For many years, 35mm SLRs were the mainstay of "general" photographers. Medium- and large-format offered better image quality through their larger film sizes, and held sway among fine-art, architectural, landscape and even wedding/portrait photographers for that reason (and, in the case of large-format and studio still-life, for the cameras' perspective-correcting movements). But, by far, the most camera sales were 35mm SLRs for their compactness, speed, versatility and wide range of lenses and accessories, along with lower cost.
When digital took over, camera manufacturers essentially adapted their film camera designs, and we had DSLRs, medium-format digital cameras and digital backs that could be attached to medium- and large-format cameras.
It bothered some people, though, that there wasn't more new thinking in terms of camera designs. DSLRs didn't need room for film cassettes, and with the development and improvement of electronic viewfinders, the DSLR's bulky, complex and costly SLR moving-mirror/prism-finder assemblies were no longer required. By doing away with the latter, manufacturers were able to produce much smaller cameras featuring the same sensors used in the DSLRs, thus providing the same image quality in a much smaller package. In 2008, Panasonic introduced the first mirrorless interchangeable-lens camera, the Lumix DMC-G1, which looked like a "mini-DSLR"—much smaller, but containing the same Four Thirds System image sensor used on Four Thirds System DSLRs: the Micro Four Thirds (MFT) System was born (Micro referring to camera size, not sensor size). This was soon followed by Olympus' MFT E-P1, which featured a "flat" design like a compact digital camera and thus was even more compact, then models from Samsung and Sony containing larger APS-C sensors used in those companies' DSLRs. Sony introduced the first full-frame, mirrorless interchangeable-lens camera, the a7, in 2013. Today, there are about as many mirrorless models on the market as DSLRs. Let's look at the benefits and drawbacks of the two camera types for the outdoor photographer.
Since they use the same image sensors, mirrorless cameras and DSLRs of a given pixel count and generation offer pretty much the same image quality. But, as with DSLRs, bigger sensors generally mean better image quality: Full-frame mirrorless cameras produce better image quality than APS-C ones, which produce better image quality than Micro Four Thirds ones, which produce better image quality than mirrorless cameras with even smaller sensors, just as full-frame DSLRs produce better image quality than APS-C ones. The fact is, all of today's full-frame, APS-C and MFT mirrorless and DSLR cameras deliver very good image quality.
Obviously, mirrorless cameras are smaller than DSLRs, especially the higher-end DSLRs (see the accompanying chart for some examples); that's their main raison d'etre. If you've become disenchanted with lugging around a DSLR, a mirrorless camera may well be the solution.
The smallest mirrorless cameras have smaller-than-DSLR sensors—a 1/1.7-inch in the case of the Pentax Q-S1 and Q7, and a one-inch in the case of the Nikon 1 series. These systems offer ultimate compactness, but there's a cost in terms of image quality due to the small sensors.
The next smallest mirrorless cameras are the "flat" ones that look like compact digital cameras. These are great when size is the main issue, but many lack an eye-level electronic viewfinder and thus can be a bit awkward to use in bright light, as the image on the external LCD monitor can be hard to see (you can attach a device like a HoodLoupe to remedy this, but then the camera is no longer so compact). The "flat"-style cameras that do have built-in EVFs are ideal when size is a major issue, but you still want convenient eye-level viewing. (Note that tilting/swiveling touch-screen external monitors can also contribute greatly to ease of use, although image visibility in bright light remains an issue.)
The "mini-DSLR"-style mirrorless cameras are the largest, some the size of entry-level DSLRs. But they're still generally lighter than DSLRs and, overall, system size is smaller, making them easier for traveling.
Sometimes, it comes down to, "Do I want to lug the camera (and lenses and accessories) with me on this outing?" If you're a photographer, of course, the answer is generally "Yes!" But many are finding it "Yes!" more often when a mirrorless system is at hand rather than a DSLR system.
Lenses for mirrorless cameras are smaller and lighter than lenses for DSLRs, but sometimes not by as much as you'd think. That's because a lens has to cover the sensor format; a lens that can cover a full-frame sensor has to be big enough to do that.
