s sequential transfers.

But the real breakthrough was found in the 2013 MacBook Air and Pro revisions, the world’s first notebooks with PCIe-attached flash drive. First using two lanes of PCIe 2.0 to break the SATA speed barrier, these have now given way in current 15-inch Pro models to four-lane PCIe 3.0. These can now comfortably reach beyond 2000 MB/s read speeds, while possessing the essential small-file processing speed that makes the Mac feel so responsive.

There’s another new development Apple pioneered earlier this year, the first to market with the latest NVMe protocol to replace AHCI (in the 2015 MacBook with Retina display). The new iMac with Retina 5K display does not feature an NVMe SSD in the flash portion of the hybrid drive however; that may well be an engineering nightmare to combine disparate storage interface protocols in a single hybrid drive.

The 2 TB Fusion Drive in this iMac comprises a 128 GB SSD (based on the Samsung SM951, using SATA-based PCIe 3.0 x4) and a 2 TB HDD (3.5-inch Seagate Barracuda SATA disk drive).

For the most part this seemed to give SSD-like performance when booting, launching applications and working on files. However we did notice some occasional disk-like lag; opening System Preferences took several seconds on one occasion, although subsequently launches opened in a flash.

Without breaking the low-level OS X Core Storage combination of the two drives it’s difficult to bench test just the flash component. But our earlier tests of the 512 GB version of this SSD in this year’s MacBook Pro (Retina, 15-inch, Mid 2015) showed it to be the fastest notebook storage in existence at the time, with 2.0 GB/s sequential reads and 1.5 GB/s sequential writes.

We did benchmark the Seagate disk in this iMac, and found it capable of recent and decent speeds, by HDD standards anyway — around 170 MB/s sequential reads and writes. But small-file random reads and writes from 4 kB to 1024 kB only averaged 23 and 35 MB/s repectively; slow but more typical for disk technology.

Less Wire

Wireless networking technology is easily overlooked in today’s PCs. Most potential buyers just assume that Wi-Fi is present, while the more canny might see which revision of 11b, 11g, 11a, 11n or 11ac is available.

Beyond the basic protocol there can be a world of difference in performance though, depending on how many wireless streams can be utilised. The most basic PCs, laptops and handheld devices use single-stream tech with but one antenna. The best Wi-Fi technology available to Windows PCs we see today is still two-stream only; Apple is the only PC brand to specify full 3×3 MIMO wireless networking in most of its personal computers.

You’ll find 3×3 802.11ac Wi-Fi adaptors in all MacBook Pro, Mac mini, Mac Pro and iMac models; all based on Broadcom chipsets.

The benefit is increased wireless throughput and greater range, when communicating with a three-stream wireless router. In contrast, many modern wireless routers feature three antennae for 3×3 operation, including of course the Apple AirPort Extreme Base Station. The iPad and MacBook Air meanwhile include 2×2 Wi-Fi adaptors.

In practical terms this means most Mac users can expect to see connection (‘sync’) speeds of 1300 Mb/s with 11ac, 450 Mb/s using 11n on 5 GHz, and 217 MB/s for 11n on 2.4 GHz.

With a two-stream solution, these figures fall to 867 Mb/s (11ac), 300 Mb/s (11n on 5 GHz) and 144 Mb/s (11n on 2.4 GHz).

Colour me impressed

Few if any other consumer electronics brands were taking display quality that seriously until Apple upped the game, notably with the first Retina display devices. One reason seems to be that most PC buyers are driven by chasing specs, and few brands publish more than the pixel resolution of their displays, leaving potential buyers to obsess about processor and memory before being left to stare at a downright lousy screen image.

In the Windows world, manufacturers still routinely pass off 15-inch laptops with 1366 x 768-pixel displays as ‘HD’, when nothing could be further than the truth. Such display have only 100 pixels per inch to render the UI, resulting in rough, bitty text and furry images.

But there is a lot more to a good picture than cramming in pixels, even if that is a good start. Colour quality has eroded since the very first iMac, as the industry as a whole started to migrate to flat-panel TFT displays in the late 1990s.

Early generations of LCD had terrible response times, leaving trails behind moving images. Colour gamut was shrunk to a small fraction of the already downsized sRGB colour space agreed as the PC standard in 1997. Viewing angles were severely limited, such that looking from more than a few degrees away from perfectly on-axis provided at best a faded image or sometimes a negatised inversion on the screen. And contrast ratios plummeted, making screen images seem flat and washed out.

Many of these limitations were addressed in time, especially with the introduction of in-plane switching (IPS) LCD technology in the early 2000s. But these had limited appeal while the price was so much higher than budget LCDs using twisted-nematic (TN) technology. The IPS screen — along with similar but less popular variants like vertical alignment (VA) — offered hope, and has allowed us to view again from any angle, experience contrast ratios up to around 1000:1 and see all colours within the sRGB colour space.

That sRGB colour space is only a small proportion of the spectrum of light that we can experience though, even if it contains enough to satisfy everyday online duties. Graphic designers and photographers who want a more accurate rendering of natural colour may turn to Adobe RGB, which expands the colour palette, showing more realistic green hues, for example. Meanwhile, the push to digital cinema in the high street has led to a new standard for high-definition moving images — the Digital Cinema Initiatives (DCI) P3 standard.

