21.5-inch 3.1GHz iMac with Retina 4K display (2015) full review

The 21.5in iMac has finally gained a Retina display. It’s a 4K display, as opposed to the 5K display that the 27in Retina iMac gained in 2014. The 21.5in Macs hadn’t been updated since 2013 so this update was long overdue.

Apple has finally updated its range of 21in iMacs, and at the top of the line is a iMac with a 4K Retina display, which starts at £1,199. This isn’t the cheapest Retina display Mac, but it is the same price as the equivalent 21in Mac cost previously, so at least Apple hasn’t upped the price in recognition of the new display. The move is essentially what Apple did back in October 2014 with the 5K iMac, and while that Mac was £1,999 at launch, the price came down over time and eventually the whole range of 27in iMacs was given a 5K Retina screen.
Along with the Retina display in this top-of-the-range 21in model, you get a collection of much-needed internal upgrades, including Intel's Broadwell CPUs and GPUs, Thunderbolt 2, and faster storage and RAM. These upgrades are also available on the refreshed non-Retina 21.5-inch iMacs).

Read more about the new range of iMacs here: Apple launches 21.5in Retina iMac, updates 27in iMacs with Skylake and the 2015 27in range of Retina iMacs is reviewed here.

New 21.5in iMac (non-Retina)

While this is mainly a review of the 4K Retina iMac, here are a few words on the other 21.5in models. First things first, the 21in iMac really isn’t great for one key reason - it ships with an exceptionally slow 5400RPM hard drive as standard - that’s the kind of hard drive you find in a laptops (except these days you’ll be hard pushed to find a laptop with a HDD rather than an SSD/flash drive). To be frank, we don’t think Apple should be shipping any iMacs with hard drives in this day and age, the company has a Fusion Drive build-to-order option that combines a flash drive with a large hard drive, and that only costs £80. We can’t fathom why Apple wouldn’t just include that as standard across the range. The hard drive in the iMac rather undoes any of the benefits of the faster processor and graphics card. If you are getting a 21in iMac it goes without saying that you should upgrade to the Fusion Drive at point of purchase, or if you don’t need 1TB of space (you probably don’t) then go for a flash drive.

There are three 21in models and as we said above only the top of the range model ships with a Retina display.
The entry-level model costs £899, has a 21.5in Full HD display, a 1.6GHz Core i5 processor, 8GB of RAM and that positively prehistoric 1TB 5,400rpm HDD 1TB hard disk. Regarding the RAM, it’s soldered onto the motherboard, so you can’t upgrade it in the future. If you don’t look at a Retina display, you probably won’t find fault with the display on the 21in iMac, it is very good (if you’ve read about how iMac screens are too reflective that story is years old, Apple rectified it around 2011).

While £899 might sound like an attractive price if you are just looking for a basic, everyday computer, it’s not the cheapest Mac Apple sells, and to be honest we don’t think that you would be getting much for your money, for just £150 more (£1,049) you'll get an iMac with a 2.8GHz processor as opposed to the 1.6GHz processor, so why wouldn’t you pay the extra… Mind you the 3.1GHz Core i5 processor and a Retina 4K display only costs another £150 (£1,199) so frankly you should just go the whole hog. Mind you, on top of that £150 (or £300) extra we’d also recommend spending that extra £80 on the 1TB Fusion Drive upgrade - which is absolutely necessary in our view. So, if you are buying a 21in iMac, you’ll be spending £1,279 to get the best Retina display option, or £1,129 if you don’t really care about the screen, but recognise that a hard drive will just annoy you.

New 21.5in iMac with Retina display: Display

Only the top of the range 21in iMac model gains the 4K display, the other 21in models retain the older display - and the prices don’t change.

The price of the top of the range 21in iMac hasn’t changed either, as we discuss below, but at £1,199 it’s not the cheapest Mac with a Retina display you can buy. The 13-inch Retina MacBook Pro costs £999. But that Mac has a smaller display and a slower processor, so it’s certainly not a bad deal. [Read more Retina MacBook Pro reviews here]. The 21in Retina display is also a vastly broader work area than an that offered by any of Apple’s Retina laptop screens.

One thing that can’t be denied, however, is the fact that the 4K display on the new 21in iMac is impressive. It has a resolution of 4096×2304, that’s 9.4 million pixels, which actually adds up to 14% more pixels than most 4K monitors. If it’s an even bigger Retina display you are after, the entry level 27in iMac now offers one, but it costs £1,449, that’s £250 more. [Read more iMac reviews here].

Interestingly, Apple has chosen to match the pixels per inch of the 27in Retina iMacs - so they all share the same 217 PPI pixel density. Apple could have given the 21.5-inch version of the Retina iMac a 3840×2160 screen but it chose not to, and should be commended for giving it a higher-than-standard resolution, it essentially means you can see as much information on the 4K iMac as you can on the 5K iMac. In order to drive all these pixels Apple is using the custom timing controller from the 5K iMac to drive the 4K display.

