Mirrorless Cameras: A Primer

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.

Until fairly recently the only real options have either been compact cameras which have very small image sensors, or the considerably larger - in both sensor and body dimensions - DSLR design.

That middle ground has finally been filled, with the emergence of 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 immediately 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 Olympus PEN, left), and those which resemble scaled-down DSLRs, like the Panasonic G1 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 (shown above) 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, Samsung's NX, Sony's NEX, Pentax's Q and Nikon's 1 systems have also been introduced, with Fujifilm confirming that it too will join the party in early 2012.

What's to gain?

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. 

The advantage 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 have significant drawbacks in terms of either speed or flexibility.

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.

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.

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).

What are the disadvantages?

There are currently two major disadvantages we've experienced with the Mirrorless cameras we've encountered so far. The first is the inability 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 Mirrorless cameras cannot track moving subjects (specifically those moving towards or away from you) as well as DSLRs, if at all, and so are far less well suited to sports or other action photography. For the average target user of systems like this the most obvious consequence of the slow low-light focus and inability to keep up with movement comes when shooting even moderately active children indoors in anything but perfect light.

The camera makers we've discussed this with admit that offering continuous and tracking AF on Mirrorless is challenging, so we're not expecting a radical improvement in the short term (though Nikon may have found the answer with its 1 System). As a result, if shooting fast-moving subjects is one of your reasons for buying a new camera, then you'll probably be better served with a DSLR, though you'd still have to consider investing in a more expensive lens to get the best results. There is also 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).

The other drawback we've encountered is that the Mirrorless cameras we've tested so far can struggle to focus in low light to a greater degree than most DSLRs do. 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 few mirrorless cameras can 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.

Waiting for focus means the picture is taken a fraction too late. Pre-focusing is the only solution.	Most mirrorless cameras 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 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).

The final limitation for Mirrorless systems at present is a distinct lack of native lenses. For many buyers, this is likely to be a moot point - the majority of DSLR buyers don't ever take off the lens that comes with their camera. However, if you are planning to throw yourself into photography, it's worth looking to see how extensive your options are.

As the oldest Mirrorless system, it's no surprise that the Micro Four Thirds lens mount has the most comprehensive range available: stretching from wide-angle zooms through to a selection of telephotos and including a handful of enthusiast-friendly prime lenses. However, this can't yet compete with the vast ranges of DSLRs lenses offered by the camera makers and third-party manufacturers.

Meanwhile the newer Sony E-mount (used by its NEX cameras), currently offers only six lenses, and Samsung has a slightly broader choice for its NX system. This will undoubtedly change and all the Mirrorless manufacturers are being open about the lenses they have planned.

Are there any other options?

Mirrorless cameras aren't the only attempt to bridge the gap between compacts and DSLRs. Sony has, characteristically, created its own solution. Its SLT range offers some of the best features of a DSLR with compact-camera style live view, giving a camera that's capable of fast continuous AF, even when shooting video. You lose the size advantage of the Mirrorless systems, however.

Is the technology ready yet?

Mirrorless cameras have only been on the market for around three years, rather than the 15-or-so years over which 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, despite being able to inherit a lot of know-how from both DSLRs and compact cameras, are still a developing technology. But, even though most of these systems are only just reaching their second or third generation, we've already seen considerable improvements in focus speed and user interfaces designed to make it easier to access their full capabilities.

Even in their relatively unrefined state, unless you need the fast-subject shooting capability of a DSLR, we think the Mirrorless cameras' size, accessibility and (increasingly) 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?'

To further help your decision, we'll be publishing a buyer's guide to the best Mirrorless models in the coming weeks.