Sony ALPHA TIP # 107: RX10 Review dpreview/previews/sony-cybershot-dsc-rx10/ *** Interesting read on Imaging-Resource about the NEW Hybrid-Drive AF Actuator imaging-resource/camera-reviews/sony/rx10/ luminous-landscape/reviews/cameras/sony_rx_10_review.shtml cameralabs/reviews/Sony_Cyber-shot_RX10/ youtube/watch?v=cqCv6AUIriE And interesting note on the Imaging-Resource review was the part of Sonys all NEW Hybrid-Drive AF Actuator: Focusing. When they briefed us on the RX10, Sony called particular attention to the new autofocus drive system it contains. Its a contrast-detect AF system, but the company has made major strides in AF speed, thanks to the combination of a CMOS sensor with very fast readout, a greatly-enhanced image processor, and an all new hybrid-drive AF actuator. The basic idea behind contrast-detect AF is that crisp detail means the maximum brightness difference between adjacent pixels, wherever theres a contrast edge in the original subject. Think about it - when an image is out of focus, everything is soft and blurry, and nothing changes very rapidly as you move across the image. On the other hand, when an image is crisply focused with fine detail, the brightness will change much more abruptly as you look across the image. This is the basis of contrast detection. The camera looks across a row of pixels, and adds up the differences from one pixel to the next, all across the image (or all across the area its concerned about evaluating focus for). The summed differences between pixels is a measure of the overall contrast of the image, which in turn is a measure of how well the image is focused.To understand what this is all about, it would help to take a moment to understand how contrast-detect AF works in the first place. This is a relative measurement, though, since the summed contrast will vary enormously depending on the subject. The only way the camera can tell whether the focus is optimum, then, is to measure the contrast, adjust the focus slightly, and measure the contrast again. If the contrast number increased, the focus improved. If it decreased, it worsened. Most people understand that being able to make the contrast measurement quickly will be key to being able to focus rapidly, but fewer realize how critical it is to be able to move and stop the lens quickly. After each contrast measurement, the camera must move the lens a precise amount, and then have it stop moving before the next measurement can be made. It turns out that this last requirement is one of the most difficult to achieve. Its (relatively) easy to make a lens motor more powerful, so it can get the lens moving quickly, but its often another matter to bring it back to a stop with equal quickness. Sony developed a special hybrid actuator for the RX10, making dramatically faster AF cycles possible. The RX10 uses a standard ultrasonic-wave motor to get the lens moving, but then uses a special piezoelectric actuator (borrowed from Sonys sensor-shift image stabilization technology) to stop the motion quickly and precisely. As it was explained to us, the piezo actuator in this case isnt so much operating as a driver, but rather as a brake. Apparently, the ultrasonic-wave motor is good at getting things moving, but not so good at stopping them, hence the need for a brake or friction clamp, in the form of the piezo element. The NDA briefing was a little sparse on deep technical details for this, but if we understood correctly, Sony says that this hybrid system can complete 50,000 start-move-stop cycles per second(!) Thats apparently how many cycles the focus-actuator system can perform per second, but its not clear how many cycles the camera actually performs in practice, as clocking the image data off the array and performing the contrast measurement will add at least some time to the process, and we dont know whether the 50K/sec spec included the contrast measurement process or not. Its likely that contrast measurement is very fast, though, given how fast we know the array can be read (from the video processing mentioned elsewhere), and that we know the new BIONZ X processor has an enormous amount of processing horsepower to bring to bear. Regardless of the cycle rate, though, this new system clearly works. Our lab testing found autofocus on the Sony RX10 to be as fast as most consumer DSLRs using phase detection. And this was also borne out in our real-world shooting with the camera, too: It focused quickly and confidently, and nailed focus right where we wanted it most of the time.
Posted on: Wed, 05 Mar 2014 11:05:42 +0000