Sony A33 Video Recording

Over the last couple of years, high-definition video capture has become a fairly common feature for interchangeable-lens cameras, available in many single-lens reflex and single-lens direct view cameras at all price points. In all cases, though, focus adjustment during video capture has either been restricted solely to manual focus, or rarely, via contrast detection autofocus based on data streamed from the image sensor. Phase detection AF -- the system used for still image capture in an SLR camera -- simply isn't possible during video recording for a traditional SLR, because once the mirror is raised to expose the image sensor, there's no way to redirect light from the optical path to the dedicated phase detection AF sensor. Pulling focus manually isn't for everyone, though, as it's a notoriously difficult skill to master. Contrast detection AF isn't perfect either, as compared to phase detection AF, it's generally rather slower, and less able to cope with moving subjects. It also requires focus adjustment in repeated small steps, with some hunting around the exact point of focus, and tends to lag briefly behind subjects with unpredictable movement.

The radical design of Sony's Alpha A33 changes all that, allowing phase detection autofocus during video capture for the first time. This is achieved with a pellicle mirror -- or in Sony parlance, a Translucent Mirror -- which reflects around 30% of incoming light to a phase detection autofocus sensor located in the flash / electronic viewfinder housing, and transmits the remaining ~70% of the light to the image sensor. This means that the phase detection sensor receives its share of incoming light at all times, although if autofocus is in use, aperture must be controlled automatically, and is kept at or near its open position. Potential drawbacks of the approach are two-fold. The A33 must increase its gain (and hence, noise levels) to provide the same exposure as a traditional SLR, and since the aperture must remain open when autofocus is available, it must control exposure using shutter speed and ISO sensitivity alone. In bright scenes, the higher framerates used to control exposure can lead to somewhat choppy-looking video.

Of course, since the A33 uses a standard Sony Alpha mount, available lenses will have been designed with still image AF in mind. This means that AF drive noise is more of an issue than would be the case when using lenses designed for video, such as in Sony's NEX-series single-lens direct view cameras. With that said, the phase detection autofocus means less hunting, and --- since phase detection systems can provide immediate information on the size and direction of AF adjustment required -- more direct AF operations, perhaps resulting in less noise than would be the case with a traditional SLR using the same lenses for contrast detection AF. For the amateur videographer desiring autofocus during video capture, the AF noise is likely a relatively small price to pay. It can also be further mitigated thanks to the A33's external, stereo microphone connection, which allows the use of a quality directional mic and some form of isolation mechanism that will also reduce the impact of body handling noise.

Its lack of manual control over video exposure parameters means that the Sony A33 may not be a choice for serious video aficionados, but it does offer a modicum of control over the look of videos, with the ability to adjust exposure compensation and lock exposure during recording, and to select a creative style, white balance mode, metering mode, or AF area before recording. (If the AF area is set to local, it is also possible to change the active AF point during recording.) All things considered, the Sony A33 (like its simultaneously announced siblings, the A33V and A33) offers an interesting first among interchangeable lens cameras, making phase detection video autofocus not only a possibility, but a reasonably useful proposition. Let's look at its video capabilities in a bit more detail:

Sony A33 Basic Video Specs

• 1080i (1,920 x 1,080), 59.94i (interlaced) fields/second, 17 Mbps HD recording (50i in European versions) in AVCHD format
• HDV 1080p (1,440 x 1,080 rectangular pixels), 29.97 fps, 12 Mbps, HD recording (25 fps in European versions) in MPEG-4 format
• VGA (640 x 480), 29.97 fps, 3Mbps standard-definition recording (25 fps in European versions) in MPEG-4 format
• Movies don't use full sensor area, and so increase effective focal length slightly.
• Maximum clip length is 2GB or 29 minutes
• Full-time phase detection autofocus functions during movie recording, with any autofocus-capable Alpha-mount lens
• Automatic exposure with aperture locked at or near wide open position if using autofocus (exposure brightness controlled with shutter speed and gain adjustment)
• Automatic or Aperture-priority exposure if using manual focus
• Exposure compensation and exposure lock are available both before and during movie capture
• SteadyShot image stabilization, white balance, creative style, metering mode, and AF area can be set before movie capture begins
• Maximum clip length is greatly reduced to just 11 minutes, if SteadyShot is enabled
• If AF area is set to local, AF point can be changed during movie capture
• Stereo audio recording via built-in microphones or via a standard 3.5mm external mic jack
• Audio recording can be disabled altogether
  
