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Sony Alpha SLT-A33 Gordon Laing, February 2011

Click here to find out about the Sony Alpha SLT-A33's Autofocus, sensor, processing and burst shooting

Sony Alpha SLT-A33 Movie Mode


  Sony Alpha SLT-A33 capture area: stills vs movies
  Crop marks indicate area captured during video
The Sony Alpha SLT-A33 offers Full HD video with stereo sound and continuous autofocus, and you can start recording in any mode by simply pressing the dedicated record button to the right of the viewfinder. When you start recording you'll notice the field of view shrink a little, so clearly the A33 isn't using its sensor's full area for video. This means wide angle lenses won't be quite as wide when filming video, although at least the Grid display option previews the cropped area that will be captured once you start recording - see right.

You can choose to record in AVCHD format at 1920x1080 pixels, or MP4 format at either 1440x1080 pixels (stretched out to 1920x1080 for playback) or 640x480 pixels. AVCHD footage is recorded as MTS files with interlaced video at either 50i or 60i depending on the region of the camera, while MP4 files are encoded as progressive video at 25fps or 30fps, again depending on the region; the frame rates cannot be adjusted.

Audio is recorded using stereo microphones built-into either side of the viewfinder head, or you can alternatively connect an external microphone to a 3.5mm jack on the side of the camera. The port can power compatible microphones, but it also works with those employing a phantom source; we tried the SLT-A33 with a Rode SVM and it worked fine. Limiting the usefulness though are the non-standard accessory shoe on the top of the camera and lack of manual recording levels.

The maximum recording time in any mode is 29 minutes, but this reduces to around 11 minutes on the A33 when SteadyShot is enabled - presumably due to overheating issues. Full HD AVCHD files consume around 120 Megabytes per minute, so you'll hit the 29 minute mark with a file measuring around 3.5GB. The MP4 files are additionally limited by size and stop recording when they hit 2GB, which will get you about 20 minutes in the 1440x1080 mode.

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Sony quotes a formatted 16GB card as being able to record just under two hours of Full HD AVCHD, just under three hours of MP4 at 1440x1080, or ten and a half hours at VGA resolution - although remember those earlier limitations of 2GB for MP4 files and 29 minutes for all modes. If you're using an SD memory card, you'll need Class 4 or faster to support Full HD.

The movie inherits the current White Balance, Creative Style, Exposure compensation, AF area and Metering mode. By default the exposure is fully automatic with no control over aperture, shutter or ISO, although you can adjust the exposure compensation both before and during filming. Aperture control is possible in Aperture Priority, but only when the camera and lens are set to Manual focus, the reason for which will become clear in a moment.

By default the focusing is set to continuous, and rightly so since the A33's unique combination of Live View composition with full-time phase-change AF delivers far better results than any video-equipped DSLR we've tested.

As explained earlier, the SLT-A33 employs a fixed semi-reflective mirror which always reflects a small portion of the incoming light to a phase-change AF sensor, while allowing the rest to pass through to the imaging sensor. This allows the camera to continuously autofocus using its quick and accurate phase-change system while filming video - a pretty unique capability in today's market.

Literally in contrast, existing DSLRs and EVIL cameras are forced to use contrast-based autofocus while filming video as the latter don't have alternative phase-change systems and the former bypass theirs during filming. Unfortunately most contrast-based systems are insufficiently quick to deliver discreet and continuous autofocusing while filming, leaving most cameras to either deliver a disappointing AF experience during video or simply not bother and force you to focus manually instead.

This is where the SLT-A33 really scores, confidently, quickly and continuously adjusting its autofocus while filming. Better still, since phase-change systems (under ideal conditions) know which way to turn the focus to achieve a sharp image, the SLT-A33 can lock onto subjects without the usual searching back and forth. With the right subject matter, it's very satisfying to watch the SLT-A33 refocus in one move while rarely over-shooting or searching. You can see an example of this in the clip below, which simply cannot be matched by video-equipped DSLRs at the time of writing.

Sony Alpha SLT-A33 sample video 4: indoors, handheld, continuous AF and shallow DOF
Download the original file (Registered members of Vimeo only)

In the example above we chose the Local AF area option and manually selected the central focusing point, so we could force the camera to focus on the desired object simply by pointing at it. We found this much more successful than the default Wide area option which automatically selects from the 15 available AF points and as such may end up concentrating on the incorrect subject.