There is more to the Retina display than being able to see subtle details that you were blind to on a low res screen. Because the pixel count is so high, there’s a lot of flexibility with a Retina display. These screens can be set to alternate resolutions without sacrificing image quality. For example, if you want everything to be a little bit bigger you can set the 4K iMac to emulate a smaller monitor in the Display preference pane. Or if you want, you could set the iMac to display a higher resolution giving you more room to work. You can adjust the displays scaling to 2304×1296 and 2560×1440 display modes.

If you want to plug another display in you can, but the 4K iMac can only drive one 3840×2160 screen because it is limited to DisplayPort 1.2 compatibility. It won’t be possible to drive another 4K display until Apple adds DisplayPort 1.3.

According to Apple this display also offers an expanded colour space thanks to new red-green phosphor LEDs that enable the displays to display a wider range of red and green light than before, allowing them to display 25 percent more colours. Hence, these displays are capable of displaying 99 percent of the P3 colour space. Not bad, although it probably won’t matter to you unless you work in the video or graphics industries.

There was a very tiny difference in colour accuracy between the 5K and 4K display, with the 5K coming out on top. The DeltaE result was 1.1, and brightness was 446cd/m2, it managed 86.5% of the Adobe RGB gamut and the contrast ratio was 1,247:1. We found the backlighting to be more even than the 5K model for more uniformity across the panel.

Read our Mac buying guide

New 21.5in iMac with Retina display: Price

The new top of the range 21.5in iMac gains this new 4K Retina display, and we wouldn’t have been surprised if Apple had put up the price - but the good news is that the price remains the same, so you can now purchase a Retina iMac for £1,199. You can buy the new 21in Retina iMac from Apple's online store here.

New 21.5in iMac with Retina display: Processor and speed tests

Another factor letting down all the 21in iMac models, including the Retina display version is the fact that they are stuck using the older Broadwell architecture rather than the Skylake chips being used by the 27in models.
It has to be said that this is an advancement on the previous models, which having not been updated for two years were still using 2013 Haswell chips. Sadly, however, the performance improvements achieved with the Broadwell processors, which were delayed so much that they were overtaken by the newer Intel processor Skylake (as seen in the 27in model), isn’t worth writing home about.

You may be wondering why Apple didn’t just add Skylake to the 21in iMac - or at least the 4K Retina version. Unfortunately, while the desktop versions of Skylake suitable for the 27-inch iMacs were available, the series of Skylake chips that feature their own integrated graphics processors, which Apple needed for the 21in iMacs weren’t available when the 4K 21in iMac launched.

If it’s not frustrating enough that these 21in iMacs ship with last generation chips, the entry-level model ships with a 1.6GHz Core i5-5250U processor, which is a processor normally destined for a laptop, so don’t expect much from it.

The top of the range iMac with 4K display does at least gain a 3.1GHz quad-core i5 processor, up from 2.9GHz. There is a 3.3GHz quad core i7 option for an additional £160.

Our colleagues at Macworld US have run Geekbench on the new 3.1GHz 21.5in 4K iMac and found it to be even faster than the 27in 3.3GHz 5K iMac launched in the summer of 2015 (and updated again in October 2015). It’s also considerably faster than the 21in iMac model it replaces. These gains are despite the fact that the 4K Retina display will be taking a toll, powering four times as many pixels.

The results were as follows:

Geekbench 3 single-core 64 bit
3.1GHz 21.5in 4K iMac : 3,783
3.3GHz 27in 5K iMac (mid 2015) : 3,691
2.9GHz 21.5in iMac (late 2013) : 3,543
2.7GHz 21.5in iMac (late 2013) : 3,175

Geekbench 3 multi-core 64 bit
3.1GHz 21.5in 4K iMac : 12,799
3.3GHz 27in 5K iMac (mid 2015) : 11,769
2.9GHz 21.5in iMac (late 2013) : 10,683
2.7GHz 21.5in iMac (late 2013) : 10,199

New 21.5in iMac with Retina display: Graphics and speed tests

Apple offers no dedicated GPUs in its 21.5-inch iMacs, Retina and non-Retina alike. All three models (and all build-to-order configurations) ship with Intel's integrated GPUs.

The top of the range, 4K iMac features the Intel Iris Pro Graphics 6200 - the same graphics chip is in the 2.8GHz iMac without the Retina display.

Apple's decision to move to all-integrated GPUs in the 21in range of iMacs explains why the 27-inch iMacs feature Skylake CPUs but the 21.5-inch models are lumbered with the last-generation Broadwell CPUs. Intel hadn't released the Skylake chips with integrated GPU when Apple launched the new iMacs.

