Why did Sony eliminate the traditional reflex mirror?

October 1, 2010 at 6:16 pm  •  Posted in Q&A by

I read an article about the new Sony a55 and a33 D SLRs indicating that the reflex mirror in these camera does not move at all. How is that possible? And why would Sony do this? Isn’t the traditional SLR system just about perfect?
Weldon Lamb

You’re right, Weldon, Sony has eliminated up/down reflex mirror action—in these two D SLRs only—by installing a fixed semi-transparent mirror. In truth, Canon was the first to use a similar concept, starting in 1965, as discussed in a Wikipedia article. Of course, Canon abandoned this concept long ago. Sony now employs new technology in both Alpha models with a translucent mirror that allows 70% of the light to reach the CMOS sensor. The rest of the light is reflected up to the 15-point AF sensor.

As this illustration indicates, the translucent reflex mirror allows light to reach the CMOS imaging sensor, but also reflects light to the AF phase-detection sensor at the top of the mirror chamber. Photo Courtesy of Sony Corporation

This ensures that autofocus—with phase-detection AF even in Live View and in Movie mode—is maintained at all times. (This type of AF is faster than contrast-detection AF and is more reliable in low light). Automatic focusing is not interrupted between every shot in a series by reflex mirror up/down motion as it is with other SLR cameras. This should ensure even better continuous AF while shooting a long series of images and it allows for Continuous AF in Movie mode.

Because the semi-transparent mirror technology would allow little light to reach an optical viewfinder, the a55 and a33 employ an electronic viewfinder. The EVF display boasts ultra high resolution of 1.152 million dots and it provides a 100% field of view plus incredibly high magnification of 1.1x that’s impossible with an optical finder. Both the LCD screen in Live View and the EVF receive the image preview display directly from the CMOS imaging sensor. There are benefits to this arrangement.

The electronic viewfinder provides 100% scene coverage and 1.1x magnification with an ultra high resolution display. The EVF also provides more shooting information than a conventional optical finder. Photo Courtesy of Sony Corporation

In Movie or Sweep Panorama Mode, you do not need to compose on the LCD; that’s ideal on sunny days when glare can obscure the external display. The live preview in the EVF allows you to see the effect that will be provided as you change exposure compensation, WB and picture styles. You can also use the EVF to view images in playback mode. And finally, the EVF allows for more data to be displayed than an optical finder. So, while some long-time photographers may prefer a more traditional D SLR like the a850, the a55 and a33 provide advantages; for a full appreciation of these, be sure to check out the new Sony models at a well-stocked camera store.

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