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Sony a33 and a55V: Pellical mirror returns, digital style

Sony a33 and a55V: Pellical mirror returns, digital style

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Sony’s new digital cameras are not the first with fixed translucent mirrors, but they may be the best so far.

September 2, 2010

Sony’s announcement last week of the SLT-A55V and SLT-A33 DSLR-ish cameras claims super-fast burst rates, no black-out during exposure, and video shooting while viewing the optical image through the prism finder. How did they swing this? By using a rarely-employed technology that goes back 45 years.

So, let’s go back 45 years—to 1965. Canon introduced the Pellix, the first 35mm focal plane shutter SLR with TTL metering but that’s not important for the purpose of this narrative. What is important is the Pellix's fixed pellicle mirror, which used a thin, semi-transparent film that simultaneously bounced light up into the reflex mirror housing and allowed light through to the film plain—an SLR first.


The advantages of this design included no viewfinder black-out during exposure, no camera vibration caused by mirror slap since there was no mirror movement, less camera-generated noise, and the ability to increase high-speed motor drives speed for sports photography.

The disadvantage? A loss of around 1/3 of a stop. An f/1.4 lens would act like an f/1.8, an f/4 would be like an f/4.5, etc. Additionally, the image in the viewfinder was darker, and more than the loss of exposure, the dim images in the finder turned many potential buyers away.

A novelty feature becomes a specialty item
 
Sports shooters, however, embraced this technology as Canon eventually produced the F1 High Speed Motor, a limited-edition camera (only 100 were produced) which captured much of the action at the 1972 Olympics in Sapporo, Japan and boasted a 9 fps motor drive. A later version could shoot up to 14 fps. Nikon also produced versions of its venerable top-line F cameras with pellicle mirrors. The F, F2, and F3 all had “H” (High Speed Motor) versions. The last one, the F3H, could churn out up to 13 frames per second with an MD-4H battery pack.

Later on, Canon placed a pellicle mirror in its EOS RT 35mm camera, the first such AF model. In 1995, Canon announced its first digital camera with a pellicle mirror, the EOS-1N RS. But since then, the pellicle mirror had been virtually ignored until Sony’s announcement.

Enter Sony

I’m surprised it took so long for someone to try this approach in a digital camera. Advantages in the digital world of using a translucent, fixed mirror include are many, even more so than with film:

•    Without the moving mirror, the cameras can be more compact while retaining an optical finder.
•    The area around the mirror can be sealed, forming a natural barrier against sensor dust
•    High-speed capture can be even faster
•    New generation sensors more than make up for slight loss of light
•    Simultaneous still and video capture
•    New-generation translucent reflective surfaces should help image appear relatively bright.

(On the last point: I’ve seen plenty low-end and mid-range DSLRs with mirror housings that produce relatively dim finder images that likely rival the Canon Pellix’s finder. Advances in translucent mirror light transmission technology might help here.)

Indeed, the Sony a55 offers a stunning 10fps high-speed frame rate with continuous AF, the fastest burst rate of any APS camera and a great feat considering it will only cost $750 for the body. Sony says it took advantage of the lack of mirror motion to make the cameras more compact, and that focus can take place as the camera shoots, which promises sharper images of fast subjects.

However, both Sony cameras’ mirrors reflect not into a mirror housing and optical finder, but into a high-resolution electronic viewfinder. That's right, these cameras are EVILs, not DSLRs. Yes, the EVF can compensate for any light lost from the translucent mirror reflected image—and that’s a big advantage—but for many photographers, the ability to see an uninterrupted optical image while shooting rapid-sequence stills (or, in the case of these cameras, even HD videos) would be a huge selling point.

Translucent mirrors are a good feature for digital camera makers to explore. I hope there's more to come.

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