  

Sony A33 Video: Image Size, Frame Rate, and Encoding

The Sony A33 records at three different video resolutions, and can record high-definition movies in either AVCHD or MPEG-4 formats. Audio is encoded during movie recording in Dolby Digital (AC-3) for AVCHD and AAC-LC for MPEG-4. No spec is provided for the sampling rate of the audio tracks, though video players report 48 kHz at 256 kbps for AVCHD and 48 kHz at 128 kbps for MPEG-4.

The table below shows the specs for various video recording options.

Sony A33 Video Options
AVCHD Format (.MTS files)
Format	Resolution	Frame Rate	Card Capacity
AVCHD	1080i 1,920 x 1,080	60i (interlaced) (59.94 fields/second actual) 17 Mbps	2.1 MB/second (~15.2 minutes on 2GB card)
MPEG-4 Format (.MP4 files)
Format	Resolution	Frame Rate	Card Capacity (very approximate)
MPEG-4	HDV 1080p 1,440 x 1,080 (16:9 aspect ratio, 1.33:1 aspect rectangular pixels)	30 fps (progressive) (29.97 fps actual)	~~1.6 MB/second (~21 minutes on 2GB card)
MPEG-4	VGA 640 x 480 (4:3 aspect ratio)	30 fps (progressive) (29.97 fps actual)	~~0.4 MB/second (~83 minutes on 2GB card)

As noted above, the Sony A33 offers two video recording formats, either the HD-only AVCHD format or the less space-efficient but more computer-friendly MPEG-4. The MPEG-4 file format is a bit less efficient in its use of memory card space for a given image quality level, but is more widely supported, and seems to be a bit easier for older computers to read. AVCHD is the best choice if your primary output is going to be directly to a HD television, but MPEG-4 probably a better choice for your computer, particularly if it's more than a year or two old.

In AVCHD mode, the pixel resolution is 1,920 x 1,080, and the data stream on our test sample seemed to be recorded at an average rate of 2.1 MB/second, the maximum rate according to the "Main Profile" spec for AVCHD. No options are offered for lower bit rates, but in our experience, there wouldn't be much point to them: We've generally found lower AVCHD bit rates to result in poor detail and excessive artifacts, especially in higher-resolution and higher frame-rate cameras. At ~17 Mbits/second, file sizes are modest enough that we see little or no benefit in greater compression. (You'll definitely want to buy a really big SD card for use with your A33, though: Even with AVCHD compression, video files take up a lot of card space.)

With a frame rate of 60 fields-per-second (interlaced), the Sony A33 produces smooth motion, though not as smooth as could be possible with a sensor running natively at 60i. The A33's sensor is read at 30 frames-per-second (progressive) and video is interlaced to 60i by extracting alternate field pairs of each 30p frame. (The AVCHD standard does not define 1,920 x 1,080 at 30p, so providing 60i from a 30p sensor makes perfect sense.) While this is not true 60i from the sensor, it does have the advantage of making de-interlacing much easier, though sophisticated de-interlacers can certainly produce more fluid looking motion from true 60i video.

MPEG-4 mode offers a choice of two resolutions both recorded at 30 frames/second (progressive), with data rates and compression ratios as detailed in the table above. The MPEG-4 formats on the Sony A33 actually use slightly less memory card space than does its AVCHD format. This may in part be due to their lower frame rate, but also reflects the reduced resolution and significant compression that the A33 applies to these files.

There's an interesting difference in the high-def video capture functionality between the Sony A33, and its siblings, the A55 and A55V. Where the higher-res cameras use the entire effective area of the image sensor to capture movies, the A33 uses a window that's somewhat smaller than that used for still imaging. The result is a slight increase in the focal length crop when shooting movies, although Sony doesn't provide a precise figure for the increase. The LCD and electronic viewfinder previews match this cropped area, and so users may well not even notice the difference, but it's worth noting in that it will make truly wide-angle high-def video just slightly harder to achieve with the A33, compared to its siblings (and conversely, making it slightly easier to record at longer effective focal lengths). The difference is likely due to a limitation in how the A33 clocks a video stream off its sensor, preventing Sony from using the entire sensor area at the required frame rates. Note that VGA movie recording doesn't suffer this additional focal length crop.

