Viewfinder: Optical for DSLRs, Electronic for Mirrorless
A DSLR gets its name from its viewfinder. The acronym stands for digital single-lens reflex, which means that the camera has one lens that works to both take pictures and project an image into the eyepiece, either via a solid glass pentaprism or a set of mirrors.
Because of this, you actually see the light coming through the lens, which means there's no perceptible lag (the speed of light being the speed of light). This also means that the quality of the viewfinder itself, and the lens, influence its apparent size (measured numerically with a standardized magnification rating), brightness, and framing accuracy. The lens you attach matters, too—an F3.5-5.6 zoom gathers less light than an F1.8 prime, so the image in the viewfinder appears brighter with the latter.
Entry-level DSLRs usually use pentamirror finders, which are relatively small, dim, and don't show the entire view of the lens. Midrange models use solid glass pentaprism finders, which show a larger, brighter image to the eye than a pentamirror. In the DSLR era, full-frame sensors were reserved for upmarket cameras, all of which shipped with pentaprism finders that are noticeably larger to the eye than those in the entry and midrange classes.
Mirrorless cameras do away with the mirror box and optical viewfinder. They send a live view from the sensor to a rear LCD and eye-level electronic viewfinder. When mirrorless systems debuted in 2009, the EVF tech was fairly rudimentary, but it has gotten much better over the years. Newer mirrorless cameras include EVFs with smooth refresh rates, wide color gamuts, and high magnification. Some are better than others, and more expensive cameras typically come with larger, sharper EVFs. It's important to read reviews to see how a particular camera's viewfinder stacks up against competitors.
Choosing between an optical viewfinder and an EVF comes down to personal preference. As EVFs have gotten better and my eyes have gotten older, I've noticed it's easier to see smaller details in a scene and set manual focus with a good EVF.
Autofocus: Newer Mirrorless Cams Trounce DSLRs
Digital SLRs use a dedicated phase detection sensor module to find focus, along with a color RGB meter to gauge light and, in some cases, provide rudimentary face detection. Most DSLRs restrict focus to the central area of the frame, and a few high-end options go further to the edges, but none provide full-frame focus coverage.
Using a separate phase detect unit than the sensor created some issues as tech evolved. Some DSLRs would fall out of calibration, which resulted in images that were focused very slightly ahead or behind a target. This is especially apparent when using a telephoto lens or a bright prime with a shallow depth of field, and is even more noticeable on DSLRs with high pixel sensors. Because of this, focus calibration became a fact of life for photo enthusiasts and pros using DSLRs, to the point where calibration assistants like the Focus Pyramid, a piece of cardboard origami that I reviewed in 2015, were real things you could buy.
Mirrorless cameras focus and meter a scene directly off the sensor, so there's no chance of falling out of calibration. In the early days, all mirrorless cameras delivered slower autofocus than DSLRs because on-sensor focus relied entirely on contrast detection, a method that requires the focus system to slightly overshoot its target before locking in to find perfect focus. On the plus side, because focus is done from the sensor, all but the oldest mirrorless cameras can find focus nearly to the edge of the frame.
Phase detection would soon follow, but it was a little while before mirrorless cameras beat out DSLRs for tracking focus. The introduction of Sony's real-time tracking focus with the a6400 marked a sea change and put a mid-priced mirrorless camera on par with the fastest DSLRs in focus acumen. Since then, other brands have developed similar tracking systems. Further, improvements in processing power and sensor scan speed have introduced improved subject detection for common photographic subjects, including people, wildlife, and motor vehicles.
In short, DSLRs are hampered by a relatively narrow area of interest for autofocus, and only a few have rudimentary face detection support through the viewfinder. Mirrorless cameras leverage nearly the full area of the sensor for focus and include the processing power to reliably detect and track moving subjects at faster speeds and with more precision. Thanks to the shift to mirrorless tech, manufacturers have made huge leaps forward in autofocus in recent years.
