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Key Features / What's new?

The mirror in the A55 is semi-transparent. According to Sony, it allows approximately 70% of the light that strikes it straight through to the 16.2MP CMOS imaging sensor. The remaining proportion is reflected upwards to a large 15-point phase detection AF array. This equates to a roughly 1/2 EV reduction in light transmission for live view/image capture but means the AF sensor is always receiving light.

This isn't an entirely new idea - fixed, semi-transparent mirrors have been built into SLRs before, but the system in the A33 and A55 is a major new development. What makes the A33 and A55 unique is that they can maintain phase-detection AF at all times, even during an exposure.

They manage this by removing the optical viewfinder entirely and only having to direct light to the phase-detection AF sensor array. For packaging reasons this has been moved from its conventional position at the base of the camera's mirror box, to its roof. Roughly 30% of the light that strikes the main mirror is reflected upwards, onto the AF sensor array. Meanwhile, in the absense of an optical finder, the main imaging sensor provides a full-time live view image via an electronic viewfinder in its place.

There is still a conventional mechanical shutter so operation isn't silent, however.

In detail

The optical system of the A33 and A55 is unique, and represents a significant step forwards from conventional SLR design. Both cameras are built around semi-transparent mirrors, which allow light to pass through them, for the purposes of live view and exposure, but also reflect a portion of the light onto a new phase-detection AF array. Here's a detailed look at this and other new technology used in the SLT A33 and A55.

*From this camera position, the mirror appears to be fully reflective. In fact, only around 30% of the light which strikes it is reflected (onto the A55's phase-detection AF sensor) and approximately 70% of the light which strikes it is transmitted...
*...a change of camera position reveals the mirror as a 'window' through which around 70% of the light which enters the camera passes - falling on the imaging sensor.
This view shows the semi-transparent mirror in its normal (down) position and manually flipped up for sensor cleaning. In the down position, you can see the 15-point AF array reflected from its position in the roof of the mirror box. A newly developed 15-point AF array is recessed inside the roof of the A55's mirror box. Three of the 15 AF points are of the cross type (shown here in red) for more accurate focus with large-aperture lenses.
The A55's EVF is a Sony-manufactured, 1.44 million dot field-sequential display which provides a 100% field of view. This image shows a simulated view through the finder when shooting in the 16:9 aspect ratio. In a first for Sony's Alpha series the 912k dot TruBlack LCD on the rear of the A55 is fully articulated.
The A55 features Sony's established Super SteadyShot INSIDE image stabilization system to reduce the effect of camera shake. Sony claims a benefit of between 2.5 and 4 stops in useable shutter speed. The A55 has a built-in GPS receiver, which can automatically tag photographs with location data. This data can then be used in conjunction with Google Maps using the supplied Image Data Lightbox software.

* Please note: these images are for illustration and do not show the exact position/angle of the mirror as it exists inside the A55/A33.

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Total comments: 7

I can only confirm that at least the first generation model is totally unreliable.
As others described often enough the camera would just turn off mid shooting, with only the first half of the shutter sound (shutter count 32k).

Sony insists on charging you for replacing the Shutter unit, which will likely die again within the next 15-50k.

Design flaws can happen, but Sony Customer Service is just terrible. Definitely not going to buy into their systems.


It seems Sony A55 is very unreliable camera.

While I was taking photos, camera suddenly went black Fully dead. I verified battery is 100% charged.

I tried to contact the repair centre and no feed back. I am very much frustrated as my friend who took Cannon at the same time with me has had no issues with his and I have many other friends ho use Nikon never had issues with their cameras. I feel sorry for my decision to go for SONY DSLR.


I think that the fixed mirror means the cameras are relatively small -- 23 per cent smaller and 26 per cent lighter than the Sony Alpha DSLR-A550, to be precise. They're very light, but with a solid and comfortable rubberised grip.

1 upvote

It does, or rather, did. Strangely, Sony decided, presumably for sound commercial reasons, to delete this body shape when they released the current model, the A58, which is bigger than the A55, 33, 35, and 37 which were all this shape. I like my cameras small (as well as light) so this shape definitely interests me more than the A58.


I love it. It is easy to use and I have never had any mechanical issues with it. I especially love the rapid shooting feature. My grandchildren never stay steady for a second. With this feature, I can go back and delete the frames I don't like. I also used it at professional sporting events at a great distance using the zoom lense with great success. I've also used the portrait function for stills. Great camera!


I have not had a good time with this camera. Just over a year after buying it the camera stopped working and SONY charged me £117 for the repair. Now just 7 months later the camera has a different fault and will not focus or take photos. Sony want a further £117 for repairs!! I feel this is not what i expected from a camera at this price and SONY are not interested in the fact that possibly I have a poor quality camera. I would NEVER recommend a Sony camera to anyone

1 upvote

hey, sorry for your bad luck. Sony manufactures good, but sometimes quite unreliable cameras, I agree.

Total comments: 7