Keeping lens size down is also why there are few truly fast lenses for mirrorless cameras. While there are many faster than ƒ/2.8 available for DSLRs, few are offered by the mirrorless camera makers. However, third-party lenses can be attached to most mirrorless cameras via adapters, and some of these lenses are very fast (albeit manual-focus only).
DSLRs also offer a wider range of native (AF, and designed for the camera) focal lengths: Canon, from an 8-15mm superwide zoom to an 800mm supertele, Nikon, from a 14-24mm superwide zoom to an 800mm supertele, and Sigma, from an 8-16mm superwide zoom to an 800mm supertele, for example. If you're into supertelephoto or superwide-angle photography, or want superfast lenses, you'll want a DSLR.
On the other hand, while the native AF lenses offered by their manufacturers are limited in range and speed compared to what's available for DSLRs, mirrorless cameras can use a tremendous range of lenses from other manufacturers via widely available adapters. The short flange-back distances (the distance between the lens mount and the image plane) made possible by the elimination of the SLR mirror box mean you can use just about any lens for which an adapter can be found. You lose autofocusing and sometimes automatic exposure control, but you have access to a tremendous range of lenses.
And, of course, the native lenses offered by the mirrorless camera makers do cover the most popular focal lengths, and are suitable to a wide range of outdoor photography, including landscapes and macro work. If some mirrorless models intrigue you, check out what lenses the manufacturer offers.
Most mirrorless cameras use contrast-based autofocusing (CDAF), while DSLRs use phase-detection autofocusing (PDAF) except in Live View mode, when they must use CDAF, because PDAF requires the SLR mirror to be in the "down" position, while Live View requires it to be "up."
Mirrorless contrast-detection AF is very fast in terms of establishing focus on a stationary subject, and more accurate than phase-detection AF because CDAF reads focus right off the image sensor, while PDAF depends on moving parts that must be aligned precisely. However, PDAF can determine in a single reading whether the subject is in focus, and if not, by how much it is out, and in which direction, while CDAF requires multiple readings. This makes PDAF better for predictive AF on quick action subjects like birds in flight. Some mirrorless cameras (and newer Canon DSLRs in Live View mode) offer hybrid AF systems, which place PDAF sensors on the image sensor itself. This makes for better performance on action subjects than CDAF alone, but still isn't as good as PDAF.
Many mirrorless cameras offer touch AF on their external LCD monitors, terrific for video focus changes and handy for still work, especially for odd-angle shooting with cameras whose LCD monitor tilts and swivels.
For manual focusing, electronic viewfinders are often better than DSLR finders because you can zoom the image and adjust its brightness, and many cameras offer focus peaking, which highlights in-focus edges. And the focusing screens in DSLRs aren't optimal for manual focusing—unlike those of film SLRs, DSLR focusing screens are designed for AF use, not manual focusing.
DSLRs use pentaprism or (in lower-end models) dimmer pentamirror optical viewfinders, while mirrorless cameras use electronic viewfinders. Some mirrorless cameras don't have eye-level finders; on those, you must compose and focus via the external LCD monitor, which can be difficult to see in bright daylight (DSLRs used in Live View mode have the same problem).
Which type of viewfinder is better is the topic of many a heated online debate. Some (especially longtime film SLR users) prefer DSLR optical finders because you're seeing the image the lens is sending to the film. These photographers don't like the electronic "video" image of the EVF being between them and their subject. Of course, the SLR finder image briefly blacks out as the mirror flips up so the image can be recorded, so it's not really "continuous," either.
Conversely, fans of the EVF like the fact that they're seeing the image produced by the image sensor—the image as it will be recorded. EVFs can show the effects of white balance adjustments, exposure compensation and special effects, and can be adjusted for brightness in dim light (be aware that the viewfinder image gets noisier when this is done).
