Review: Apple iMac with 4K Retina display (21.5-inch, Late 2015)

Introduction, design and display

Fans of compact all-in-one desktops rejoice: Apple has finally upgraded its 21.5-inch iMac with a 4K Retina display. If you’ve owned one of the company’s non-Retina iMacs in the past and grimaced at its jagged fonts and wonky lines, an upgrade to the newest, sharpest, most eyeball-pleasing iMac on the block might be just what you need.

Apple’s 27-inch iMac has received not one, but two upgrades since 2014. The first introduced a 5K Retina display, while the most recent refresh brought Intel’s sixth-generation Skylake processors into the mix.

The 21.5-inch iMac with 4K Retina display features a screen that’s equally as impressive as the one on the 5K iMac – only more compact. However, on the inside it’s only seen an upgrade to Intel’s fifth-generation Broadwell processor.

4K iMac


Just as the 5K iMac was pretty much the only affordable option if you wanted a 5K display in 2014, Apple’s 21.5-inch 4K iMac stands out from the crowd for several reasons.

First off, it’s the smallest 4K monitor around. Most Ultra HD panels are 24-, 27- or 28 inches in size. AOC markets its 23.6-inch U2477PWQ as a “space-saving” 4K monitor, which gives you some idea of just how compact Apple’s 21.5-inch 4K iMac is. Its unibody aluminium chassis remains unchanged from the non-Retina model, which was slimmed down along with the 27-inch iMac in 2012.

In my eyes, the appeal of Apple’s 21.5-inch iMac has always been its compact nature. It’s probably a stretch to call a slab of aluminium and plastic “cute”, but the iMac’s small footprint makes it feel like a computer that could I could tuck away in the corner of any bedroom or office without worrying about it not blending in. It’s basically the cool metallic chameleon of the desktop world.

Faults in the design are hard to find, but if I had to pick one out, it would be the display’s bezel. While it would be difficult to slim down the computer’s rear any further – after all, the components have to go somewhere – I can see Apple eventually making the black bezel around the display smaller.


Packing four-and-a-half times the pixels of the non-Retina 21.5-ich iMac, the new iMac’s 4K (or Ultra HD) display is stunning. Sure, there are other 4K monitors on the market, but few have the sharp quality of the iMac’s. The increased clarity makes text incredibly sharp and easy to read, and high-resolution photos pop out of the display. Videos are even more impressive – you haven’t lived until you’ve loaded up a gang of snow monkeys on YouTube larking around on the top of a mountain in 4K.

Those extra pixels make for more than just a visual feast – they also provide more space to work with on the desktop. The 4K iMac ships with Apple’s default scaling option (200%), which gives you roughly the same amount of real-estate as a 1,920 x 1,080 pixel-resolution monitor. Of course, you can choose one of the “more space” or “less space” options to increase the amount of room you have to work with.

4K iMac

Whichever scaling setting you use, OS X 10.11 El Capitan scales brilliantly – you never have to worry about fonts or menu icons being too small to see comfortably. You can download a third-party tool like SwitchRes to increase the resolution further all the way up to the native resolution which, if you have decent eyesight, makes 4K iMac a formidable tool for productivity work, graphic design, editing Ultra HD resolution and browsing the web with multiple windows open.

At 4,096 x 2,304, the 4K iMac has a slightly higher pixel-resolution compared to conventional Ultra HD monitors. Apple likely raised the resolution to give it 219ppi (or pixels per inch) across the display’s 21.5 inches to match the 218ppi on the 5K iMac. In other words, if you opt for the 4K iMac, you won’t feel short-changed when it comes to clarity: they’re both on equal footing there.

As with the 5K iMac, the 4K iMac’s display supports the DCI P3 colour space, which supports a wider range of colours than non-P3 displays through the use of red-green phosphor LEDs. The support can prove particularly useful to photographers who shoot using a camera’s RAW setting, as images appear more “life-like” with more accurate colours and a greater vibrancy. For everybody else, the difference is subtle – unless you’re a professional, it’s unlikely that you would notice it.

Specifications and peformance

The 4K iMac starts at £1,199 (US$1,499/AUS$2,299) – just £300 (US$443/AUS$606) short of the cost of the entry-level 27-inch iMac with 5K Retina Display. Apple’s larger iMac gets you a lot more screen and much more powerful components including a dedicated graphics card, so you’ll have to consider whether going large represents better value for your wallet.

Alternatively, you could pick up an entry-level 11-inch MacBook Air (£899/US$899/AUS$1,399) and connect it to an affordable 4K monitor such as the Acer CB240HYK (£300) for roughly the same cost. The other option is hooking up the latest Mac mini to a 4K display, which isn’t recommended due to that machine’s 30Hz limitation when outputting 4K.