DCI P3 is an expanded colour gamut that Apple is aiming to cover with its new ‘P3’ displays in this year’s iMac. When comparing CIE colour charts this gamut can be seen to roughly approximate the same space as Adobe RGB (the latter actually encompasses 93.6 % of DCI P3).

The DCI P3 standard is relatively new and is not included as a recognised colour space in typical screen measurement tools such as those from Datacolor and X-Rite. That presents a problem when quantifying output from this iMac, so instead we’ll discuss some findings made with a Datacolour Spyder4ELITE colorimeter.

The new iMac display comfortably covers the standard sRGB colour space — but then so did the first Retina 5K panel. That original model covered 78 % Adobe RGB according to the same colorimeter, while this Late 2015 iMac with P3 display indicated 90 % coverage.

Apple iMac (Retina 5K, 27-inch, Late 2015) Adobe RGB

Contrast ratio remains extremely impressive, between 980:1 and 1070:1 when set between nominal 25-100 percent brightness output. The first Retina 5K iMac reported fractionally higher figures, very close at 1040:1 to 1160:1 over the same brightness range.

Colour accuracy seems to have tightened even further, from an average 1.66 down to 1.33 Delta E. A few displays manage to get below 1.0 in this test but less than 2.0 is certainly very good.

For the first time ever, Mac OS X now seems to be able to work with 10-bit per pixel colour, previously the preserve of high-end workstations with matching graphics cards.

But, the one-billion colour question is — does the new iMac completely cover the DCI P3 colour space? And to that we don’t yet have an answer, without the right tools to had. Although it’s likely ‘no’, given the stringent requirements to fulfill even Adobe RGB which few if any white-LED backlit LCD panels can accomplish. Apple itself promotes the panel as ‘achieving over a 99 percent P3-based color gamut’, and for once we’ll have to accept that specification untested.

We do know that Apple discusses using a backlighting technique using modified red and green phosphor LEDs rather than standard white LEDs. And judged by eye the display can reveal colours and detail hitherto unavailable to most displays, although to experience this you’ll have to meet various stringent conditions.

Firstly, the original image needs to be better than sRGB, which usually means an image file format other than JPEG, which typically strips out these additional colours. So from a digital SLR you’ll need to set capture to RAW mode, for example.

Then the application on the Mac with which to view the media must be capable of better-than-sRGB performance. It’s not entirely clear which apps support 10-bit colour, although some are suggesting Apple’s own and are the only ones at the moment.

For a photographer shooting Adobe RGB photos, or a film maker working to professional standards, the iMac should allow a 10-bit workflow. It’s early days for the complexities of wide-gamut throughout the system, but even if not everything is in place today, you could think of this deep-colour display as more future-proofing until there are more stills and video cameras and software applications can accommodate better colour. For most people, it’s worth remembering that the photo and video output of an iPhone, for example, is resolutely sRGB-only for the moment.

Missing in action

The move to a Skylark-generation processor with Z170 chipset should also unlock another breakthrough in the form of third-generation Thunderbolt.

Thunderbolt 3 moves theoretical sync speed from 20 Gb/s to 40 Gb/s. It should also allow the connection of two rather than just one external 4K monitor; or alternatively a single 5K monitor can be attached with a single cable for the first time.

Problem is, this iMac with Retina 5K display does not include Thunderbolt 3. Apple has left the latest and greatest of the Thunderbolt ports it co-designed with Intel out of this Mac. That’s a shame as there’s space enough out back for the new-fangled UB Type C port, as found on the 2015 MacBook.

27in iMac 27in Late 2015 - rear

The 100 Word or Less

Apple has moved the goalposts once again, this time by introducing its first wide-gamut display. The new Skylake processor shows around 10 % improvement in synthetic benchmark tests, while the revised AMD Radeon R9 395 graphics provide around the same very competent performance as the original R9 290X – faster in some games, slower in others, and with potentially more gain in OpenCL acceleration. The end result is substantially the best desktop PC money can buy in build, design and ease of use, while screen image quality – when factoring in the resolution, size and gamut – is quite literally second to no other. AH

Price £1849


27-inch (5120 x 2880 pixel) IPS display with anti-reflective finish; OS X 10.11 El Capitan; 3.3 GHz Intel Core i5-6600; AMD Radeon R9 M395 with 2 GB GDDR5 memory;  8 GB (2x 4 GB) 1866 MHz DDR4 memory; 2 TB Fusion Drive (128 GB PCIe 3.0 x4 SSD and 2 TB 3.5in SATA HDD; 2x Thunderbolt 2; 4x USB 3.0; gigabit ethernet; 802.11ac Wi-Fi; Bluetooth 4.0; 1.2 Mp FaceTime HD webcam; dual mics; Bluetooth Wireless Keyboard and Magic Mouse; SDXC card reader; 3.5 mm headset jack with Toslink optical digital output; stereo speakers; Kensington lock slot; 650 x 516 x 203 mm; 9.5 kg