Updated September 2013

The eternal question, 'What camera should I buy?' became more complicated with the fairly recent emergence of a new breed of cameras promising to fill a gap in the market. Previously it was a fairly simple decision - you bought a DSLR if you were most concerned about image quality, operational speed and taking control of what the camera did, or you bought one of the various sub-types of small-sensored compact camera if you prioritized pocketability, ease-of-use, price or zoom range. There were some attempts to bridge the gap - superzoom compacts and high-end enthusiast models often offered extensive manual controls - but there was no real middle ground.

Traditionally, the only real options for digital photographers have either been compact cameras which have very small image sensors, or the considerably larger - in both sensor and body dimensions - DSLR designs.

That middle ground is where you'll find Mirrorless Interchangeable Lens Cameras. These cameras take the large sensor and interchangeable lenses that help DSLRs produce such good images, and combine them with the technologies that underpin compact cameras - providing a shooting experience that will be nice and familiar to compact camera users.

Mirrorless cameras have tended to fall into roughly two basic designs. There are models that resemble the classic Rangefinder style (like the Fujifilm X-E1, above...  ...and those which resemble scaled-down DSLRs, like the Panasonic Lumix DMC-GH3 on the right.

Mirrorless cameras (also known as Compact System Cameras, amongst other names) first appeared with the launch of the Micro Four Thirds system from Panasonic and Olympus in 2008. The first generation cameras from these manufacturers reflect two distinct body styles that are still prevalent today; a rangefinder-inspired aesthetic and the mini DSLR look-alike. Of the two, rangefinder-style bodies have a smaller form factor by omitting a built-in electronic viewfinder, instead depending on the rear screen for image composition, much like current compact cameras.

Since the Micro Four Thirds launch, most of the major manufacturers have waded-in. These range from Samsung's NX, Sony's NEX, Fujifilm's X and Canon's EF-M mounts, that are based around APS-C sensors, via Nikon's smaller-sensor 1 System down to Pentax's Q, that uses sensors usually seen in compact cameras.

Why go Mirrorless?

For all of their outward differences, it's important to understand how much Mirrorless cameras have in common with DSLRs. In most cases, their sensors are the same or similar in size to most popular DSLRs. By comparison, most compact cameras have sensors that are one twelfth of that size - and this is a big deal, since sensor size is probably the largest determinant of image quality (decidedly more relevant than the number of megapixels). As a result, the image quality of the Mirrorless models we've tested has either been identical or very similar to that of contemporary DSLRs.

In addition to low-light image quality, large sensors tend to result in systems with greater control over depth-of-field, meaning that you can think about using large aperture lenses to get photos with soft, defocused backgrounds.

Putting aside for a moment the oddity that is the compact-sensored Pentax Q, the only real exception to this statement is Nikon's 1 system (surprisingly) goes against this trend and uses a sensor two-thirds smaller than most DSLRs or Mirrorless cameras. This is still four times the size of the sensors in mainstream compact cameras, but does mean the low light image quality and control over depth-of-field isn't on the same level as most of its peers.

Overall, the result is a significantly smaller camera that combines DSLR image quality with a more compact-camera-like user experience, offering 'point and shoot' users a huge upgrade in image quality without the need to change their shooting behavior.

Removing the mirror and optical viewfinder found in the DSLR allows a considerable reduction in bulk as can be seen by comparing the size of a Sony DSLR with one of its Mirrorless NEX cameras.

Putting most lenses on the front prevents them being truly pocketable but they can still be less obtrusive.

The large sensors used in Mirrorless cameras mean that their bodies and lenses don't really count as pocketable, but they are substantially smaller than even the most compact DSLR. This is an advantage not just in that there's less weight and bulk to lug around but also in that using one can be substantially less obtrusive. There are plenty of situations in which pulling out a large 'professional-looking' DSLR will influence the way your would-be subjects respond.

The smaller bodies also, in principle at least, allow for smaller lenses - though this mainly tends to be true for wide-angle and normal zooms, with telephoto lenses being essentially the same size as those used on DSLRs. The small body designs have also prompted most of the manufacturers to create at least one small 'pancake' prime lens for their system. These are fixed focal length lenses and, with the right combination of price, aperture and focal length, can be a great addition if you're looking to really engage with photography. 

Advantages of Live View

Traditionally the divide between compact cameras and DSLRs has been that digital compact offer a fast, live preview of the scene you're shooting on the rear display - often called 'live view'. By contrast, the film-era heritage of the SLR design has made adding a fast preview on the camera's rear screen has proved difficult. Several DSLR live view solutions exist but most of them still have significant drawbacks in terms of either speed or flexibility.

If you're stepping up from a compact camera, the excellent live view experience offered by a mirrorless system camera will make the transition seem a lot less daunting.

Mirrorless cameras have all been designed specifically to use the main imaging sensor to provide autofocus information and the preview of what the camera is about to shoot, just as is the case with compact cameras, rather than having this option added on later. Likewise, the lenses for Mirrorless systems have been designed for the different demands that this focus acquisition method requires, making them quicker to focus than when most DSLRs try to focus in live view mode.

This offers not just familiarity, but also another potential advantage: it means you don't need to raise the camera to eye-level to take a photograph. It means they can be used more discretely and do not require the camera to be imposed between the photographer and the subject to nearly the same degree. This feature, combined with their smaller size, results in a camera significantly less intimidating than a DSLR.

Automatic focus - contrast and phase-detection

Almost all compact and mirrorless cameras use contrast-detection autofocus - in its simplest form, this racks the lens through its range of focus distances and picks the one that delivers the highest contrast at the selected AF point. Once upon a time this was painfully slow, but focusing algorithms and lens designs have improved substantially over the past few years, and it can now be extremely fast and accurate. But it struggles in some situations - most notably with moving subjects. That's where 'Hybrid' autofocus comes in.

