The Good: Apple upgrades the 21.5-inch iMac with a stunning 4K-resolution display, along with faster processors and Thunderbolt 2. The redesigned keyboard, mouse and trackpad all have rechargeable batteries and lightning connectors. The Bad: The new processors are still one generation behind Intel's latest and there are no discrete graphics options. No HDMI input means it can't double as a TV monitor. The Bottom Line: The smallest Apple iMac trades up to a 4K display, and jumps to newer, but still not the latest, processors. While the design hasn't changed, newly crafted accessories with rechargeable batteries and Lightning connectors add flair and convenience. Last year's addition to the line caught our attention by cutting the starting price to $1,099 (\u00a3899 or AU$1,349) and positioning itself as essentially a MacBook Air recast as a desktop system; the new version for 2015 aims higher.The flagship device in Apple's just-updated late-2015 iMac line is the configuration reviewed here, stepping up to a gorgeous 21.5-inch 4K resolution display and starting at $1,499 in the US (\u00a31,199 or AU$2,099). Two other base models, at $1,099 and $1,299, retain their 1,920x1,080 displays. It's worth noting that this is the more literal implementation of 4K in the new 21.5-inch iMac. The display resolution is 4,096 pixels wide, rather than the more common consumer version (sometimes called ultra-high definition or UHD), which has a just slightly lower resolution and is 3,840 pixels wide. That says to us that this system is targeting professional (and high-end enthusiast) photo and video users, who may shoot at that higher version of 4K resolution. Your big, fancy 4K television set, and every other 4K computer we've seen to date, all adhere to the more consumer-targeted 3,840x2,160 version of 4K. The 4,096x2,304 on this new iMac screen will play any of your 4K content just fine, although the tiny difference in native resolution can have a small scaling effect, which we really only noticed when viewing 3,840-resolution test patterns. The big secret about 4K is that a lot of people buy a 4K TV, monitor or computer, without really planning on viewing much 4K content on it. It's just the latest spec upgrade to strive for, and most of your video content, and even video games, aren't going past standard 1,920x1,080 high-definition anytime in the near future. But higher-res screens such as the 4K display here have other advantages. As we saw with the , individual pixels are practically invisible to the naked eye. On-screen text looks sharper and clearer than on lower-resolution screens, and Apple's expert scaling always keeps apps, menus and icons at a decent size, while giving you the option to mimic the look and feel of several different scaled resolutions. It's the same system Apple has used in all its Retina display models, going back to the MacBook Pro with Retina display. But there's more to the new iMac than just a higher resolution. The new Retina display supports the wider P3 color gamut versus the more common sRGB (standard Red\/Green\/Blue) version. Translated for the rest of us, that means the display can show more of the green and red color spectrum (blue, the third leg of the color triangle, is already fairly maxed out under RGB). Apple says it adds up to 25 percent more available colors to display. P3 is the standard for digital cinema projection in theaters, and for certain photo professionals and film and video experts, this is potentially a big deal. For photo hobbyists, you're unlikely to be able to appreciate the difference, as very little consumer-grade equipment is going to give you files that can take advantage of the wider P3 color gamut (some dSLR cameras, however, can). However, in side-by-side testing using some sample images, the effect was subtle but impressive, with richer reds and greens, and Apple's photo apps as well as third-party programs such as Photoshop, all support color far beyond standard sRBG. It's little surprise that the new 21.5-inch Apple iMac looks the same as models from the past several years. The basic design language of the iMac has changed little since 2012, when it adopted the current setup of a slim, bowed screen sitting on top of an aluminum stand and minimalist base, a look that still manages to feel fresh years later. But inside the familiar convex chassis of the iMac, there are some important spec changes for 2015. All three 21.5-inch models jump from Intel's fourth-generation Core i-series processors to newer fifth-generation chips. That's important because these iMacs were previously two generations of CPU behind. However, only the 27-inch iMacs, now all featuring the high-res 5K display, move up to the very latest CPUs, from Intel's recent sixth-generation of Core chips, sometimes known by the codename Skylake. It's a shame the