It’s taken me a long time to get round to writing a review of the Alpha 55. You don’t get to use a new type of camera very often, and this camera blends elements which have all been used before in a completely new way. This review is pretty from the point of view of the still photographer not the video shooter. This is a multi page report. There’s a lot of it. Please use the navigation for pages 1, 2, 3, 4, 5 and on to keep the pages a reasonable size – even if it’s rather hard to spot it… or click the Read More link to get it as one big scrolling monster.
First of all, it has the basic geometry of an Alpha system DSLR – the lens mount, the size of the darkchamber (also known as the mirror box), the APS-C 1.5X sensor. It even looks like an SLR with a top bulge shaped as if there was a prism there.
In fact, there is no prism. This bulge is much like the one found on the Konica Minolta Dimage A2, a familiar profile used to house a tiny electronic display screen and a powerful magnifying eyepiece. The display screen imitates a reflex focusing screen, and gets its image directly from the CMOS sensor. The experience is planned to be as much like looking through a conventional SLR optical finder as possible.
Cameras like the A2 used (and still use) contrast detection autofocus, where the sharpness of the image received by the sensor is measured while the lens adjusts its focus setting. This requires a focus motor which can move in very small increments both ways, so that the sharpness can be ‘homed in’ on using a fast cycle of overshooting the maximum contrast in both directions. With a high contrast target, this oscillating ‘wave’ rapidly finds its peak response; with a poor target, it may take longer by performing this micro-hunting process for longer.
The lenses made for the Alpha A-mount don’t have the kind of focus drive or motor which can perform this cycle as efficiently. Those with screw drive can not do it smoothly, sometimes not at all. Those with SSM or the new SAM (a motor designed to overcome the problem) can home in, but nor very quickly. They are all really designed to work with a separate focus detector, the AF module known as a Phase Detector.
Phase Contrast Detection (its full name) works very differently. It compares two images, which are formed by using secondary lenses that ‘see’ the image from two positions behind the taking lens. The luminance value of the image is measured using a sensor strip – a row of pixels. You could compare the result with a graph. Each lens creates a nearly identical graph, but when the image is out of focus the contour of these two simple forms is at a different position on that lens’s row of pixels.
By comparing the two linear images, the system can tell not only whether they are offset or coincident, but in which direction to focus the lens to bring them into phase. This is what Contrast Detection can’t do and Phase Detection can do. It can, for the same reason, be programmed for Predictive AF which adjusts the lens in line with the measured speed of phase shift. Modern AF systems use multiple sensors, some with crossed or multiple parallel linear arrays and secondary lenses which respond to different aperture taking lenses.
The challenge facing Sony was to design a camera which could shoot live video, and continuously refocus any Alpha system lens – that meant having a Phase Detection sensor. This would have to be able to receive the image forming light at the same time that the main sensor was doing so. In regular DSLR cameras, that does not happen. A system of mirrors diverts some light to the AF module, the rest goes to the optical viewfinder, and when the picture is taken both of these are bypassed and all the image goes to the sensor.