Ease of Use: EVFs and Autofocus Give Mirrorless a Leg Up
Both DSLRs and mirrorless cameras support everything from fully manual to entirely automatic operation, so both beginners and experts can get the results they want. I give an edge to mirrorless cameras, however, because an EVF provides instant feedback of the exposure level of an image, while with a DSLR, you'll have to have some know-how to set exposure compensation to overcome scenes that are difficult for a camera to meter properly, notably backlit scenarios. In addition, mirrorless cameras are a little smarter when it comes to autofocus, which results in fewer out-of-focus pictures than with a DSLR.
Burst Rates: Electronic Shutters Outpace Mechanical
When using the optical viewfinder, a DSLR relies on a mechanical focal plane shutter to expose an image. These shutters can only open and close so fast, so even the speediest DSLRs like the Canon EOS-1D X Mark III top out at 16fps. Most entry-level DSLRs put up burst numbers in the 3 to 6fps range, with midrange cameras getting faster; the Nikon D500 hits 10fps, for instance.
As mentioned, early mirrorless cameras were on the slow side for focus, which led to slow burst rates. But newer cameras blow them out of the water. Canon's entry-level full-frame EOS R8 is capable of focusing and tracking subjects at 40fps for short durations, and Sony's high-end a9 III goes to 120fps. Mechanical rates are in line with DSLRs for the most part, with more affordable (and older) mirrorless cameras delivering slower rates, and newer ones hitting 10 to 12fps.
Image Quality: Not Much Movement in Recent Years
The early days of DSLRs marked a race to improve resolution and picture quality. I remember starting off with a 6MP DSLR in 2005 and upgrading to a 10MP model a few years later, which was a huge jump at the time. An industry move from CCD to CMOS sensors netted huge improvements in picture quality at higher ISOs, and improvements in sensor architecture expanded dynamic range, giving creators huge leeway to control color and tone.
Sensor tech hit a plateau in picture quality around 2015 with the release of the first camera with a full-frame BSI CMOS sensor, the Sony a7R II. Newer chips may pack more pixels (the latest a7R V uses a 60MP sensor, for instance), but PCMag's tests show that, for noise and dynamic range, sensor tech has stood relatively still for a decade now.
There have been improvements, however. Newer sensors, exclusive to mirrorless, scan more quickly than older DSLR chips. A fast scan speed has some practical benefits, including less chance of seeing motion distortion in video, the option to sync with a flash using the electronic shutter, and burst capture without any loss of view of a scene. We've even seen one camera that reads out every pixel of its sensor simultaneously, the Sony a9 III, and it's natural to expect others to follow.
Video: Huge Gains in the Mirrorless Era
Early DSLRs recorded stills only. Video debuted in 2008 with the Nikon D90, which recorded 720p with an APS-C chip, but a few months later, Canon stole all the thunder with its full-frame, 1080p-equipped EOS 5D Mark II. Hollywood filmmakers started paying attention to DSLRs after the 5D Mark II was used to film a 2010 episode of the Fox medical drama House, M.D., as it was able to squeeze into tight spaces where the 35mm film cameras typically used for the show wouldn't fit.
Mirrorless cameras debuted in late 2009, so again, video capability depends on the age of your camera more than anything else. DSLRs and mirrorless cameras kept up with each other in terms of resolution, frame rates, color depth, and encoding quality as the years progressed, but mirrorless cameras took a huge lead in autofocus with the advent of on-sensor phase detection. Sensor stabilization, which is frequently included in mirrorless cameras but rare in DSLRs, is a useful feature for handheld recording.
In the past few years, video tech has made huge leaps, and it's all been in mirrorless cameras. More and more support ProRes and Raw video recording with 12-bit color depth, offering cinematographers ample leeway to edit color. 8K resolution and 120fps slow motion at 4K are also increasingly common.
Size: Mirrorless Cameras Are Slightly Slimmer Than DSLRs
Mirrorless cameras have fewer parts than DSLRs, which makes for a smaller, lighter design. Since there's no mirror box or optical viewfinder to account for, engineers are able to create truly small models like the Panasonic Lumix GM5.
