Sony A33 and A55: Old Technology Put to New Use

Sony A55

by Miserere


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:

  1. 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).
  2. 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.
  3. 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:

  1. The camera is quieter as there will be no mirror slap.
  2. You should be able to hand-hold slower shutter speeds, as with a rangefinder, do to the lack of a moving mirror.
  3. 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.
  4. Phase detection AF can now work at all times, as when shooting videos or following a moving subject.
  5. 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.
  6. You can compose (both stills and video) either with the rear LCD or looking through the electronic viewfinder (EVF).
  7. Video can be shot with smooth autofocusing thanks to the always-on phase-detection AF, but there is a price to pay (see below).
  8. 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.
  9. 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.
  10. 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:

  1. There is no OVF, which to some is a drawback.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

Sony A55


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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  1. Excellent overview, with one small quibble. For someone shooting single images, the lack of a mirror blackout means that these cameras are the closest thing to a rangefinder camera experience you’ll get digitally outside a $7,000 Leica M9. With a little experience, the ability to see the frame as the shutter is tripped lets you immediately see if someone blinked, or lets you see if something changed in the composition, etc. — all without seconds of chimping. It is possible that these are stopgap cameras, but I think they could be excellent street cameras. (The EVF has lag issues in 6-10 fps mode, but few street photographers would be using that mode anyway.)

    • Good point, elliot, although there is still a mechanical shutter, so there will be a brief interruption of service to the EVF/LCD, and it might be longer than with a DSLR and OVF. It’s the same issue with micro-4/3, NEX and NX cameras.

      • there is still a mechanical shutter, so there will be a brief interruption of service to the EVF/LCD

        Are you sure? My understanding was that the beam splitter separated the image sent to the EVF so that, like Canon’s similar Pellix film cameras, there is no blackout whatsoever.

        • Yes, I’m sure. Canon’s Pellix had an optical VF, so the 1/3 reflected light went to it. In the Sony SLTs, that 1/3 reflected light goes to the AF sensor, while 2/3 goes to the CMOS, which provides the live view for both the rear and EVF screens.

          This following snippet is from the DPR review of the A55:

          Roughly 30% of the light that strikes the main mirror is reflected upwards, onto the AF sensor array. Meanwhile, in the absence of an optical finder, the main imaging sensor provides a full-time live view image via an electronic viewfinder in its place.

          There is still a conventional mechanical shutter so operation isn’t silent, however.

          As long as there’s a mechanical shutter, there will be “something” whizzing vertically past the sensor, although it probably moves fast enough that you won’t actually see it. The blackout, however, happens because after the exposure is taken the sensor needs to be read out, during which time it cannot give a live image. This happens with the Oly Pens, Panny G-series and Samsung NX series. I suspect it will also be the case with the A33/55, but DPR doesn’t specifically mention this issue.

          In the near future we’ll get sensors that can be read out in a small fraction of a second and won’t necessitate mechanical shutters. I dream of that day! :D

  2. I am seriously considering this camera. I’m a long time Pentax user who recently bought and E-P2. I haven’t really used my Pentax gear (except lenses) since purchasing the E-P2. This looks like the best of both worlds and apparently the are readily available adapters for K mounts.

    I was actually going to skip the next Pentax body anyway. I find the K7 and KX (to some degree) more than adequate for my purposes. This on the other hand is incredibly tempting.

    Now is it this or the new Voigtlander 25mm for M 4/3?

    • Steve, I’m in a similar boat, still shooting my K10D…but only occasionally, favouring the NX10 (that B&H kindly loaned me for an extended period) most of the time. After Photokina I’ll have to decide which way to go with my systems. Pentax could make my life a lot easier by releasing a MILC :D

      • I played with an NX10 and it seemed like a decent enough camera but the EVF was much, much grainier than the EVFs in the Panasonic cameras, which apparently use a different display of RGB (or something). This is something you unfortunately cannot see in online screenshots (at least I never saw it) but it is immediately clear when handling the two brand lines side-by-side. From the video (and reviews) of the Sony a33/a55 they are among the better EVFs around but I want to see for myself.

  3. I bought a new Canon Pellix-QL 35mm camera with a Canon FL-P 38mm f/2.8 lens, a very compact pancake design, about 1970 or so. The camera body had a lightning fast shutter response and a quiet shutter mechanism, but the viewfinder was so dim. The lens was good. Together the body and lens were compact and easy to carry. Years later I bought a Canon EOS RT 35mm camera which also had a pellicle mirror. Again, the shutter response was immediate and the camera was quiet, but the dim viewfinder was a bother. For both cameras, a pellicle mirror provided continuous viewing, but the benefit was useful only at wide apertures; otherwise, the image was too dim. Sony’s EVF seems to get around the disadvantages that afflicted my Canons.

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