Sony recommends use of a Memory Stick PRO Duo Mark 2, Memory Stick PRO-HG Duo, or SD / SDHC / SDXC Class 4 rated memory cards for movie recording, to ensure that card write speed isn't a limiting factor in clip length. (Slower cards will likely still work to some degree, but with a reduction in clip length, especially in AVCHD mode.)

Here are some examples of video shot with our sample of the Sony A33:

Sony A33 Video Samples
AVCHD 1920 x 1080, 60i (28.0 MB)	MPEG-4 HD quality 1440 x 1080, 30p (15.9 MB)
	MPEG-4 VGA quality 640 x 480, 30p (4.3 MB)
Focus Test Example 1 MPEG-4, HD quality (23.9 MB)	Indoor Lighting Example MPEG-4, HD quality (10.1 MB) [White balance set to Incandescent.]
Focus Test Example 2 MPEG-4, VGA quality (4.9 MB)
Rolling Shutter Artifact Example 1 AVCHD, 1080i (9.7 MB)	Night Video, MPEG-4 MPEG-4 HD quality (21.7 MB)
Rolling Shutter Artifact Example 2 MPEG-4, HD quality (7.2 MB)
Rolling Shutter Artifact Example 3 MPEG-4, VGA quality (2.2 MB)
Note: we hope to add some commentary and analysis when we get the chance, but for now, click on the links to download the videos and see for yourself. Enjoy!

Sony A33 Video-Mode Focusing

Live autofocus during video recording has been one of the main areas in which SLRs and SLDs have lagged dedicated camcorders, ever since video first appeared as a feature on SLR cameras a couple of years ago. It's perhaps not as much of a problem as one might expect, because the lower resolution of video images results in greater effective depth of field than with high-resolution still images, but it's still an issue for many consumers. Arranging camera angles so the subject will stay in the plane of focus for the duration of a clip can help to some extent, as can using smaller apertures to increase the depth of field, but neither works in all situations, and provides an adequate replacement for live autofocus. Of course, you can also learn to "pull focus" while the camera is recording, moving between preset focus points manually, but doing that well requires a surprising level of skill and experience -- and it can be made even trickier with lenses that weren't designed for video use.

Without live AF, consumers for the most part are reduced to only shooting subjects at a constant distance from the camera, or to having to settle for a lot of poorly-focused video -- rather defeating the purpose of recording at high definition resolution in the first place. A lot of video-capable SLRs are being sold to consumers these days, and having some video capability is certainly better than none, but for most consumers to make full use of a video camera, it really needs to be able to focus on the fly. Some single-lens direct view (SLD) cameras, including Sony's own NEX-series, have been able to offer improvements in certain respects, thanks to opportunities arising from their need to create new lens mounts (and compatible lens designs) from the ground up, but they still can't entirely get around the fact that contrast detection autofocus by its very nature requires focus changes to be made gradually in a series of steps, followed by some hunting around the point of focus.

The Translucent Mirror design of the Sony A33 allows it to avoid these issues, by providing full-time phase detection autofocusing during video capture, but the system brings some compromises of its own. Perhaps most significantly, the Sony A33's autofocus functionality during movies relies on the lens aperture being kept at or near the open position, and for this reason, manual aperture control isn't available when autofocusing is enabled during movie recording. Since smaller apertures can't be used to control exposure, this further means that when autofocus is enabled, a combination of adjustments to ISO sensitivity and shutter speed must be used to control exposure in bright lighting. When higher shutter speeds are selected by the Sony A33's autoexposure system, the result can be a noticeable choppiness, because each video frame retains less motion blur on moving subjects. (If you're intending to extract stills from your video, though, the reduced motion blur can potentially be an advantage, with moving subjects more sharply rendered.) This issue can be worked around using neutral density filters to bring the shutter speeds back down, but that of course relies on your having the appropriate selection of filters for your chosen lenses.