The GM5 is an outlier, and for the most part, mirrorless cameras are simply a little bit slimmer than comparable DSLRs. Entry-level models with APS-C sensors like the Canon EOS R10 are light and best used with slimmer lenses, while midrange options like the Fuji X-H2 have deeper grips and work better with telephoto lenses, but neither are as tiny as the GM5.
The story is similar in full-frame. Entry and travel-friendly models like the Canon EOS R8 and Sony a7C II run pretty small and work best with lighter-weight zooms and prime lenses. Midrange and pro options like the Canon EOS R6 Mark II and Nikon Z 8 have larger bodies and grips for better balance with larger lenses.
Battery Life: Where DSLRs Sip, Mirrorless Chugs
If there's one area where DSLRs run circles around mirrorless cameras, it's in battery life. This is by virtue of how the cameras work. A DSLR uses energy to drive focus, open and close its shutter, and process an image. A mirrorless camera's viewfinder draws power whenever the camera is in use. That, plus more demanding processors, higher burst rates, and video modes, means mirrorless cameras get less battery life than DSLRs. In the DSLR era, it'd be reasonable to expect to get through a weekend on a single battery. With mirrorless, you're often left changing power packs midday.
Lenses: DSLR Lenses Are Moving to a Used Marketplace
At the height of the DSLR era, every major player had a full set of lenses to go with its cameras, and third-party makers added to the selection. Today, there's no one making new lenses for DSLRs. It's now been a few years since Pentax, the last brand sticking with DSLRs, released a newly developed lens, the HD DA 21mm Limited. Even with no new glass coming out, there's a huge amount of lenses still available at photo shops, new and used. Canon DSLRs work with lenses dating back to the 1980s, and Nikon and Pentax digitals support glass going back to the '70s.
Mirrorless cameras use different lenses than DSLRs, and because they put such a short distance between the lens mount and sensor, they can use DSLR system lenses with an adapter, too. Made for mirrorless lenses have some advantage over older glass, however. Because there's no optical view through the lens, camera makers are able to lean on digital corrections to remedy optical distortion, which in turn makes smaller lens designs with fewer glass elements possible. Since you're looking through an EVF, you'll never see any distortion when taking photos.
It's not all digital either. Manufacturing has gotten better, so there are more types of corrective lenses to include in optical formulas. And there's physics: Because the rear of a lens is so close to the sensor in a mirrorless camera, light has a shorter path to travel, which makes it possible to create lightweight lenses with extremely wide angles of view, like the Canon RF 10-20mm F4 L IS STM, and previously unheard of formulas like the Sony FE 50-150mm F2 GM.
So, with a DSLR, you'll have plenty of lenses from which to choose, though new options will disappear over time, so be sure to be comfortable with the secondhand market. Mirrorless cameras have been around long enough to have full sets of native lenses and include the latest and greatest optics, plus they can adapt older glass if you want that vintage look.
Availability: Few New DSLRs Are Made Today
DSLRs had their day in the sun, but since Ricoh Pentax is the only brand that's stuck with the tech, the number of new options in the market is stark when compared with the heyday of DSLRs. Even so, you can still buy a new DSLR from Canon, Nikon, or Pentax, at least while supplies last. At press time, it's been more than two years since the most recent DSLR release, the Pentax K-3 Mark III Monochrome. Nikon's most recent is the D780 (2020), and Canon's last DSLR was the 90D from 2019.
In contrast, new mirrorless cameras come out on a regular schedule. At the time of publication, there are several systems from which to choose from including Canon RF, Nikon Z, Fujifilm X, L-Mount Alliance, Micro Four Thirds, and Sony E, all of which use lens systems that are under active development. If you are interested in getting the latest and greatest, mirrorless is the path forward.
Final Thoughts: Go With Mirrorless for the Latest Tech, or a DSLR for the Optical Viewfinder Experience
DLSR and mirrorless cameras are quite similar in form and function, but with mirrorless, you'll have access to the newest lenses, autofocus systems, and enjoy video features that simply weren't available when DSLRs dominated the marketplace. However, photographers who prefer an optical viewfinder, want to save money by getting older (but still quality) gear, and whose creative work doesn't require the next-generation autofocus or video, will find that a DSLR to be a more than capable tool.