What are mirrorless and DSLR cameras, and do you need one?
Mirrorless and DSLR cameras are a step up from compact cameras in performance and price. They have larger image sensors than compact cameras, perform better at high ISO settings and in burst mode, and have more dynamic range and better control over depth of field than compact cameras. They accept interchangeable lenses, so that shooters can adjust their choice of lens to shooting conditions.
If you haven't thought before about the features in the preceding paragraph, buy a compact camera. When shooting non-challenging subjects in good light, today's high-end compacts produce results that are indistinguishable from mirrorless or DSLR cameras.
If you need a more capable camera to shoot challenging subjects in poor light, then read on. Buying either a mirrorless camera or a DSLR will improve your photography.
Mirrorless cameras show the output from the camera's sensor on an LCD screen, just like compact cameras. Some mirrorless cameras also have electronic viewfinders (EVFs), which provide the same information as the LCD screen, but which are shaded and easier to use in strong sunlight. Compact camera upgraders will be comfortable with composing pictures on a mirrorless camera.
DSLRs use a mirror to reflect light coming through the lens up through an optical viewfinder. Some DSLRs also have live view, which allows photographers to use the LCD screen to compose photos. However, the implementation of live view in DSLRs is often clunky. The mirror technology in DSLRs is the same technology that high-end 35mm film cameras have used for decades, so most people migrating from film to digital will be comfortable with DSLRs.
Some manufacturers are beginning to introduce hybrid viewfinders, which have some of the functionality of a mirror and some of the functionality of an EVF.
The mirrors in DSLRs allow photographers to shoot many frames per second while keeping track of action-filled scenes through the viewfinder. EVFs can't do this because there is a slight lag while the image preview is routed from the sensor to the LCD. Also, EVFs sometimes freeze while the camera is writing picture files to the memory card.
DSLRs use phase detect autofocus (PDAF). Most mirrorless cameras use contrast detect autofocus (CDAF), which is slower than PDAF. In my experience the difference is not great. I have had no trouble focusing on birds in flight with CDAF, for example. Photographers who need the absolute last millisecond of focus speed, however, may want to choose a camera with PDAF over a mirrorless camera.
With the EVFs in mirrorless cameras, what you see in the viewfinder is what you get when you snap the shutter. That's not always true of DSLRs, which do not necessarily show a 100% view of the frame.
EVFs are also better for manual focus. Most EVFs have some sort of focus assist, where you can zoom in on the image for critical manual focus at wide apertures. DSLRs do not. Also, many DSLRs struggle with providing an accurate preview for lenses with large apertures, because of microlenses that are built into DSLR focusing screens. Anybody who plans to use manual focus consistently, should think long and hard before buying anything other than a mirrorless camera with an EVF.
Mirrorless cameras are smaller. End of story.
Actually, there's more to it than that. Mirrorless cameras are so small in some cases, that the ergonomics are not suited to users with large hands or big fingers.
Mirrorless cameras are roughly divided into “rangefinder” form factor and “mini-dslr” form factor cameras. The rangefinder cameras are small, sometimes very small, and rectangular, like compact cameras. They are not quite pants-pocketable, but they are definitely jacket-pocketable, provided you don't stick a big lens on them. They generally do not have an included EVF. The mini-dslr cameras do have an EVF, as well as a grip which makes the body of the camera larger. You need a small bag or a strap to carry a mini-dslr camera. I prefer the mini-dslr form factor, because I have big hands.
Lens size is another issue. Olympus and Panasonic have teamed up to promote the popular micro four thirds format, which has both rangefinder (Olympus Pen series, Panasonic GF and GX series) and mini-dslr cameras (Panasonic G and GH series). Because the micro four thirds sensor size that Olympus and Panasonic use is smaller than the Sony Nex series sensor, for example, the micro four thirds lenses are also smaller. The difference in size is substantial and matters in real life. Sony makes some very small bodies for the Nex series, which look tempting in the store, but the Sony lenses are much larger than micro four thirds lenses, so the camera and the lens together are big.
DSLRs, well, they're just big.
Of course, bigger can be better. My brother, who photographs weddings, says that his number one rule is to unload all the equipment that he has, all the tripods, all the light stands, the huge camera, the huge flash, everything, whether he needs it or not. It makes everybody think he knows what he's doing.
High ISO, Dynamic Range
DSLRs have less noise at high ISO and are able to capture a wider dynamic range of light then mirrorless cameras. It is fair to say that DSLRs have at least a stop ISO advantage over mirrorless cameras on average, and that DSLRs have a similar advantage in dynamic range, which allows the camera to capture both the highlights and the shadows in scenes with difficult lighting.
There are exceptions. The Sony Nex matches DSLRs in both high ISO and dynamic range, but without the small size advantage of the Panasonic and Olympus micro four thirds cameras or the Nikon 1 series cameras.
High ISO and wide dynamic range matter a great deal to some photographers, and not at all to others. If you do street photography at night, buy the most capable DSLR that you can find. If you shoot mostly during the day, or use flash at night, and don't print bigger than 13x19”, buy a mirrorless camera.
Both DSLRs and mirrorless cameras can shoot HD video. In general, mirrorless cameras are more useful for video because they can accept a wider variety of legacy lenses via adapters, EVFs can be used for framing during video capture (the mirrored prisms in DSLRs can not, and because some of the mirrorless cameras, the Panasonic GH line in particular, have a very high video specification including manual control over most settings and the ability to attach an external microphone.
Still Image Quality
Both DSLRs and mirrorless cameras are capable excellent image quality, allowing prints up to 13 x 19” or larger under most conditions. DSLRs offer slightly better image quality under difficult conditions, especially low light. This difference is not apparent under the vast majority of shooting conditions.
Most people will be served well by a high end compact compact. The best are made by Fujifilm, Panasonic, and Olympus.
If you need better image quality than a compact camera can give you, but aren't sure how to get it, buy a mirrorless camera, one of the Panasonic or Olympus models. The Sony Nex cameras are too big. The Nikon 1 cameras are too expensive for their specification. In addition, Olympus and Pansonic offer more and better lenses for the micro four thirds system than Sony and Nikon offer for their respective mirrorless systems.
If you know exactly how to get the image quality you want, and exactly why a mirrorless model will not be sufficient for your needs, then buy a DSLR. Any of the mid-range models from Canon, Nikon, or Sony would be a good place to start.
Mirrorless cameras are known by other acronyms, including EVIL (Electronic Viewfinder Interchangeable Lens) and MILC (Mirrorless Interchangeable Lens Cameras). The EVIL acronym may reflect reluctance by photographers of a certain age to embrace mirrorless technology.