This cross section of the Fujifilm X-Trans CMOS sensor shows the following:

1) Microlenses

2) X-Trans color filter

3) Left/Right light interception filter

4) Phase detection sensor / green filter pixel

5) Photodiode

[Diagram courtesy of Fujifilm]

So-called 'hybrid' AF systems are becoming more common in Mirrorless cameras, and combine contrast-detection AF with phase-detection, which was traditionally used in DSLRs. A phase-detection AF system is able to tell from a single measurement exactly how to adjust the lens to achieve an in-focus image. As such, the main advantage of phase-detection AF is that it can support predictive focus tracking - a boon for all sorts of photography, from social shots of fast-moving kids to sports and wildlife. 

Hybrid AF was first employed successfully by Nikon's 1 System cameras, but other manufacturers such as Sony, Samsung, Fujifilm and Canon use similar technology, with varying degrees of success. 

In the Fujifilm design shown above (currently used in the fixed-lens X100S and X20, but representative of other similar systems in competitive Mirrorless ILCs) the PDAF sensors are localized towards the center of the frame, and use pixels that are masked to receive light from the left- and right-sides of the lens's exit pupil. The difference between the images coming from these two paths allows the focus distance to be determined. 

Manual Focusing and Lens Options

A side-benefit of their mirrorless design is its ability to assess exposure and show a magnified preview for accurate focusing of old, manual focus lenses, including some that need to be mounted very close to the sensor. This has resulted in a wide range of adapters appearing for the various Mirrorless mounts, allowing the use of a wide variety of manual focus lenses in obscure and obsolete mounts. It may not be as easy as using a native, autofocusing lens but it's a great way for budding photographers to experience fast, fixed focal length lenses without too much financial risk.

The ability to assess exposure and easily magnify the electronic preview make it easy to experiment with second-hand manual focus lenses.

It provides a cheap way to experience shooting with fast, fixed-focal-length lenses (which every budding photographer should get a chance to try).
More recently, manual focus aids such as focus peaking have become almost-standard on Mirrorless cameras. Focus Peaking provides a highlighted 'contrast' view of your subjects, allowing for accurate manual focus. This is especially useful when combined with live view magnification (shown above). 

What are the disadvantages?

Mirrorless cameras are getting better all the time, and there are far fewer disadvantages to using them now then there were in the recent past. That said, there are currently two major disadvantages we've experienced with some of the Mirrorless cameras we've encountered so far. The first is the inability of many models to match the autofocus speed of DSLRs when conducting continuous or predictive AF. Single-shot AF tends not to be so much of an issue, with most recent Mirrorless models able to match or, in the case of Nikon, Olympus and Panasonic, exceed the single-attempt focusing speed of most DSLRs. 

In general though, most Mirrorless cameras cannot track moving subjects (specifically those moving towards or away from you) as well as DSLRs. There is considerable variation between the different Mirrorless cameras in terms of autofocusing time so it's worth reading the reviews of the individual models before parting with any cash (the newer ones tend to be better than the first generation models, as you'd expect, and we anticipate newer 'hybrid' AF technology to quickly become standard and continue to improve).

The other drawback we've encountered is that some of the Mirrorless cameras we've used can struggle to focus in low light to a greater degree than we'd expect from an equivalent DSLRs. And, as the result of the way they focus, they don't always see the same benefit from using an AF illuminator as DSLRs do.

AF illuminators can be distracting but they can also give cameras (DSLRs in particular) an advantage when trying to focus in low light.

Most cameras, by default, won't take a picture unless it is in focus ('focus priority'), so the result of their inability to lock onto a moving subject is less likely to be an out of focus shot than either no shot at all (if the camera can't find anything to focus on), or such a long delay that you miss the moment entirely. Even in continuous shooting mode many mirrorless cameras struggle to keep a moving subject in focus, meaning the results will hit and miss - with a lot more miss than hit. It gets worse as focal lengths increase and as light levels drop, and as the amount your subject moves increases.

DSLRs aren't immune from this issue either, of course, and whether you're using a Mirrorless or DSLR camera, fast, bright lenses help a lot when it comes to AF performance in poor light. 

Waiting for focus means the picture is taken a fraction too late. Pre-focusing is the only solution. Many mirrorless cameras still fail to focus on a subject is moving towards (or away from) you.
Shooting in burst mode simply allows you to take more out of focus and/or mis-framed shots as the camera struggles to keep up. With the right settings, good technique and a bit of light you can take great action shots using a mirrorless camera.

Another result of small camera bodies is that the batteries have often also been pared-back. This capacity reduction, combined with the need to always use the LCD or electronic viewfinder means the battery life of some Mirrorless cameras isn't quite in the 'shoot-all-day' range that many DSLRs offer (that said, entry-level DSLR batteries have also been getting smaller, so it's worth factoring-in the cost of a spare, whichever system you choose).

What comes next?

Mirrorless cameras have only been on the market for around five years, rather than the 15-or-so years over which enthusiast-oriented DSLRs have been developed (plus, of course, more than half a century's worth of refinement of the underlying SLR mechanism), so it's understandable that there is more obvious room for improvement.

Mirrorless cameras are still a developing technology, but even in the past couple of years we've seen considerable improvements in focus speed and user interfaces designed to make it easier to access their full capabilities.

Unless you need the fast-subject shooting capability of a DSLR (most Mirrorless models are getting there, but not all), we think that Mirrorless cameras' size, accessibility and (increasingly) attractive price make them pretty compelling. In fact, if you're someone looking to upgrade from a compact camera, it's certainly worth asking yourself the question 'do I really want a DSLR?'