The Sony A33's phase detection system functions identically for both still image and movie shooting. There are a total of fifteen AF points, of which three are cross-type points, sensitive to both horizontal and vertical detail. While these points cover a fair portion of the image frame, and it's possible to manually select which point you want to use, they cannot be moved around the frame at will, as you can do with a camera using contrast detection autofocus. (They also don't reach as near to the edges of the frame as is possible when using contrast detection.) Three AF point selection modes are available -- Wide (which automatically selects any of the 15 AF points), Spot (which uses only the central, cross-type point), and Local (which allows direct selection of any individual point). The focusing mode must be selected before movie capture commences, but when set to Local, it's possible to change the active AF point during movie capture -- useful if your subject is moving around the frame, or you want to direct the viewer's attention between multiple subjects in the scene.

Regardless of the autofocus servo mode selected for still image capture, movies always use continuous servo autofocus, so it isn't possible to manually trigger single AF cycles as desired during movie capture. There is a workaround for this, in that you can simply disable and re-enable autofocus during movie recording, using the physical switch on the lens or camera body, as appropriate. Unless the camera's mounted on a tripod, though, this approach is liable to cause camera shake, and of course the sound of the switch being moved will also likely be picked up by the A33's internal microphone. Of course, it's also worth noting that noise from the autofocus drive mechanism is also likely to be picked up on the audio track, unless using an external microphone mounted remotely, or on some form of shock mount.

For more advanced users, manual focus during video recording is certainly an option, just as it is on many video-capable, interchangeable-lens cameras -- and unlike video-capable DSLRs, the A33 allows framing of video not only on its LCD display, but also in its electronic viewfinder (an important advantage when shooting under bright sunlight, when LCD panel glare can make manual focusing even more challenging). The Sony A33's very high resolution LCD display and electronic viewfinder both make this somewhat feasible, but without magnification, even displays this sharp don't quite hold up to 1,920 x 1,080 video resolution. While the A33's LCD and electronic viewfinder are towards the upper end of what's available in terms of resolution, neither provides even half the resolution of a Full HD video frame, and so the AF system could almost always do a noticeably better job of focusing than we could manually. The Sony A33 does offer its users a potential solution for any shortcomings in their manual focusing abilities, thanks to the availability of aperture control when focusing manually. Presuming that there's enough light for an adequate exposure, you can simply stop down the lens further to increase the available depth of field, giving yourself a margin of error around your selected point of focus.

The availability of full-time phase detection autofocusing during movie capture is perhaps one of the most significant benefits of the Sony A33's Translucent Mirror design, and it arguably provides a hugely important benefit for consumers, offering relatively fast autofocusing without the obtrusive hunting that's common in contrast detection AF systems. Since its Alpha-mount lenses weren't designed for video, autofocus drive noise is unfortunately quite obtrusive, but that's an issue that the Sony A33 shares with most video-capable DSLRs. Probably the biggest issue in the design is the requirement that the lens aperture be held open when autofocus is active, which can result in choppy video when shooting in bright conditions. This, coupled with the inability to place a focus point outside the area covered by the phase detection AF points, are probably the most significant drawbacks of the design. Unfortunately, with the availability of full-time phase detection AF in the A33, Sony has chosen not to provide any contrast detection AF capability. Given that both methods have different strengths and weaknesses, we'd have preferred to see Sony offer both systems, allowing the videographer to decide which AF mode was best suited to their subject.

Sony A33 Video Exposure Control

While the Sony A33 lets you record movies directly from any of its still-image exposure modes, including aperture-priority, shutter-priority, and full manual exposure modes, the shutter speed, aperture, and ISO settings for video recording are always automatically controlled whenever autofocus is enabled. Thus, while the camera's menu system might suggest full PASM (programmed, aperture-priority, shutter-priority, and full manual exposure) exposure control for videos, there in fact is no direct control over aperture, shutter speed, or ISO during movie recording.

When focusing manually, however, the Sony A33 offers a choice of either completely automatic exposure, or Aperture-priority exposure. The latter is available only when the Mode dial is in the Aperture priority position, and the camera retains control of both ISO sensitivity and shutter speed, but the ability to stop down the aperture is a particularly useful one when focusing manually, providing a means to increase depth of field (and hence, the chance that your chosen subject is in focus.)

Exposure compensation adjustments made in still-capture modes do carry over into movie recording, provided that there's enough light. (That is, if it's too dark, boosting the EV adjustment won't make your video any brighter.) It's also possible to adjust exposure compensation during movie recording, and to lock the exposure level by holding down the autoexposure lock button. (When released, exposure metering and adjustment will immediately resume.) White balance settings also carry over to video mode, as do the metering modes and creative styles..

