Sony's only full-frame DSLR, the Alpha SLT-A99 features a 24.3-megapixel sensor and it measures 5.8x4.4x3.1 inches and weighs 25.9 ounces. The A99 can use all Sony A-mount and legacy Konica Minolta lenses; when a DT (APS-C) lens is attached, the camera automatically crops to APS-C format. Full-frame lenses range from a 16-35mm superwide zoom and a 16mm full-frame fisheye to a 500mm supertelephoto, including 1:1 macro and 1.4X and 2X teleconverters.
The A99 features Sony's Translucent Mirror Technology (TMT), with a fixed semitranslucent mirror rather than the moving mirror found in conventional DSLRs. The TMT mirror transmits most of the light to the image sensor, while directing a portion up to the phase-detection AF sensor. This enables the Sony SLT cameras to provide constant phase-detection AF during live view and video shooting. The nonmoving mirror also does away with SLR mirror vibration, which is important for landscape work at longer shutter speeds. SteadyShot INSIDE sensor-shift image stabilization works with any lens. The A99 can shoot full-res images at 6 fps with continuous AF and 10-megapixel APS-C images at 7 fps (8-10 fps in Tele Zoom Continuous Advance Priority mode). Normal ISO range is 100-12,800, expandable to 50-25,600. With the TMT system, the A99 has an OLED Tru-Finder instead of a typical DSLR optical viewfinder—the same unit used in the SLT-A77, that permits eye-level viewing for video as well as still shooting. The 3.0-inch, 1229K-dot LCD monitor tilts in almost any direction for easy odd-angle shooting. The body is sealed against dust and moisture, and the shutter is rated at 200,000 cycles. Slots are provided for SD/SDHC/SDXC cards and Sony Memory Stick PRO/PRO-HG Duo media. Video capabilities include 1920x1080 at 60p and 24p, with full-time phase-detection AF (or manual focus, if you prefer) and eye-level viewing.
Mirrorless vs. DSLR
Mirrorless interchangeable-lens cameras have become popular over the past few years, largely owing to their combination of DSLR image quality and truly small size. Most feature APS-C or Four Thirds sensors, and many will—with "pancake" lens attached—fit in a jacket pocket. The mirrorless cameras get their small size—and class name—from the fact that they do away with the DSLR's bulky and complex SLR mirror box and pentaprism (or pentamirror) viewfinder, replacing them with an eye-level electronic viewfinder (EVF) or just using the external LCD monitor for composition. The shortened distance from lens mount to image plane also allows for less bulky lenses (considerably less bulky for Micro Four Thirds cameras, and somewhat less bulky for APS-C models).
The big benefit of the mirrorless cameras is the ease of carrying them around. If you just want to have a camera with you—one capable of delivering DSLR image quality—at all times, it's much easier to take a mirrorless camera with a zoom lens or two than to lug a big DSLR and an equivalent lens. Mirrorless models make great hiking cameras, especially when covering terrain that requires use of your hands as well as feet. Some serious outdoor shooters find themselves taking their mirrorless cameras along more often than their DSLRs.
The main drawbacks to mirrorless cameras are lack of an "always-on" optical viewfinder and relatively short battery life. Some mirrorless cameras have eye-level electronic viewfinders (either built in or available as clip-on accessories), but even though EVFs have improved a lot of late, they still aren't nearly as good as a DSLR finder for tracking action subjects like birds in flight. Mirrorless cameras are in Live View mode all the time, so they go through batteries more quickly than DSLRs, and since they generally have smaller batteries to keep camera size down, this can be a problem. Also, there are fewer lens choices available for mirrorless cameras than for most DSLRs.
Bottom line: If you shoot a lot of wildlife or sports action, a DSLR is a better choice, otherwise a mirrorless camera is certainly worth considering if minimizing camera size is important.