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Tuesday, November 26, 2013

Sony Alpha a7 And Alpha a7R


The age of full-frame interchangeable-lens mirrorless has arrived

Labels: Cameras
This Article Features Photo Zoom
One of the main reasons why mirrorless interchangeable-lens cameras were designed was to provide DSLR image quality without the bulk. From the very start (the first mirrorless interchangeable-lens camera was introduced about five years ago), serious photographers wondered when—even if—there would be full-frame mirrorless interchangeable-lens cameras. In 2012, Sony introduced the RX1, a truly compact camera with a full-frame sensor (the same one used in the company's SLT-A99 DSLR), but with a fixed wide-angle lens.

Now, Sony has introduced the first full-frame mirrorless interchangeable-lens camera. The two models are almost identical with the following exceptions: The Alpha a7 features a 24.3-megapixel, full-frame Sony Exmor CMOS sensor, and the Alpha a7R has a 36.4-megapixel unit with no OLPF (optical low-pass, or anti-aliasing, filter). The a7's sensor also provides Sony's Fast Hybrid AF.

Fast Hybrid
The Sony a7 has the company's on-sensor phase- and contrast-detection AF for improved performance on action subjects: the a7 can shoot at 5 fps with nonstop AF tracking in Speed Priority Continuous mode. First, PDAF, via 117 phase-detection AF points in a 13x9 grid in the center of the frame, moves the lens to quickly establish focus on the subject; then contrast AF, via 25 points in a 5x5 grid that covers the entire frame, fine-tunes focus.
The a7R's higher-res sensor is contrast-detect AF only, and its contrast-based Fast Intelligent AF is about 40% faster than that in the NEX-7 camera: the a7R can do 4 fps in Speed Priority Continuous mode, which is quick, especially considering the huge 36.4-megapixel sensor.

No Low-Pass Filter
The image sensors in most DSLRs and mirrorless interchangeable-lens cameras incorporate an OLPF to do away with the moiré and artifacts that are produced by the demosaicing process needed with conventional Bayer-array sensors. Since the photodiodes in image sensors can't detect colors, only brightness, color is obtained by fitting a filter grid over the sensor, which allows each pixel to receive only red, green or blue light. The missing colors for each pixel are obtained by interpolating data from neighboring pixels using complex proprietary algorithms. This demosaicing process produces moiré with finely patterned subjects and artifacts. The OLPF, or anti-aliasing filter, does away with these, but slightly blurs the image in the process.

When pixel size becomes small enough, diffraction functions as an anti-aliasing filter of sorts, eliminating the need for the blurring OLPF. This seems to occur with a pixel size of just under 5 microns, the size found in 16-megapixel APS-C sensors and 36-megapixel full-frame ones. Thus, some newer cameras feature sensors without the OLPF effect, gaining sharper image potential (at the slight risk of moiré with some finely patterned subjects): Nikon's D800E, D7100 and D5300; Pentax's K-5 IIs and K-3; and Sony's RX1R and now a7R.

Image Sensors
Sony's 24-megapixel, full-frame Exmor CMOS sensors have been impressive from their introduction, appearing in the SLT-A99, RX1, RX1R (without OLPF) and now the a7 (with on-sensor PDAF). There are always questions about the Sony and Nikon sensors being one and the same because of their many similarities on paper.

The image sensors might be close siblings, if not identical twins, but the in-camera processing engines are definitely unique to each company. Sony uses its Bionz processors (Bionz X in the a7 and a7R), while Nikon uses its EXPEED technology (EXPEED 3 in recent models). Nikon certainly had input in its versions of the sensors, and might well use different filters in the RGB array, or different microlenses, or different AD converters, but the important thing is, the sensors are very similar in spec and performance, and according to some tests, they're the best that are available today.

Very likely Sony had a hand in the 24-megapixel, full-frame sensors used in Nikon's D600 and D610. The 36-megapixel, full-frame sensor that was introduced in Nikon's D800 and D800E (the latter without OLPF) looks to be very similar to the sensor in the Sony a7R (also without OLPF).

Lens Mount
The new Alpha a7 models feature a new FE lens mount, and Sony introduced five FE-mount lenses with the cameras: the 28-70mm ƒ/3.5-5.6 OSS and 70-200mm ƒ/4 G zooms, and Carl Zeiss Sonnar T* 55mm ƒ/1.8 ZA, Sonnar T* 35mm ƒ/2.8 ZA and Vario-Tessar T* 24-70mm ƒ/4 ZA OSS. You can also use existing Sony A- and E-mount lenses, via adapters.

Regular Sony E-mount lenses were designed for the NEX cameras, which feature smaller APS-C sensors, so they'll vignette if used in full-frame mode on the new a7 cameras, but can be used (via adapter) in APS-C mode. Full-frame Sony A-mount DSLR lenses (and legacy Konica Minolta Maxxum lenses) can be used, also via adapter. When used with the LA-EA2 or LA-EA4 adapters, you get phase-detection AF via the Translucent Mirror Technology AF system in the adapter. Sony DT A-mount lenses were designed for APS-C sensors, and like the E-mount lenses, will vignette if used in full-frame mode on the a7 cameras. But the cameras will automatically switch to APS-C crop when such a lens is attached (or you can make the switch manually with any lens). In crop mode, there's a 1.5X crop factor, and image sizes drop to 10 (a7) and 15 (a7R) megapixels, respectively.
 

Shared Features
The two new a7 cameras share a number of features, including compact bodies (5.0x3.7x1.9 inches, 16.7 ounces for the a7, and 16.4 ounces for the a7R), 1200-zone metering, ISO settings from 100-25600 (plus 50), a 2359K-dot eye-level Tru-finder, 3.0-inch 920K-dot tilting LCD monitor, and built-in Wi-Fi with NFC for easy connection. Both can do 1080p video at 60 fps and 24 fps,
with stereo sound via built-in or external microphone, and headphone port. Images are stored on SD/SDHC/SDXC cards or Sony Memory Stick Duo/PRO Duo/PRO-HG Duo media. An optional battery grip doubles the cameras' shooting capacities.

Specifications

Estimated Street Price: $1,699 (a7); $2,299 (a7R)
Contact: Sony, www.sony.com



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