In terms of CPU speeds, the new 21.5in iMac can’t even compete with the older 27in iMacs. Which isn’t a big surprise given the fact that the GPU is integrated on the processor in the 21in models and separate in the 27in iMacs. Our colleagues at Macworld US ran Cinebench tests and found that while the new 3.1GHz 21.5in iMac performed better than it’s predecessor, despite the fact that it is driving four times more pixels, but it was left behind by the 5K 3.3GHz iMac that launched in mid 2015.

The results were as follows:

Cinebench R15 CPU
3.1GHz 21.5in 4K iMac : 72 (less is better)
2.9GHz 21.5in iMac (late 2013) 3,543 : 88
2.7GHz 21.5in iMac (late 2013) 3,175 : 100

Cinebench R15 OpenGL
3.1GHz 21.5in 4K iMac : 47
3.3GHz 27in 5K iMac (mid 2015) : 87 (higher is better)
2.9GHz 21.5in iMac (late 2013) : 55
2.7GHz 21.5in iMac (late 2013) : 29.2

Unigine Heaven 1920x1080 OpenGL
3.1GHz 21.5in 4K iMac : 18.5 (higher is better)
2.9GHz 21.5in iMac (late 2013) : 10.6
2.7GHz 21.5in iMac (late 2013) : 7.2

Unigine Heaven 1280x720 OpenGL
3.1GHz 21.5in 4K iMac : 47.3 (higher is better)
2.9GHz 21.5in iMac (late 2013) : 43
2.7GHz 21.5in iMac (late 2013) : 37.2

If you are a gamer, these are not the Macs you are looking for.

New 21.5in iMac with Retina display: RAM

The new Mac comes with 8GB RAM and is configurable to 16GB when you purchase it from Apple (you can't update it later as the RAM is soldered on).  If you choose to opt for the 16GB RAM it will cost an additional £160 - and if you think you will need it you should buy it because the 21.5-inch iMacs don't have user-accessible RAM slots.

New 21.5in iMac with Retina display: Storage

As we said above when we were criticising the 21in iMac range, Apple’s big mistake is lumbering the 21in models with hard disk drives, and even worse, 5400RPM hard drives. Unacceptable in this day and age, in our opinion. For that reason we recommend that if you are buying one you upgrade to either the Fusion Drive, or an SSD. Opting for a hard drive if you are used to flash storage will make the Mac feel slow while Fusion Drives combine a hard drive with faster last storage for the best of both worlds.

We recommend opting for an SSDs if you don't need 1TB of space, the SSD in the new iMacs actually gets a nice speed bump compared to the previous generation because Apple has moved from two PCI Express 2.0 lanes to four PCI Express 3.0 lanes, that add up to  a huge amount of storage bandwidth. You can get 256GB or 512GB SSDs as a build-to-order option. The 256GB Flash upgrade costs £160 or 512GB is £400 extra.

Alternatively, if you think you need the space, buy yourself a Fusion Drive for £80 - you’ll get a 1TB hard drive coupled with 24GB - it’s another annoyance that this has been reduced to 24GB from the128GB found in previous generations of Fusion Drives, but at least the price has come down and it should be enough to speed up boot times and launch times for most of your apps and files. There is a 2TB Fusion Drive option which still includes 128GB of flash (that costs an extra £240).

Regarding the lower price for the Fusion Drive we can’t help but think Apple could afford to stick that drive inside every single one of these iMacs.

Our advice: avoid the hard drive, if you really need all that storage get an external drive and use that.

Find out more about which storage to add to your Mac here: How to add more storage to your Mac | Hard drives, flash storage, NAS - find out which storage option is best for you and SSD vs hard drives: which is the best storage to have in a Mac

New 21.5in iMac with Retina display: Ports

The new iMacs look identical to the old iMacs, the one big change is the addition of Thunderbolt 2 , the previous models were so ancient they only offered the older Thunderbolt 1 ports.

Next time we'd like to see Thunderbolt 3 or USB Type-C.

Find out what port does what here: Guide to Mac and iPhone ports: what is USB Type-C, Thunderbolt & Lightning

New 21.5in iMac with Retina display: other peripherals

The new iMac ships as standard with the new Magic Mouse 2 (or you can switch it to the Wired Apple Mouse if you prefer).

If you're happy to pay a little more, the new Magic Trackpad 2 costs an additional £44 as a build to order option, or you can get both the Magic Mouse 2 and the Magic Trackpad 2 for £109. In this case you are essentially paying for the Magic Trackpad 2; if you were to buy them separately the new Magic Mouse 2 costs £65 and the Magic Trackpad 2 costs £109 - the mouse is bundled with the iMac so you don’t have to pay for it in this instance.  You can purchase the Magic Mouse 2 on the Apple Store here, and the Trackpad 2 is available from the Apple Store here. There is also a new Magic Keyboard (Apple's getting a bit carried away with the Magic). The Magic Keyboard is available to buy here.

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