With its use of translucent mirror technology in the Alpha 33, Sony has evolved the way that its digital cameras focus. But how much does the new system really improve on the SLR? We put the Sony Alpha 33 to the test
- 14.2 million effective pixels
- 7fps continuous shooting
- New 15-point AF system
- New 1,200-zone evaluative metering system
- Street price: £569.99 for body only
It seems at odds with an apparently ‘forward-thinking’ camera industry that Sony should look to an idea from the past to solve a problem in the present. Yet that is exactly what the company has done with its new Alpha 33 and Alpha 55 SLT (single-lens translucent) cameras.
The problem centres on how to the improve the phase-detection autofocus system to make it faster between shots and allow it to work in Live View or video capture modes. Sony’s solution is, in fact, the evolution of an idea first used by Canon in 1965, when it introduced the Canon Pellix.
Instead of a moving mirror in the Pellix SLR, Canon designed and used a pellicle mirror (see Geoffrey Crawley explains… on pages 58-59 of this issue), which was fixed into position. This very thin mirror split the light that entered the camera, redirecting a portion to the viewfinder while letting the rest through to expose the film. The camera wasn’t a success, as the amount of light reaching the viewfinder made it dark and difficult to focus. Although in the 1980s and ’90s a small number of professional SLRs were fitted with pellicle mirrors, the technology was largely forgotten – until recently.
With the introduction of video capture, DSLR manufacturers face the problem of how to focus the lens without the use of the usual AF system. Phase-detection focus relies on a camera mirror being down to reflect light to an AF sensor. However, that mirror has to be flipped up in video-capture mode so the image-focusing light can reach the sensor. Until now, the only way to focus during video capture has been either to focus manually or to use contrast-detection focus, which is comparably slow and fidgety.Sony is the last of the major DSLR manufacturers to introduce video in its cameras, with the company insisting that it wanted to give users the same experience when focusing during video capture as when taking still images. This meant making continuous phase detection possible.
The answer to the implementation of phase detection during video capture comes in the form of Sony’s translucent mirror technology (TMT). As with a pellicle mirror, this uses a piece of glass with a metal coating that is fixed in position in the Alpha 33 and its sibling, the Alpha 55. The mirror lets around 70% of light through it, while reflecting the remaining 30% to the phase-detection AF sensor. This allows phase detection to be uninterrupted, regardless of whether the camera is in Live View or video-capture mode.
However, the benefits of TMT don’t stop there. With no moving mirror (and no film wind-on mechanism) the shooting rate can be increased significantly – the Alpha 33 offering up to 7fps, and the Alpha 55 up to 10fps while still autofocusing.
Having learnt from the Canon Pellix, Sony has resolved the problem of a dark viewfinder by replacing the optical unit with a 1.15-million-dot (equivalent) electronic viewfinder.
With the removal of the mirror box and reflex system for the viewfinder, the Alpha 33 and Alpha 55 are no longer SLR cameras. Instead, they are described by Sony as SLT or single-lens translucent cameras. Consequentially, I was keen to find out exactly where the Sony Alpha 33 camera would sit within the Alpha range.