Sony’s new Translucent Mirror Technology – a misnomer as the mirror is optically transparent and doesn’t perform the function of a net curtain or a Perspex light table – creates a new class of camera that looks like a DSLR but is being called an SLT – Single Lens Translucent. Since there’s very unlikely to be a Twin Lens Translucent, SLT is just as much a semantic sin as the use of translucent itself.

Has Sony forgotten why SLRs were named such? It describes the use of one lens and a moving mirror, instead of a viewing lens with a mirror and focusing screen (the “reflex” part) above a taking lens and film; the once-ubiquitous TLR. But that was half a century ago. Really the camera is an ILT – Interchangeable Lens Trans-mirror. Neither twin, nor single, nor translucent! And indeed, Sony’s instruction manual simply calls it an Interchangeable Lens Digital Camera on the front.

A semi-silvered glass mirror – not a pellicle plastic skin like the old Canon Pellix or EOS RT – sits at a slightly flatter-than-normal angle in front of the image sensor, which receives around 70 percent of the image-forming light. The other 30 percent is directed at a phase-detect AF sensor module that sits more or less where the focusing screen would be in an SLR. The appearance of a “prism housing” keeps the camera looking familiar, but this bulge on top houses some electronics and a 1.14 million-pixel optical viewfinder display, fed with a live image from the 16.2-megapixel CMOS sensor and viewed through a conventional finder eyepiece.

Pros and cons
Although it takes regular Minolta and Sony Alpha mount lenses, the A55 and the lesser-resolution (14.2-megapixel) A33 have more in common with electronic viewfinder/rear-screen display models in the Micro Four Thirds system. The key difference lies in using the semi-silvered mirror to feed the 15-point AF sensor, which is tuned for a combination of high speed and acceptable accuracy, especially when lenses with built-in ultrasonic focus motors are used.

Unlike contrast-detection focus off the main sensor, used by the Micro Four Thirds cameras and by Sony’s pocketable NEX range, phase detection can track fast-moving subjects and give responsive, fast, predictive AF during video capture. It is the first system camera design to allow this with legacy SLR lenses, and it even works acceptably with screw-drive focusing glass from a quarter of a century ago, give or take some impressive focus noises recorded by the internal stereo microphones. This is not a big issue as the A55 and A33, despite being sub-£600 cameras, do have the vital stereo mic jack input – and they supply 5V phantom power, which means a wide range of microphone types can be plugged in.
The other appealing feature of the A55, for the professional user, is that is has integrated GPS, which tags raw files, JPEGs, or both. It is very accurate and records latitude and longitude as normal, but also the height above sea level. It does not record orientation (compass direction) or camera tilt.

Although the EXIF data shows direction as an angle, this seems to mean the direction in which the recorder has been moving. While it takes a while to lock on every time you turn the camera on, when you leave the camera switched on, shifts of a few feet in position are recorded clearly. But this is also the downfall of the SLT concept, and every camera that depends on sensor-fed live view for a viewfinder image. If you do leave it turned on, it’s feeding either the rear 2.7-inch rotate-articulated LCD screen, or the electronic viewfinder (EVF), which uses much more power. The sensor is getting warm, or even hot, just as it would if you kept filming video.

Flare with the translucent mirror system: depending on the exact position of a light source, strong flare patches can appear. A faint patch is visible in the shot of the house [top]. To track the cause down, I used an LED source in a darkened studio, and moved the light round the image area. The small flare patch [above] is created by the lens rear element and the sensor cover glass, and is present most of the time. The three bands of light are only present when the source is within a small area just above the axis. 
This flare is unique to the SLT design. Illustrative images © David Kilpatrick.

Fast and slow
Leave it switched on, and you can shoot fast. Raise the camera to your eye, and a surprisingly large clear 100 percent view of the composition is there instantly. There is no mirror to move, so the double close-open-close-open shutter action is relatively quiet and slick. Turn the mode dial to the continuous shooting setting and it kicks in a remarkable 10fps, aided by the biggest memory buffer ever fitted in a camera of this type, apparently 320MB. It can store 19 raw images around 16MB each in size and write them with surprising speed to SD or Memory Stick Pro Duo cards, preferably of the fastest and best quality possible. A 19-frame burst of raw 16-megapixel images at 10fps puts the A55 in a class of its own.

During 10fps shooting the A55 will autofocus if set to C-AF (single AF locks on the first frame). No other camera – not even the Canon EOS 1D Mk IV or the Nikon D3s – can autofocus during a burst at this shooting rate. But the A55 is almost irrelevant for full-time sports action work. It’s only got a light duty shutter speeded 30s to 1/4000s, with synch at 1/160s.

