It seems like just yesterday that we were trying to come up with a sensible name for mirrorless cameras but 2016 will mark the eighth anniversary of the first Mirrorless Interchangeable Lens Camera. In that time they've gone from being an interesting new development to being a credible alternative to APS-C and full-frame DSLRs.

Mirrorless cameras are interchangeable lens cameras that don't have the mirror and optical viewfinder that define a DSLR. In most other respects, they're extremely similar: with only a few exceptions, most mirrorless cameras these days are built around the same sized sensors as DSLRs, increasingly have similar lenses available, and can offer the same image quality.

Other than the presence of a through-the-lens optical viewfinder, the only other significant difference is that DSLRs have a separate, dedicated autofocus module, whereas mirrorless cameras use their main imaging sensor for focus. Beyond that, there's not a whole lot of difference between the two: mirrorless and DSLR cameras are available for a variety of needs and budgets and there isn't a major manufacturer that doesn't have some mirrorless cameras in its lineup.

What are the advantages?

Size

As mirrorless cameras have matured they've also diversified, making it harder to generalise about their strengths and weaknesses. It remains true though that mirrorless cameras are generally smaller than their DSLR peers. Taking the mirror out means that the lens can be mounted closer to the sensor, helping to reduce the size of the system. Furthermore, without the need to be backwards-compatible with film-era systems, the lenses tend to be designed to match the size of the sensor, meaning many of these are smaller, too. Just as in the DSLR world though, more expensive, faster lenses - especially those designed for Sony's full-frame Alpha a7-series - are still pretty big and bulky.

Autofocus

Another advantage is autofocus. Once considered a weakness of mirrorless, hybrid autofocus systems (using a variant of the phase-detection method used in DSLRs to determine how far away a subject is) have essentially closed the gap between DSLR and mirrorless focus performance for all but the most demanding applications. Not all mirrorless models offer hybrid AF though, so it's worth doing a little research if you need a camera with good continuous autofocus.

The Sony a6000 is much smaller than a DSLR but offers impressive autofocus tracking performance

In fact, because all mirrorless cameras assess focus from their imaging sensor rather than a separate module, they are able to be more accurate and consistent, especially when focusing wide-aperture lenses. Combined with awareness of the scene that allows cameras to track and maintain focus on a subject's eye, it's increasingly the case that autofocus is one of mirrorless's strengths.

Video

Mirrorless cameras already use their sensors' continuous output to constantly preview the scene, so it's not a big leap (or change in behavior) for them to capture this output as movie footage.

The most obvious advantage of mirrorless cameras is that there isn't a mirror blocking the sensor. This means that unlike a DSLR, you don't have to completely change the way that the camera works in order to shoot video: you can continue to use the same means of previewing and shooting when you switch from stills to video. As a result, it's mirrorless cameras that have led the charge towards offering better and better video and the clever photographic tricks that can stem from it.

System

Many mirrorless systems are now mature enough to include a broad range of lenses, such as Fujifilm's X-mount lineup, pictured here.

Because all mirrorless systems are less than eight years old, their systems aren't as fully developed as legacy DSLR systems. The flip-side of this novelty is that most of the lens designs are relatively new and are well optimised for use on the latest high-resolution models. It's worth checking that the lenses you might want are available for the system you're considering, but you may find that there are more modern, more interesting or more affordable lenses available for some mirrorless systems than are available for the big DSLR brands.

What are the disadvantages?

The disadvantages of mirrorless are decreasing with every new generation of camera and only a few of the remaining drawbacks are directly related to the lack of a mirror. The relative novelty of mirrorless mean that manufacturers are still finding their feet, so there's less consistency of performance and behaviour across brands: ergonomics and user interface are still the subject of experimentation and not every company has worked out how to design lenses that autofocus quickly.

Furthermore, the need to constantly power the sensor and screen while shooting, combined with attempts to capitalize on the size benefits of mirrorless mean that battery life is much more limited on most mirrorless cameras - sometimes to a restrictive degree. We'd strongly recommend you read our reviews or roundups to find out about these details before making a purchase.

Most mirrorless cameras allow SLR lenses to be mounted using adapters. The Sony a7R II is the first that can autofocus other brands' DSLR lenses successfully.

Finally, of course, there's the absence of an optical viewfinder. While electronic viewfinders are constantly improving, aren't limited by the size of the image format and can more accurately preview the image you're going to get, most contemporary cameras' live view feeds still lag behind the action when continuous shooting. On top of that, some people simply prefer the 'part-of-the-scene' feeling that an optical finder gives. These factor, as much as the vast number of photographers already invested in DSLR systems, pretty much guarantees that mirrorless cameras won't totally replace the DSLR in the near future.

Just as mirrorless cameras have started to compete in areas the DSLRs have traditionally dominated, we've also seen DSLR makers beginning to respond to that challenge, with mirrorless-style on-sensor phase detection being added to offer a faster live view experience and better autofocus during video. As such, it's getting to the point that personal preference for one particular feature or one specific lens might sway your decision more than the presence or absence of a mirror in the camera. Whichever you choose, advances in technology and a broader range of choices can only be a good thing.

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