iMacs are still one chip generation behind, especially with nearly every consumer PC hitting stores this holiday season moving to Skylake chips. But, for a desktop system it's less important, as actual performance changes between generations of Intel chips are fairly modest. Most of the advantage comes in battery life, and for an all-in-one desktop, that's not going to be an issue. Also new in the 21.5-inch iMac are Thunderbolt 2 ports for faster data transfer (if you have any Thunderbolt-equipped accessories) and some new hybrid hard drive options -- Apple calls them Fusion drives -- combining a small amount of solid-state memory for quick access of frequently used data, with a larger standard platter drive. Besides the higher screen resolution and new CPUs, the biggest obvious change to the iMac line is the revamped collection of accessories bundled with it. The Apple wireless Keyboard, Magic Mouse and Magic Trackpad -- all familiar sights on Apple users' desks around the world -- have gotten their first overhaul in years, and it might be the new feature I'm most excited about. All three lose their reliance on disposable batteries, instead moving to internal rechargeable lithium-ion batteries. That allows the keyboard and trackpad to slim down, removing the bulbus battery compartments that dominated the previous designs. The Magic Keyboard is smaller and flatter, but has slightly larger key faces. The Magic Trackpad 2 has a larger surface area -- it looks huge compared to the original version -- and now supports Force Touch, just like the pads in the MacBook and MacBook Pro. The Magic Mouse 2 looks the same, but is a hair lighter with better rubber tracks along the bottom. Overall, this is an important update to the iMac line, as a better-than-HD display feels like table stakes these days for any premium laptop or desktop computer, and the improved color gamut support in the 4K and 5K displays will help keep the iMac line as a top choice for creative pros. For everyone else, even if you don't regularly view 4K content, a sharp-looking Retina-level display is one of those things that's nearly impossible to give up once you get used to it, and the new 4K iMac is competitively priced with the handful of 4K-display Windows PCs we've reviewed. If you have a model from the past few years, this isn't a must-have upgrade, but it may certainly be worth picking up the new keyboard and mouse or trackpad to give your older iMac a facelift. \tDesign and featuresYou can read any of our iMac reviews over the past three years to get a feel for the long-standing design of this high-end all-in-one. It still manages to look current, although some newer trends seen on the Windows side, from thinner bezels to touch screens, are absent here. The iMac is dominated by its display, which also houses all the system components. It's still just 5mm thick at the edge, gently bowing out in the back. It looks almost paper-thin when viewed from the correct angle, and still pretty svelte even in full profile, where the rear panel bows out in the center into a gentle bowl shape.That top section is connected via an adjustable hinge to a curved one-piece stand. If you're not connecting any external USB, Thunderbolt 2, or Mini-DisplayPort devices, and using Wi-Fi instead of a wired Ethernet connection, this is essentially a one-cable setup with a single white power cord in the lower middle of the back panel. As with most Apple computers, this is a sealed system, with no user-accessible components -- unlike the 27-inch iMac, which has an access port for the RAM slots.Thinner, lighter accessoriesThe biggest physical change is in the new packed-in accessories, named the Magic Keyboard, Magic Mouse 2 and Magic Trackpad 2. As before, the keyboard and mouse are included by default, but you can choose to swap in the Trackpad instead of the mouse. With these new versions, however, the adding the Trackpad costs extra. All three are also sold separately, at $79 for the Magic Mouse 2, $99 for the Magic Keyboard and $129 for the Magic Trackpad 2 (all prices in US dollars). Previously, all three first-gem accessories were sold by Apple at $69 each.That's a big jump in price for the trackpad, but it's easily the most impressive of the new accessories. The pad goes from aluminum-colored to off-white, and looks and feels massive. Apple says the surface area is 29 percent larger. Without the the cylindrical battery compartment, the pad has an even more minimalist look, lying flat on the tabletop with a slight wedge shape. The Magic Trackpad 2 supports Force Touch, the new touchpad mechanic found in the MacBook and MacBook Pro laptops (a variant is also now in the iPhone and Apple Watch). Force Touch uses four corner sensors to replace the hinged "diving board" mechanism found in most touchpads, including Apple's previous ones.