Note that many mirrorless cameras, when used at their fastest advance rates, don't show a real-time live image in the finder—there's a delay between frames, which can make it difficult to track quick, erratic action. That said, there's at least one photographer we know of who shoots birds in flight with EVF mirrorless cameras using old manual-focus SLR supertelephoto lenses and focusing manually (birdsinaction.com), so it can be done. Still, if you tend to specialize in birds in flight, we'd recommend a DSLR and AF lenses.
Image stabilization is especially important with mirrorless cameras that don't have an eye-level EVF because holding the camera out at arm's length to use the external monitor for composing and focusing isn't very stable. Some mirrorless cameras have in-body image stabilization (IBIS), where the sensor shifts to counter camera shake (Pentax and Sony DSLRs also use this method). With others, you can get stabilized lenses (Canon and Nikon DSLRs don't have IBIS, but many of their lenses have optical stabilization built in). Ironically, while Sony's DSLRs have IBIS, most of their mirrorless cameras do not (you need an OIS lens to get stabilization with these). However, Sony's new a7 II has five-axis IBIS. IBIS has the advantage of working with any lens, while in-lens stabilization can be optimized for that particular optical formula. Both types work (the five-axis systems in the Olympus OM-D E-M1 and E-M5 Mark II, and Sony's a7 Mark II are especially effective); the main point is, you want stabilization—either in-camera or in-lens—if you shoot handheld a lot (which presumably you do with a mirrorless camera), especially if your camera doesn't have an eye-level EVF (which makes for more stable handholding).
Mirrorless cameras tend to be better video tools because they provide eye-level viewing in Live View/video mode, something DSLRs don't do—with a DSLR, you must use the external LCD monitor to compose and focus videos. Mirrorless cameras also tend to have better AF performance for video—while not up to birds-in-flight standards, mirrorless CDAF is better than the CDAF employed by DSLRs in Live View/video mode. Three affordable mirrorless models can do in-camera 4K video (Panasonic's $1,699 GH4 and $799 G7, and the $1,299 Samsung NX1), while the $8,000 Canon EOS-1D C is the only DSLR that can do 4K in-camera. Sony's a7S mirrorless camera can do 4K video, but requires an external recorder to do so.
Many of today's mirrorless cameras have built-in WiFi. This allows you to control your camera remotely from your smartphone or tablet (using the camera maker's photo app), transfer images wirelessly from camera to smart device or computer, and even geotag your images via the smartphone's GPS. While built-in WiFi is common with mirrorless, few DSLRs have it.
Since mirrorless cameras are in Live View mode all the time, and often use smaller batteries to keep size down, they won't get as many shots per battery charge as a DSLR. This isn't a huge deal, though, as extra batteries are relatively cheap and easy to carry (and you should carry spares even if using a DSLR).
Mirrorless cameras tend to give you more "bang for the buck" than DSLRs, sensor-wise, assuming you like EVFs and can live with the smaller choice of AF lenses. Sony's full-frame, 36.4-megapixel a7R mirrorless camera currently sells for $1,999, while Nikon's full-frame, 36.3-megapixel D810 DSLR with similar sensor sells for $1,000 more. And Sony's original full-frame, 24.3-megapixel a7 (introduced late in 2013) currently sells for $998, about $500 less than Nikon's full-frame, 24.3-megapixel D610 DSLR, which came out about the same time with a similar sensor. The highest-priced, non-full-frame mirrorless models sell in the $1,200 to $1,700 range, and are competitive with the top APS-C DSLRs in everything but action AF and lens selection (and offer some features the DSLRs lack, including 4K video).
Mirrorless cameras offer DSLR image quality in a smaller, lighter package, near-real-time viewing of the image as it will be recorded, quick AF on stationary subjects and subjects moving across the frame, and generally better video capabilities. DSLRs offer a wider range of lenses, better AF on quick subjects moving toward the camera, bigger bodies (which could be advantageous when using bulky telephoto lenses) and "pure" optical viewing of the subject. You'll have to decide for yourself which type of camera better suits your needs. It's definitely a good idea to try out each potential purchase at a camera store (or by renting it) to see how you get along with its benefits and drawbacks.