Its burst speed is in stark contrast to the slowness of the camera if not left switched on. 
You lift it to your eye – after all it looks and feels like an SLR – and there is no viewfinder, just black. You then have to switch it on, and there is a distinct delay while you wait for the system to start up and provides a finder image. You can have grid lines, information display and a virtual spirit level, but you can’t get truly instant reaction. If you set the menu to give the shortest two-second review of each image shot, it’s considerably more than two seconds as a practical shooting interval.

All this is made up for by the 1080/50i (1080/25p) video capture. But unlike the little NEX-5, which offers 29 minutes of shooting with an optically stabilised lens, the A55 has clip-length times more in the 7-10-minute range found on other DSLRs with 1080p HD video. In theory, it can achieve longer, but with sensor-based stabilisation instead of in-lens, the whole system heats up rapidly. The plastic body does not dissipate heat too well, and overheat warnings are likely to shut down the camera. However, swivelling the rear LCD away from the back can help with cooling.

While Canon has adopted a side-hinged articulated rotating LCD in the 60D, Sony has gone for the same basic module used by the Nikon D5000 (it even has the same circular markings to show how it turns). This hinges at the bottom, and repeats the Nikon mistake of only providing a view to the front of the camera for self-filming if the LCD hangs below the body behind your tripod head. The virtual spirit level works for both horizontal and vertical compositions, and has a neat OK signal where the markers at each of the horizon lines light up green when you have the camera level. This makes it easy to use, even with the rotated screen viewed from arm’s length at an odd angle.

EVF restrictions
This camera is also very small indeed, and very light. It feels more like a bridge camera, although the grip is big enough for normal-sized hands. The rear screen can be turned to face the body, for protection, to use just the EVF. It is then always turned off. Eye sensors switch between EVF and rear screen if you leave it turned to view. But it is almost impossible to use the A55 or A33 with studio flash, or things like manual macro ring flash, because the live view gain only works in auto mode with a dedicated flash attached.

Manual mode, needed for many flash systems and triggers, simulates actual exposure. Often this means almost solid black shadow. The translucent mirror produces faint but visible secondary imaging; ghosts of point sources may appear, slightly offset. In strong backlight with bright sources included in the frame, flare patches occur, which are due to the mirror and internal hardware. 
The EVF fails to correctly show high-contrast areas, such as bright skies, rendering a plain white 
or grey instead.

It also has some issues with polarising filters. The finder brightens in a way that corrects exposure for polarised sky shots, making rotation hard to judge. Semi-silvered mirrors also cause polarisation of the transmitted light, and this will have some effect on reflections and blue skies, and on linear polariser use. A circular polariser is the only type safe to 
use on these “SLT” models.

Despite the true 100 percent view, the EVF was described by one user I passed the camera to as a test as “like watching a very bad home video”. Most photographers disliked it – despite it being one of the better examples of its breed. It is impossible to see outdoor colour, light or contrast correctly. On the other hand, it can see in the dark. Composition in dim interiors has never been easier.

The A33 is similar, but lacks GPS and only shoots at 7fps despite its smaller file size, having a much smaller memory buffer for only seven raw frames. Both have fairly exceptional high ISO performance. The 55’s 16 megapixels are not super-smooth, even at the minimum of ISO 100, but remain very clean right up to 6400, and usable at 12,800. Sony is now producing some of the lowest-
noise CMOS sensors. Processing from raw using ACR or Lightroom helps; in-camera JPEGs are very coarse by comparison.

There is an issue with many Sigma lenses, which either won’t work at all, or stop the shutter release working (turn off, remove battery, remove and refit lens, switch on). Sigma will re-chip recent lenses. Many don’t need it, but I tested the 70-300mm f/4-5.6 OS and the 8-16mm f/4.5-5.6 HSM DC, and both occasionally locked out the shutter. Both cameras have sweep panorama, 3D panorama 
and 16:9 modes, face detection, smile-shutter triggering, multi-shot in camera HDR and low-light shooting. They also offer burst shooting with selection of the 
best frame in-camera for saving.

Conclusions
So why buy it? Size, weight, resolution and the GPS function make the A55 an ideal travel camera – free from unsynchronised camera and GPS clock settings – embedding data in raw files, movies or JPEGs.

Manual live view magnified focusing and the lack of a mirror action, plus the articulated rear screen, make the A55 an option for microscope or instrument recording, macro, and via HD 
cables for remote photography.

Also, it’s cheap enough to introduce into hazardous environments as a disposable live view-enabled, high-resolution body. It is not much use for astrophotography because of 
the ghosting that creates a secondary image of every star.

One thing remains to be seen – will that fixed trans-mirror stop the sensor getting dust spots? It can be lifted to clean them. So far, no dust has found its way behind the mirror. If it doesn’t that would be a worthwhile bonus in an idiosyncratic choice of compact, high-resolution camera.

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