4K iMac side


Here is the spec sheet of the review model provided to techradar:

  • Processor: 3.1GHz Intel Quad-Core i5 (Turbo Boost up to 3.6GHz)
  • Operating Systems: OS X 10.11 El Capitan
  • Memory: 8GB 1867MHz LPDDR3
  • Display: Retina 4K 4,096 x 2,304 IPS display
  • Graphics: Intel Iris Pro Graphics 6200
  • Storage: 1TB Serial ATA Drive @ 5400 rpm
  • Dimensions: 45cm x 52.8cm x 17.5cm (H x W x D)
  • Weight: 5.68 kg (12.5 pounds)
  • Accessories: Magic Mouse 2, Magic Keyboard
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When it comes to specs, the iMac with 4K Retina display sits somewhere between the top-end non-Retina 21.5-inch iMac and the entry-level 5K iMac. Compared to the top-sec non-Retina iMac, the 4K model benefits from a higher-clocked sixth-generation Intel Core i5 processor (3.1GHz, versus 2.8GHz), which can be upgraded to a 3.3GHz Quad-Core Intel Core i7 processor with a maximum clock speed of 3.8GHz for a further £160 ($230) – an option unavailable on the standard model.

4K iMac rear

The 4K iMac comes with a 1TB spinning hard drive as standard. It can be upgraded to a 1TB Fusion Drive that pairs a 1TB hard drive with 24GB of flash storage (used for installing OS X, which allows for faster boot times), and further upgrade options include a 2TB Fusion Drive, a 256GB Flash Storage drive or a 512GB Flash Storage drive.

The 4K iMac’s ports remains unchanged from the non-Retina 21.5-inch iMac. You get four USB 3.0 ports, two Thunderbolt 2 ports, an SDXC card slot, Gigabit Ethernet and Kensington lock slots. Wireless is taken care of via 802.11ac Wi-FI that can reach speeds of up to 13Gbps, and there’s also Bluetooth 4.0 for hooking up peripherals.

Apple revamped its accessories with the re-launch of the 27-inch Retina iMac. The new (and much improved) Magic Keyboard ships with the 4K iMac, along with a redesigned Magic Tracpkad 2 and Magic Mouse 2. All of the revamped peripherals feature wireless charging rather than batteries.


When it comes to performance, the 4K iMac’s major flaw is apparent as soon as you’ve turned it on. The default 1TB 5400rpm drive is tortoise-like in speed and doesn’t boot as quickly as you might be used to coming from an SSD-equipped laptop.

The hard disk’s slow speed is less noticeable once you’re on the desktop. Apps load relatively quickly, and the 8GB of RAM keeps everything ticking along smoothly. However, I’d go as far to say that the 4K iMac is only worth bothering with if you’re looking at upgrading to at least the 1TB fusion drive for another £80 ($117/AUS$161), if only to save yourself sitting through longer boot and sleep resume times. If you’ve got the money to spare, upgrading again to the SSD option would ensure fastest performance all round.

4K iMac Koala


  • Xbench: Overall: 653.25; CPU: 405.54
  • Cinebench R15 Single Core: 150cb; Multi Core: 580cb; Open GL: 45fps
  • Unigine Heaven 4.0 Medium Quality (2,560 x 1,440): Score: 257
  • Unigine Heaven 4.0: Ultra quality (2,560 x 1,440): Score: 184
  • Novabench: Score 831; Graphics: 69

The 4K iMac isn’t equipped to handle games like the 5K Retina iMac is, but its Intel Iris Pro Graphics 6200 is enough for light photo and video editing in addition to running the occasional lesser demanding game. The 5K iMac, which houses a dedicated AMD Radeon R0 M390, scored 102% more in Novabench’s graphics test than the iMac 4K. If you’re looking at playing modern titles like The Witcher 3, you’re better off opting for Apple’s larger machine – or a Windows PC.

Apple thankfully chose not to mess with the non-Retina 21.5-inch iMac’s decent speakers, which once again provide loud and clear sound on the 4K iMac. They’re pleasingly loud without being overbearing at full volume, capable of pumping out decent mid-range tones and just enough bass to do rock and dance music justice.


The iMac with 4K Retina display is an excellent computer that once again offers something you can’t easily get from Apple’s rivals. Be warned, though, as you’ll want to upgrade that spinning hard drive if you don’t want to be sat twiddling your thumbs waiting out slow boot and resume times.

We liked

The 4K iMac’s display is nothing less than stunning. With bright and vibrant colours, support for the DCI P3 colour space and a good enough resolution to ensure that the desktop never feels cramped again, the machine is a pleasure to use. It’s mostly fast enough, save for the meandering standard hard drive. The 4K iMac also has excellent speakers that won’t beat a dedicated pair but are more than sufficient for listening to music, watching movies, streaming TV and light gaming.

We disliked

Apple so very nearly got the 4K iMac’s entry-level specs perfect, but the lack of a Fusion Drive as standard is a poor show. That gorgeous display soon loses its allure when you’re tapping your foot waiting for it to boot – so do yourself a favour and upgrade to the 1TB Fusion Drive – or even the 256GB SSD. It’s also about time Apple put a height adjustment stand on its iMac computers, so you’ll have to balance them on a set of books or buy an accessory to raise its height until then.

Final verdict

There’s no question that the 4K iMac is in every way an improvement from the standard 21.5-inch model. Even if you wouldn’t take advantage of the extra screen real-estate offered by its 4K display, the addition clarity it provides makes the 4K iMac a much more enjoyable computer to use. There’s less eye strain and OS X Yosemite takes on a new dimension, one that’s enriched with colour saturation, sharpness and detail. On the other hand, if you would make good use of the extra screen size (and higher resolution) afforded by the 27-inch 5K iMac and require extra graphical grunt for playing the latest games, you should strongly consider spending the extra money on upgrading to Apple’s biggest all-in-one.

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