7fps shooting rate
With no moving mirror, the frame rate of the Alpha 33 is an impressive 7fps, while the Alpha 55 is even more impressive with 10fps. Shooting rates of this speed are usually only available in cameras costing twice as much as the Alpha 33, and in the case of the Alpha 55 the Canon EOS-1D Mark IV is the only DSLR capable of this, which costs around £3,500. Usually the mirror has to lift before the shutter can open to expose the sensor. Then the shutter must close, the mirror return to its position and focusing take place again before the sequence can be repeated and the next image taken. Without a moving mirror, the shutter is free to open and close to expose the sensor.
The translucent mirror technology, however, reflects some light to the AF sensor constantly. This means that the phase-detection autofocus can be constantly active, so focusing can take place very quickly between shots. So not only is the Alpha 33 capable of a high frame rate, but the AF is also able to keep up with this high shooting rate.
Although many cameras, including compact models, have very high shooting rates, they are not able to focus between each shot – the point of focus simply remains fixed in position from the first image. This is fine for subjects that will remain roughly fixed to the same point, but moving subjects, such as someone running or cycling, can drift in and out of focus between each frame.
While this technology has been used before, it is the first time it has been used in a digital camera, and it works well. I am looking forward to seeing just how much faster it is in the 10fps that the Alpha 55 is capable of. It will also be interesting to see if other manufacturers try to compete with the frame rate offered by the Sony SLT cameras.