Tilting pentamirror enables uncompromised live view AF speed
After announcing the rather unexciting Alpha A200 DSLR at CES earlier this month, Sony has followed through with some genuinely interesting and innovative Alpha DSLR news here at PMA. Two new live view models, the 10 megapixel A300 and 14 megapixel A350 join the Sony Alpha line up and they also feature tilting screens.
Pro Alpha DSLR update
Sony also tantalised the assembled press this morning with further tidbits of information about the long-awaited ‘pro’ Alpha DSLR, with news that it will have a 24 megapixel full frame Exmor CMOS sensor. The A300 and A350 stick with Sony’s Super HAD CCD sensor technology instead of CMOS. I’m betting on a formal launch for the pro Alpha at Photokina in Cologne, Germany, later this year.
Live view to-date not good enough
But what’s really got everyone talking here is Sony’s novel live view system for the new A300 and A350 DSLRs. Sony explains that relatively poor AF responsiveness in rival live view systems was regarded as too problematical and goes some way towards explaining why the Alpha A700 didn’t feature it late last year. With live view DSLR implementations so far there is either no live view AF at all, or it’s very slow. Sony decided to solve the problem by letting ‘normal’ AF to be used in live view mode.
Normally, to enable live view mode, a DSLR’s reflex mirror has to be lifted to let the lens project directly onto the main image sensor. The problem with this is that conventional high performance AF sensors require the mirror to be down. Panasonic, Canon and Nikon utilise slower contrast-detect live view AF, using the main sensor as the AF sensor. The problem here is that contrast detect AF is slow and inefficient. However, contrast AF is very accurate.
Secondary live view CCD sensor
Sony’s clever idea to re-enable fast phase-detect AF, as used in normal DSLR AF shooting, involves tilting part of the reflex pentamirror optical path and to divert the through the lens image onto a secondary CCD. This means the live view display is captured not by the main sensor, but the secondary sensor. By doing this, the reflex mirror doesn’t have to lift, and the optimal AF sensing mode is unaffected.
Pentamirror – what’s that?
So what is a ‘pentamirror’? Conventionally, SLR cameras use a five-faced prism (hence penta-prism) to direct the image reflected from the reflex mirror to the optical finder, in conjunction with a focus screen, showing the image to the photographer’s eye correctly orientated. A pentamirror is a lighter and cheaper alternative, using mirrored surfaces instead of a prism.
The use of a secondary live view sensor was pioneered by Olympus with the E-330 DSLR two years ago. The Olympus solution relied on a semi-slivered mirror to share the image projected by the lens between the live view sensor and the optical viewfinder without having to lift the reflex mirror. The reflex mirror doesn’t have to be lifted and the fast phase-detect AF system remains deployed.
Unfortunately for Olympus, this beam-splitter arrangement results in a relatively dim viewfinder. Sony manages to do the same trick as Olympus without sacrificing light to the optical finder when live view is not in use. Sony can rightfully claim that for the first time, live view mode action photography is realistically possible without compromising the optical finder.
Sony’s live view mode means AF is just as fast and snappy as normal reflex mode AF. Sony thinks this is especially important as many present day photographers expect high performance live view with DSLRs. After all, prior to using a DSLR these photographers have been used to universal live view in digital compact cameras.
But there are disadvantages
There there are some catches, though. As the live view display is produced by a secondary live view sensor, it’s not possible to use this display for critical focus – magnifying the view – which conventional main sensor live view systems can. There is also a question mark regarding the matching of colour, contrast and tone of the secondary sensor display compared to the characteristics of the main sensor.
Tilting but not swinging
Sony has chosen to use an up/down tilting screen with the A300 and A350. The screen can’t be swung left or right and a self-portrait ‘mirror’ mode with the screen facing the same direction as the lens is not possible.
In the past, manufacturers have explained their choice of this relatively limited screen mobility arrangement because of licensing restrictions relating to the more versatile hinged screen arrangement. I asked Sony’s Sadanobu Ueda, a senior figure within Sony’s Alpha DSLR division responsible for design concepts, whether licensing restrictions had been a factor with the A300 and A350. He pointed out that Sony already uses the swing and tilt articulating screen arrangement in other camera models. Mr. Ueda told me that as far as the A300 and A350 are concerned, the decision was based purely on the merits of the tilt-only design, with the opinion that the swing-out design was not as desirable. An interesting insight…
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