Shutter speed is one aspect of video exposure we've recently become more aware of. It turns out that slower shutter speeds tend to produce more natural-looking videos than faster ones: The motion blur associated with a slower shutter speed tends to smooth-out transitions between frames with rapidly-moving subjects: Cameras with 30fps recording, but shutter speeds 1/100 second or higher produce somewhat choppy, artificial-looking movie footage, while cameras with exposure curves biased toward shutter speeds in the 1/30 - 1/60 second range show a lot more motion blur, but the end result looks more natural to the eye. The A33 is more prone to this than traditional SLRs and SLDs, because when autofocus is enabled, the aperture must remain at or near its open position to make phase detection AF possible. This results in the exposure level being controlled with only shutter speed and ISO sensitivity, where other cameras might be able to use a slower shutter speed by stopping down the lens aperture as necessary.

Sony A33 Movie-Mode Image Stabilization

Sony is unique among camera manufacturers, in having both body-based and lens-based image stabilization technology in their product lines. In the case of the A33, image stabilization happens in the body, so its available with all lenses. Sony claims that the SteadyShot INSIDE anti-shake system in the A33 provides a 2.5 to 4-stop reduction in the blurring produced by camera shake. Translating that into real-world shutter speeds, a two-stop improvement means that a shutter speed of 1/30 second would give you the same resistance to blur from camera shake that a speed of 1/120 would without anti-shake. A 4-stop improvement would mean you could shoot as slow as 1/8 second and get the same results (blur-wise) as when shooting at 1/120 second unaided. Even the lower end of the specified range of effectiveness means a pretty significant improvement in one's ability to hand-hold long exposures. Since the A33 uses sensor data to provide a live view feed on either the LCD display or electronic viewfinder, the effect of stabilization is previewed regardless of which display is being used. A SteadyShot scale on the right side of the viewfinder display further indicates the degree of stabilization.

One drawback of the Sony A33's in-body image stabilization is that, when enabled, it greatly reduces maximum clip length. This is likely an indication of heat created by operation of the sensor shift mechanism, leading to the sensor itself heating up more rapidly. When disabled, the Sony A33 can record clips as long as 29 minutes at an ambient temperature of 68F (20C), presuming the file size limit of two gigabytes isn't reached first. With SteadyShot enabled, though, the clip length is slashed by almost two thirds, to just 11 minutes.

Sony A33 Video: Audio recording

Like most competing SLR/SLD cameras with video recording capability, but the the Sony A33 goes most of its competitors one better, in that it actually has two internal microphones, located on either side of the flash / electronic viewfinder housing, providing stereo sound. It also provides for recording via an external stereo microphone, via a standard 3.5mm jack located under a rubber flap on the left side of the camera body. Sony's only published spec for the A33's audio recording capability simply says "Dolby Digital / MPEG-4 AAC-LC", so we don't officially know the sampling rate or number of bits of A/D resolution employed, although third-party MPEG players suggest a 48 KHz sample rate.

As with with most video-capable SLR/SLD cameras we've tested, the A33's internal microphones are exquisitely sensitive to noise produced when operating the camera's controls or even moving your hands on the body. Since the A33 accepts standard Alpha-mount lenses which haven't been designed with video recording in mind, autofocus drive noise is also clearly picked up by the camera's internal microphones, although the AF operations are at least brief and direct, perhaps resulting in somewhat less noise than in cameras using contrast detection autofocus.

As with most video-capable interchangeable lens cameras, the Sony A33 doesn't have any provision for manual audio level control, whether working from the internal or optional external mic. Of course, this isn't likely to be an issue for the A33's primary target buyers.

Sony A33 Movie Recording/Playback User Interface

The Sony A33 makes movie recording very easy, as you can initiate it at any time, regardless of the mode-dial setting: Simply press the prominent Movie Record button with the red dot at its center on the camera's top panel, and the camera will start recording video.

Normally, this is where we'd list the Movie-mode menu items, but the A33 has no separate movie menu. In fact, the only menu items exclusively related to video recording are the choice of file format (AVCHD or MPEG-4), the image size (1,920 x 1,080 for AVCHD; 1,440 x 1,080 or 640 x 480 for MPEG), and whether movies should be recorded with or without an audio track.