Fixing the AF point gives you control over the system, although obviously prevents refocusing on a different subject in the frame without recomposing. Considerately though, the SLT-A33 allows you to adjust the active AF point while filming by simply pressing the AF button on the back and using the rocker. In theory this could allow you to pull focus between subjects on opposite sides of the frame without recomposing. In practice they'll of course need to fall under an available AF point and you'll also need to select it quickly to prevent the camera from refocusing on something in-between. Pressing the buttons quickly can also wobble the camera, not to mention be picked-up by the internal microphones, so it's not an ideal process, but it is at least possible.

Note: since the A33 only supports continuous autofocus while filming, there's nothing stopping it from making constant adjustments, which under the wrong conditions could be visually (and audibly) distracting. You can of course take somewhat extreme action by switching the camera and lens to Manual Focus (and back again if desired) during a clip, but it's hard to do so without wobbling the camera and making a loud click.

Suffice it to say the camera is confident at tracking subjects as they approach or retreat. In the next clip we filmed this approaching gondola / cable car with the kit lens fully zoomed-in, and the SLT-A33 managed to keep it in sharp focus for the majority of the sequence. So far this has proven impossible to match with a video-equipped DSLR, and would also be very hard to achieve with manual focusing.

Sony Alpha SLT-A33 sample video 5: outside, handheld, continuous AF tracking at 55mm with kit lens
Download the original file (Registered members of Vimeo only)

If you've only filmed video with a traditional DSLR, you'll undoubtedly be impressed by the continuous AF capabilities of the SLT-A33, but there are some caveats to be aware of. Arguably the most important is that phase-change AF sensors require bright lens apertures to operate - typically f5.6 or faster.

In practice this means the SLT-A33 has to fix the lens aperture at (or close to) its maximum in order to autofocus while filming. This has the joint side-effects of a shallow depth-of-field whether you like it or not and a reliance on adjusting the ISO and shutter speed only for exposure control. The latter can cause problems in bright conditions, as with the aperture wide open, the SLT-A33 is forced to use relatively quick shutter speeds even at the lowest ISOs. This in turn can cause choppy-looking video, especially with motion or panning. The only solution to this problem is to fit neutral density filters.

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If you'd like a smaller aperture, either for slower shutter speeds or to increase the depth of field, then you'll need to switch the SLT-A33's body and lens to manual focus. This then allows the movie mode to inherit the selected f-number in Aperture Priority, although obviously you'll lose the autofocusing.

So if you want autofocus while filming on the SLT-A33 you'll need to accept a shallow depth-of-field, no control over exposure (other than compensation), and potentially choppy footage in bright conditions thanks to fast shutter speeds. It's also worth noting the kit lens is quite audible when autofocusing, and even the operation of the quietest Sony lenses will be picked-up by the internal mics.

It's also important to remember while phase-change AF during video is technically impressive, it's not the only game in town. Panasonic's Lumix GH2, arguably the A33's biggest rival in the hybrid market, may employ contrast-based autofocus, but deploys it sufficiently quickly to deliver continuous AF capabilities. In use this allows the camera to confidently refocus on different subjects during filming, and while the technology inevitably involves a little searching, it's pretty successful on the whole.

The GH2 also sports additional tricks including a touch-sensitive screen which allows you to pull-focus between subjects by simply tapping them on screen, full manual exposure control, adjustable recording levels, a 1080 / 24p option, broader AF areas including genuine face detection and tracking, Single AF modes in addition to Continuous, clean HDMI output to drive an external monitor, and a selection of lenses which focus very quietly. So while the SLT-A33 under ideal conditions can refocus without searching at all, the Lumix GH2 delivers a more sophisticated video recording experience overall which will be preferred by serious videographers or independent film makers; you can see how it compares under similar conditions on our Panasonic Lumix GH2 Video Samples page.

To be fair though, the SLT-A33 is aimed at a consumer market which should be more than satisfied by its video capabilities; indeed it's hard not to be delighted with them compared to rival DSLRs. If all you want is continuous AF while filming, it won't disappoint. You can find more movies filmed with the SLT-A33 on our Sony Alpha SLT-A33 Video Samples page. Alternatively if you've seen enough movies, find out how the SLT-A33 compares against its rivals in terms of image quality in our results pages and sample images gallery. Or if you've already seen enough, head straight to our verdict.


All words, images, videos and layout, copyright 2005-2017 Gordon Laing. May not be used without permission.

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