Sony A33 and A55: Old Technology Put to New Use
The Birth of a “New” Camera System
Sony today unveiled the A33 and A55, the first two in a new line of digital cameras that resemble traditional digital SLRs, but which Sony are calling SLTs, for Single Lens Translucent. The translucent term refers to the mirror inside the mirror box which, unlike those in SLRs, isn’t fully reflective1 but actually partially translucent. This has several implications for the functioning of the camera:
- The mirror is now stationary, so it won’t flip up and down with each exposure (but it can be flipped up for sensor cleaning).
- Sony are being tight-lipped about the exact value, but given past designs we can guess that the translucent mirror lets ~2/3 of the light through and reflects ~1/3. The reflected light makes its way to a phase-detection autofocus system, while the light passing through the mirror hits the CMOS sensor.
- Because light hits the sensor continuously, there is no need for an optical viewfinder (OVF).
The advantages of the above over a standard SLR are numerous:
- The camera is quieter as there will be no mirror slap.
- You should be able to hand-hold slower shutter speeds, as with a rangefinder, do to the lack of a moving mirror.
- A stationary mirror should theoretically keep phase-detection AF precise over longer time periods due to the mirror not becoming misaligned quite as quickly as in an SLR.
- Phase detection AF can now work at all times, as when shooting videos or following a moving subject.
- Because there is no OVF, a pentaprism (or pentamirror) isn’t required, saving space and weight, so the camera can be made smaller and lighter.
- You can compose (both stills and video) either with the rear LCD or looking through the electronic viewfinder (EVF).
- Video can be shot with smooth autofocusing thanks to the always-on phase-detection AF, but there is a price to pay (see below).
- The frames-per-second (FPS) that can be shot only depend on the shutter and the readout speed of the chip, so higher FPS can be more easily achieved.
- Without a pentaprism (or pentamirror), VF screen or flipping mirror mechanism, costs should come down, making the camera cheaper to produce and cheaper for the buying public.
- The EVF can more easily be made larger than an OVF, and according to the specs from Sony, the EVF on these two cameras is 15% larger than that of the Nikon D300s, usually considered the best VF within the APS-C segment.
Now let’s review some of the drawbacks:
- There is no OVF, which to some is a drawback.
- In order to shoot video using phase-detect AF, the lens has to be wide open. If you want to shoot at a smaller apertures you’ll be forced to use contrast-detect AF, which is not as smooth.
- Because the mirror doesn’t flip out of the way, you don’t expose the sensor to all the light coming through the lens, but only 2/3 of it, thus losing 1/3 of a stop. In theory, this degrades IQ. It’s not clear how Sony have addressed this, but I expect they’ve adjusted the reported ISO so that it shows a value 1/3 stop lower than what the camera is actually using. Given how imprecise camera makers are when setting ISO values, this may not be a big deal. As sensor performance continues to improve, it will be even less of a problem.
- Dust has always been an issue for DSLRs, accumulating on the sensors and creating the infamous dust bunnies. On an SLT design dust now has an extra surface to accumulate on: the translucent mirror, and because it’s far away from the sensor, a mote of dust will create a halo, not a spot, making it more annoying to fix in postprocessing.
- Light going through the mirror will be refracted twice: first as it goes into the mirror and then as it comes out. Not all colours of light are refracted by the same amount so this could induce a degradation in IQ by creating colour fringing effects. I trust Sony engineers worked long and hard on this issue and found a solution for it.
The Best of Both Worlds?
The rise of DSLRs as movie recording devices in the last 18 months or so has been nothing short of meteoric. And thanks to the required live-view (LV), even still photographers have benefited as LV makes shooting cameras in awkward positions a lot easier, especially with articulated LCDs. The main gripe with shooting video was AF, which relied on a contrast-detect method, and is generally regarded as being slower and incapable of smooth continuous focusing. With the new SLT system Sony is trying to bring together the best of both worlds: DSLR functionality and focusing with the added flexibility of LV.
The problem I see is that the SLT camera might be a solution to a quickly disappearing problem. As some of the latest MILCs have proven (such as the Panasonic G2 or Samsung NX10), contrast-detect AF can be fast, and is likely to keep improving with each new iteration thanks to its inclusion in the rising sector of mirrorless cameras. Once contrast-detect AF is capable of determining distance to subject, it will not only be fast, but smooth also. We may see cameras with such an AF system as early as this Autumn at Photokina 2010. When this happens, what will be the point of SLTs?
I expect SLTs to become a footnote in the history of modern camera designs, nothing but a stopgap solution to a short-term problem. I think Sony also believe this because they’ve maintained the same mount and registration distance, when they could have made the mount smaller for APS-C and shortened the registration distance by having the mirror slide to the side instead of flipping out of the way for sensor cleaning. I imagine Sony will release one or two further SLTs and then drop the line as the simple solution of MILCs takes over.
None of this should stop you from purchasing these cameras. Like I said, Sony knows they are temporary and have changed nothing essential about the Alpha system, so all Sony and Minolta lenses are fully compatible with it. If you enter the Sony system with one of these camera, any lens you buy for it will still work on any of their current or future DSLRs as well as SLTs, so there is really no risk involved in purchasing either of these cameras.
If you want precise specs, you can find them here. Both the A33 and A55 share the same body, but the A55 has 16.2MP vs the A33′s 14.2MP, 10 FPS vs 7 FPS (both very high values), larger buffer and better battery life. The A55 also has GPS while the A33 doesn’t. The A55 is US$750 (body only) while the A33 is US$650 (body only). Needless to say they both shoot RAW, have PASM shooting modes, a fully articulated LCD, shoot 1080i HD video and feature sensor-based image stabilisation. Given these specs and prices, I expect Sony to sell a boatload of them! I’d even be tempted to pair the A33 with Sony’s recently announced 35mm f/1.8 for a total of $850 and hit the streets with it.
So What Was That About Old Technology?
You may have been wondering about the title of this post, and I thank you if you’ve made it this far in order to find out. The fact is that semitransparent mirrors are very old technology, and using them inside a camera first happened in the mid 1960′s when Canon released the Pellix, so called because these mirrors are also known as pellicle mirrors. The disadvantage back then was that the VF in those first pellicle cameras was 2/3 stop less bright, because 2/3 of the light went to the film, but today, with film being replaced by a digital sensor, this is actually a benefit because thanks to the EVF, we’re now using the same device to preview an image and capture it.
While Sony is resurrecting old Canon technology, maybe they’ll reintroduce eye-controlled autofocusing, a much-requested feature that would make my life a lot easier. OK, so maybe not my life, but certainly my Photography.
1 In SLRs with autofocus (AF), the mirror isn’t 100% reflective, as some light needs to be diverted towards the AF sensor, generally located at the bottom of the camera, opposite the viewfinder screen from the mirror. However, because the mirror flips up out of the way when taking a photograph, this loss of light only affects the brightness of the image in the viewfinder.
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Tags: APS-C, Cameras, CMOS, DSLRs, Exmor, Pellicle, Pellix, Photography, Sony, Sony A33, Sony A55