Sony unveils revolutionary a55 SLT camera

I don’t write much about camera releases; in the recent years, despite incremental improvements, the DSLR segment has been largely the same. However, the new Sony a55 has the potential to radically change the market.

Some concepts

To understand what’s so special about it, we must discuss first the fundamental difference between the DSLRs and compact cameras. Just in case you didn’t know, DSLR stands for “digital single lens reflex”. While you may think that DSLRs are about interchangeable lenses, they are actually defined by their mirror.

That mirror in front of the sensor redirects the light to the top prism and it also hosts the phase detection autofocus system (more about it in a minute). When a picture is taken, the mirror flips and the shutter opens. This whole system requires precision mechanics, is noisy and induces vibrations that cause problems when doing long exposures. Compact cameras on the other hand don’t have a shutter or a mirror; the sensor is continuously exposed to the light. Because there’s no mirror, compact cameras use contrast-detection autofocus.

So why is the the clunky mirror system preferred to the simpler compact one? The short answer is that phase detection is fast and contrast detection is slow.

Phase detection works with a beam splitter that redirects light rays from opposite sides on the AF sensor, where the intensity is evaluated relative to its position on the sensor. When both images have the intensity peaks on the same position, the image is in focus. If the position is different, the phase difference (the difference in patterns peaks) tells the system how much correction is needed and in what direction.

The contrast detection system employed in compact cameras is dumber. It just measures contrast in some image areas and moves the AF back and forth trying to improve contrast.

This is why dSLRs have so fast AF, while compacts are so slow in this regard.

So how’s Alpha 55 any different?

The new Sony a55 uses a translucent mirror. 70% percent of the light is directed to the main sensor, 30% to the AF system at the top.

See? No more moving  mirror. This camera includes the best of both worlds: it’s like a traditional dSLR (technically it’s a dSLT – T for “translucent” instead of R for “reflex”), with interchangeable lenses, fast shooting speed and autofocus, but with the simpler and lighter design of the compacts.

Features

Translucent Mirror Technology for simplified design, less weight and smaller size.

Continuous hi-res live image preview with phase-detection autofocus even during movie mode.

Hi-res Electronic Viewfinder (EVF) instead of the usual optical vewfinder in dSLRs, a necessary compromise due the lack of mirror.

Hi-speed, full-size shooting at up to 10 frames per second (only the very top Canon and Nikon cameras surpass that)

Fast AF during movie shooting (the only dSLR camera capable of this)

Fully-articulated 7.5 cm (3.0″) LCD

In-body image stabilization – up to 4 stops (this is not new, but still worth noting)

Integrated GPS for geotagging

HDR features from multiple images

High ISO 25600 capabilities, with noise unnoticeable prior to ISO800

Further info

More more info, check out the excellent indepth review on DPReview.com (spoiler: they give it the Gold Award.)

Pricing and availability