Tuesday, September 17, 2013
The New Wave Of Mirrorless
Today’s mirrorless cameras offer the nature photographer DSLR image quality in much smaller packages
The original concept behind the mirrorless interchangeable-lens camera was to provide DSLR image quality without DSLR bulk. The image-quality part was easy: Just use a DSLR sensor—first, Four Thirds size, in the Micro Four Thirds cameras from Panasonic and Olympus, then, APS-C in cameras from Samsung and Sony, and later Fujifilm, Pentax, Canon and Leica. The compact part was achieved largely by eliminating the DSLR's bulky SLR mirror box and pentaprism or pentamirror viewfinder.
The big drawback of early mirrorless cameras was slow autofocusing. DSLRs use phase-detection AF, which (theoretically) can determine from a single reading whether the image is in focus, and if not, in which direction it's out and by how much. Mirrorless cameras couldn't use DSLR-style phase-detection AF because that depended in part on the mirror assembly. So mirrorless cameras used contrast-based AF, which measures contrast right off the image sensor. Contrast-based AF has to take a series of readings to zero in on correct focus, so it's slower than phase-detection. However, since contrast-based AF uses the image sensor to determine focus, it's more accurate than phase-detection AF, whose accuracy depends on the precise alignment of a number of parts, both moving and nonmoving. But initial acquisition speed and tracking moving subjects were definite problems.
The big benefit of phase-detection AF is that it works well on moving subjects. Unfortunately, with most DSLRs, phase-detection AF works only for viewfinder shooting; in Live View and Movie modes, the phase-detection system is inoperative, and the camera has to rely on a very slow implementation of contrast-based AF. (Sony's SLT cameras do employ phase-detection AF in live-view and video shooting, and thus can do videos of moving subjects very well, with eye-level viewing via the built-in EVF.)
Today's mirrorless cameras offer much quicker contrast-based AF than most of their predecessors, and some offer hybrid phase-detection/contrast-based AF systems. More and more mirrorless cameras are providing touch-screen AF—just touch the point on the LCD monitor where you want the camera to focus— especially handy for video shooting.
Because mirrorless cameras don't have optical viewfinders like DSLRs, they operate in live view at all times. This means they wear down their batteries more quickly, although today's cameras do pretty well in power management. It's always wise to carry spare charged batteries, no matter what type of digital camera you use.
Autofocus Technology. Canon (EOS M) offers Hybrid CMOS AF, in which the central phase-detection AF pixels in the center of the image sensor quickly determine the general distance to the subject, while contrast AF fine-tunes the result. Canon introduced Dual Pixel CMOS AF in the EOS 70D DSLR, in which each pixel on the sensor contains two photodiodes, so each pixel can perform phase-detection AF as well as capture light. This is considerably better than the Hybrid CMOS AF, but so far available only in the EOS 70D.
Fujifilm uses 49-point contrast-based AF in its X-series mirrorless cameras in a 7x7 grid. The new X-M1 also offers focus peaking as an aid to manual focusing.
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