The Pentax Q is the smallest mirrorless, interchangeable-lens digital camera, in large part because it uses the smallest sensor, a 1/2.3-inch unit measuring 6.2×4.6cm—a size commonly found in point-and-shoot compact digital cameras.
On the plus side, this diminutive sensor allows for a very small camera body and lenses. The Q body measures a mere 3.9×2.3×1.2 inches. Due to the small sensor’s 5.6x focal-length factor, any given focal length used on the Q frames like a lens 5.6 times its focal length on a 35mm camera; thus, a 10mm lens would effectively become a 56mm, for example. Currently, the longest lens available for the Q is 18mm (equivalent to 100mm on a 35mm camera); the shortest is the wide end of the 5-15mm zoom (equivalent to 27.5-83.0mm on a 35mm camera).
On the minus side, a small sensor collects less light (fewer photons) than a larger sensor, resulting in noisier images, especially at higher ISO settings. The Q’s sensor is a backlit CMOS type—meaning light from the lens doesn’t have to go past the sensor’s wiring architecture as is the case with conventional "frontlit" sensors. This should help with light gathering.
NIKON 1 V1 AND J1
Nikon’s new mirrorless models share a new CX-format CMOS sensor that measures 13.2×8.8mm—just over four times the area of the Q’s sensor, but about half the area of a Four Thirds sensor. The sensor’s 16mm diagonal measurement gives it a 2.7x focal-length factor (a 10mm lens on the V1 or J1 frames like a 27mm lens on a 35mm camera).
In terms of light-gathering ability, that should place the V1 and J1 somewhere in between the Q and the Micro Four Thirds cameras. Preliminary results indicate that the V1 and J1 are about equal to Micro Four Thirds at lower ISO settings, lagging a bit at higher settings.
MICRO FOUR THIRDS
Micro Four Thirds System sensors are the same size as original Four Thirds System DSLR sensors: 17.3×13.0mm, with a 2x focal-length factor relative to a 35mm camera.
Samsung and Sony offer mirrorless, interchangeable-lens cameras with APS-C sensors, the size used by most nonprofessional DSLRs. Measuring around 23.5×15.6mm, these sensors have a focal-length factor of 1.5x: a 10mm lens frames like a 15mm lens on a 35mm camera.
Because these are the largest sensors currently used in mirrorless cameras, they collect the most light and thus produce the best signal-to-noise ratios, especially at higher ISO settings.
Thus, the big advantage of APS-C sensors in mirrorless cameras is better image quality. The drawback is that bigger sensors require larger-diameter lenses. So while the bodies are very compact, some of the lenses are noticeably larger than those for smaller-sensor mirrorless cameras.
Full-frame sensors measure the same as a full 35mm image frame: 36x24mm. Due to their larger area, full-frame sensors have the potential to produce the best image quality in DSLRs, but due to their size, they aren’t suitable for compact mirrorless cameras.
DxO Labs makes image-quality testing gear used by the industry (as well as their own raw-conversion and lens-correction software for photographers) and publishes a website, dxomark.com, with ratings of raw sensor quality. Nikon’s V1/J1 sensor scored 54 and 56, respectively in DxO testing. Current Micro Four Thirds System sensors scored 51 to 60, and APS-C sensors used in current mirrorless cameras scored 62 to 77 (and the untested-at-press-time 24.3-megapixel Sony NEX-7 has the potential to score even higher). Current full-frame sensors (not available in mirrorless cameras) score 82 to 88. The top-scoring sensor on DxO is an 80-megapixel $40,000 medium-format digital back, which scored 91. (DxO had not tested the Pentax Q as of this writing.)