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Tuesday, June 19, 2012

Lighten Up! Go Mirrorless!

Mirrorless interchangeable-lens camera systems are a fraction of the size and weight of most DSLRs, and they can produce top-level images. What’s not to love?

Labels: CamerasD-SLRsGear
This Article Features Photo Zoom

Olympus OM-D E-M5
By and large, mirrorless cameras use contrast-based autofocusing right off the image sensor, as opposed to phase-detection AF used by DSLRs. Contrast-based AF used to be much slower than phase-detection AF (and it still is in most DSLRs' Live View mode). But contrast-detection AF has improved considerably over the years and now rivals phase-detection in speed, although it still isn't as good as phase-detection for action subjects such as birds in flight with long lenses. On the other hand, a number of mirrorless cameras provide touch-screen AF—just touch the spot in the image on the LCD monitor where you want the camera to focus, and the camera will focus there.

There are a few interesting phase-detect exceptions. To give photographers a phase-detect option, Sony created the unique LA-EA2 Alpha NEX Camera Mount Adapter. It lets you use Sony DSLR lenses (and legacy Konica Minolta Maxxum lenses) on NEX cameras for quick, DSLR-style phase-detection AF (see the sidebar on page 71). Also, Nikon's J1 and V1 feature a hybrid AF system that uses phase-detection AF in good light and contrast-detection AF when the light is too dim for phase-detection. When the light level is bright enough, the phase-detection provides good performance on moving subjects.

Sensor Sizes
Mirrorless cameras are available with sensors ranging in size from compact camera size to APS-C size. All other things being equal, larger sensors mean better image quality because they collect more light due to their larger surface areas. More light means less noise and a higher signal-to-noise ratio. The best mirrorless cameras equal the best non-full-frame DSLRs in image quality. The smaller-sensor mirrorless cameras are physically smaller in size than the larger-sensor models, but pay a price in image quality for that size advantage.

Besides image quality, the sensor size affects the angle of view of any lens. Compared to a full-frame sensor (36x24mm), an APS-C sensor has a focal-length crop factor of 1.5x so a 100mm lens on an APS-C camera frames like a 150mm lens on a full-frame camera, and a Micro Four Thirds sensor has a 2x crop factor, meaning a 100mm lens on a Micro Four Thirds camera frames like a 200mm lens on a full-frame DSLR. The CX sensor in Nikon's J1 and V1 cameras has a 2.7x focal-length factor (a 10mm lens frames like a 27mm lens on a full-frame DSLR), and the 1/2.3-inch sensor in the Pentax Q has a 5.5x factor (an 18mm lens frames like a 100mm lens on a full-frame DSLR). Mirrorless interchangeable-lens cameras that use APS-C sensors include all Samsung and Sony models, the Pentax K-01 and the Fujifilm X-Pro1. All Olympus and Panasonic mirrorless models use Micro Four Thirds sensors.

Mirrorless Vs. Fixed Translucent Mirror

In a conventional DSLR, the SLR mirror must be in the down "viewing" position for the AF system to function. When you fully depress the shutter button to take a shot, the mirror has to flip up out of the light path, briefly blacking out the viewfinder image and disrupting AF operation. In Live View (mirror up) mode, conventional DSLRs have to employ contrast-based AF off the image sensor, which means you have to compose images via the LCD monitor in Live View mode, and AF performance is slow—and action-tracking, continuous phase-detection AF isn't possible.

In the Sony SLT cameras, the mirror doesn't move. Instead, it's semi-translucent, transmitting most of the light to the image sensor while simultaneously directing a portion up to the AF sensor. Thus, the phase-detection AF system functions continuously, there's no viewfinder blackout when you take a shot, and you get eye-level viewing via an excellent OLED electronic viewfinder.


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