The old photo adage “The best camera is the one that’s with you” especially applies to pros who are in the business of content creation and realize that having a camera with them at all times is advantageous. For a long time, the choice was to lug your big pro DSLR everywhere, buy a lower-end DSLR that was still pretty bulky or get a compact digital camera whose image quality suffered due to its tiny image sensor—or go camera-less.
The mirrorless interchangeable-lens camera changed all that. Now, you can get a truly compact camera that can produce DSLR image quality due to its DSLR-size image sensor. By eliminating the DSLR’s bulky, complex and costly moving mirror, mirror box, focusing screen and pentaprism or pentamirror viewfinder and replacing them with full-time live viewing on the camera’s LCD monitor (and an eye-level electronic viewfinder, with some cameras), designers were able to greatly decrease camera bulk while retaining a large sensor. The lenses are also smaller than equivalent DSLR lenses, although not that much smaller with the larger-sensor (APS-C) cameras because the lenses have to be able to cover the sensor’s active area.
The mirrorless camera’s main advantage is its tiny size. Mirrorless interchangeable-lens cameras are much smaller than DSLRs, many even pocketable with a flat “pancake” lens attached. The smallest is Pentax’s Q model, measuring just 3.9×2.3×1.3 inches and weighing a mere 6.3 ounces (and containing the genre’s smallest image sensor, a compact digicam-like 1/2.3-incher). Even the largest mirrorless models are quite compact: the Fujifilm X-Pro1 at 5.5×3.2×1.7 inches, 14.1 ounces; the Olympus OM-D E-M5 at 4.8×3.5×1.6 inches, 13.2 ounces; the Panasonic Lumix DMC-GH2 at 4.9×3.5×3.0 inches, 13.8 ounces; the Pentax K-01 at 4.8×3.1×2.3 inches, 16.9 ounces; and the Samsung NX20 at 4.8×3.5×1.6 inches, 12.0 ounces. But by way of comparison, that “big” Pentax K-01 is just 61% of the volume and 72% of the weight of the company’s K-5 DSLR (which itself is one of the smaller DSLRs on the market)—yet it uses the same wide line of K-mount lenses and features a similar sensor. Sony’s NEX-7 measures just 4.7×2.6×1.7 inches and weighs 10.3 ounces, yet contains a 24.3-megapixel APS-C sensor.
Most mirrorless camera makers offer at least one flat “pancake” lens that’s a good match for the body when you want a really compact package. Note that zoom lenses, especially with the larger (APS-C) sensor mirrorless cameras, do increase package size, precluding pocketability. Even with 18-55mm kit zooms (14-42mm for the Micro Four Thirds format), however, the mirrorless models are still much smaller than DSLRs with kit zooms.
Pocketability aside, a mirrorless system, consisting of a body or even two, plus a few lenses, spare battery and memory cards, is much smaller than a comparable DSLR system, easy to carry in a small camera bag. When you want to travel light, but don’t want to give up image quality, mirrorless is the answer.
For the most part, a smaller selection of lenses is available for mirrorless cameras than for DSLRs (in part because of their relative newness). But for an always-with-you, carry-anywhere camera, you’re not looking to cart around a bunch of glass. All mirrorless cameras provide 35mm-camera-equivalent focal lengths of at least 27mm on the wide end and 90mm on the long end (297mm on the long end if you exclude the Pentax Q and Fujifilm X-Pro1). The 27-90mm range includes the focal lengths most would want for a walkaround camera.