FOCAL-LENGHT FACTORBecause mirrorless-camera sensors are smaller than "full-frame," 35mm-sized sensors, they effectively crop into the image formed by the lens, producing a telephoto effect compared to the same lens used on a full-frame DSLR.
APS-C sensors have a 1.5x focal-length factor (approximately); Micro Four Thirds System sensors have a 2.0x factor; Nikon's CX sensor has a 2.7x factor; and the Pentax Q sensor has a 5.5x factor. This means a 28mm lens used on an APS-C camera frames like a 42mm lens on a 35mm camera. When used on a Micro Four Thirds camera, a 28mm lens frames like a 56mm lens on a 35mm camera. When used on the Nikon J1 or V1, a 28mm lens frames like a 76mm, and when used on the Pentax Q camera, a 28mm lens frames like a 154mm. The primary significance of all this is that it's tough to get really wide-angle lenses for smaller-sensor cameras. Comparing the lenses currently available today for mirrorless systems, the widest you're going to go is a 24-27mm equivalent, the exception being with Micro Four Thirds cameras, thanks to Panasonic's 7-14mm, which frames like a 14-28mm.
Of course, if you're a telephoto fan, the smaller sensors are a good thing, with equivalent focal lengths up to 600mm available today. See the chart to get an idea of the range of focal lengths available for each mirrorless systems.
LENS ADAPTERSSince mirrorless cameras are much thinner front to back than DSLRs, they have a smaller "flange back distance," the distance from the lens mount to the image sensor. That means a wide variety of lenses for other cameras can be used with them via adapters.
Olympus and Panasonic offer adapters that let you mount standard Four Thirds System lenses on Micro Four Thirds mirrorless cameras, in most cases retaining all camera features. Panasonic offers adapters to mount Leica M and R lenses on MFT cameras, and Olympus has an adapter to use Olympus OM SLR lenses.
Sony offers two adapters that let you use Sony (and legacy Konica Minolta Maxxum) SLR lenses on the NEX mirrorless cameras. The new LA-EA2 adapter even incorporates a phase-detection AF system similar to the one in Sony's SLT-A65 translucent-mirror DSLR, providing quick, continuous autofocusing with these lenses.
Nikon's FT1 Mount Adapter lets you use Nikon DSLR lenses on the J1 and V1, providing autofocusing with those that contain AF motors (the AF-S lenses). Samsung's Pentax adapter lets you use Pentax SLR lenses with the NX camera. As yet, Pentax doesn't provide any adapters for the Q camera.
Independent companies such as Novoflex, Pro Optic and Redrock Micro produce adapters to attach a wide range of lenses to mirrorless cameras (mainly, Micro Four Thirds and Sony NEX). While the adapters provided by the mirrorless camera makers generally support all camera features, the third-party adapters often just let you attach the lens; there may be no autofocusing, and exposure controls may be limited to manual or aperture-priority. Be sure to check out the capabilities of the adapters for your system if you're looking to use lenses not specifically designed for your camera.