Playback mode on the Sony A33 is decidedly odd. The odd part is that the A33's playback is an either/or proposition, relative to stills and videos: If you've just shot a video and press the playback button, you'll see only movies that are stored on the card; not the still images. Likewise, if you've just shot a still image, you'll only see other stills when you hit the playback button. To switch between the two modes, you have three options -- either drop into thumbnail view and scroll over to the left, to select the correct tab on that screen, or visit the Playback menu and change the Still / Movie Select option, or simply grab a throwaway exposure of the type you want to view. We actually found it much more convenient to hit the Movie Record button twice (to record a brief video clip) than to navigate through the playback menu to change the playback mode. Sony introduced this playback mode in its NEX-series SLD cameras, where it caused us some consternation until we figured out what was going on. "Where'd all my movies/stills go?" RTM, as they say, but we can only imagine the dismay many users will experience (if only until they read the manual) when they think all their vacation photos disappeared after they'd finally shot a video. Since so few people actually do read manuals, we also suspect this is going to be the source of a lot of service calls to Sony's support centers.

The likely reason for this odd either/or choice for movie/still playback reveals itself when you try to scroll through movies you've recorded, to find a particular one you want to play back: Scrolling through recorded movies is just ineffably slow. Well, perhaps not ineffably; it's a matter of a couple of seconds per movie, but that's slow enough that navigating through more than two or three movies can be seriously painful. Why so slow? The camera is lightning-quick scrolling through still images, and doesn't it save a JPEG thumbnail of each movie, for just this purpose? This is another area in common with the NEX-series cameras that's going to produce a lot of frustration with users.

While it pales beside the slow playback scrolling, we missed one other playback option that's become increasingly common on both digicams and SLDs; namely the "trim" function. Whenever you record a short video clip, it's inevitable that you'll start earlier and keep recording longer than the action you're interested in. (If you don't do this, you're almost certainly going to miss action you're interested in.) While "padding" like this is important and necessary, we like to keep our videos concise by trimming away the extraneous material at the beginning or end of the clip after they're recorded. Many cameras support doing this in-camera these days, but the A33 does not. You could certainly import the video clips to your computer and do the trimming there, but that's awfully cumbersome. Far easier to simply trim and toss away the unwanted footage on the camera. (More grist for the mill for the first firmware update to the A33... ;-)

Rolling Shutter Artifacts

Essentially every video capable digital SLR/SLD currently on the market exhibits some level of motion-related distortion called rolling shutter artifacts. These are caused because the image data is captured and then read off the chip sequentially by rows, rather than each frame's data being captured all at once. In the case of the Sony A33, this means that image data for the last row of a given frame is captured and read out anywhere from 1/25th to 1/60th second after the data for the top row was captured. The effect on moving objects is like that of a focal plane shutter in an SLR, but more pronounced, because the video frame is read out much more slowly than the slit of a focal plane shutter moves across the sensor.

For a camera that scans video frames vertically (as all do that we're aware of), rolling shutter artifacts will be most noticeable for subjects that are moving rapidly side to side, or when the camera itself is being panned horizontally. Verticals in the scene will appear tilted to the right or left, depending on the direction of camera motion. As an example, consider the case of a camera being panned from left to right, with a flagpole or other vertical object in the middle of the scene when recording for a particular frame begins: If the top of the object was centered horizontally when the first line of the video frame is acquired, by the time the last line of the frame has been captured, the bottom of the object will have shifted to somewhere left of center: As a result, the vertical object would appear to be leaning to the right.

Computer Requirements for Viewing HD Video

A typical computer these days has little trouble dealing with still images, but high-definition video can be another matter. Depending on the file format involved, it can take a pretty beefy computer to handle HD-resolution video playback without stuttering or dropping frames. The Sony A33 supports both AVCHD and MPEG-4 recording formats. The AVCHD format is slightly more space-efficient on the memory card relative to its quality level, and displays well on HD television sets, but is less widely supported by computer playback and editing software. If most of your video playback will be on a computer, you may find MPEG-4 to be more to your liking. On the other hand, if your computer supports AVCHD fine, that would be the preferred format, given its space efficiency.