Pentax Kx Video Recording

The Pentax K-x is the company's second DSLR to feature the ability to capture video clips, after the prosumer K-7. Much like live view capability that preceded it, video capture is rapidly becoming a must-have feature in the DSLR market. The K-x's video mode shares much in common with that of the K-7, but with some of the more uncommon features absent.

Pentax K-x Video: Image Size, Frame Rate, and Encoding

The Pentax Kx records video at either high definition 1,280 x 720 resolution with a 16:9 aspect ratio, or standard definition 640 x 416 pixels with a 3:2 aspect ratio. Both video modes record at 24 frames per second; well-suited for broadcast television, but slightly less smooth than the 30 fps video offered by some DSLRs.

The Pentax Kx records its movies as motion-JPEG (AVI) files, rather than the AVCHD format favored by some manufacturers. AVCHD is more conservative of memory card space, but at the cost of some loss of image quality when a lot of the image changes from one frame to the next, as is the case when panning or in closely-cropped video of a rapidly-moving subject. AVCHD is a subset of the broader H.264 video compression spec, one that places fairly heavy constraints on the recorded bitrate (hence its smaller file sizes). Cameras with sufficient processing power can use H.264 encoding at higher bitrates to avoid some of the limitations of AVCHD, but this comes with the requirement of greater processing power, for both recording and playback.

As noted, the Pentax K-x's video suffers no compression-related quality loss during panning, but the consequence is very large file sizes: A minute of HD video can very easily produce a several hundred megabyte file, depending on subject matter.

Recording is only part of the story, though: Editing and playback are the other aspects to be considered. When it comes to editing, the Pentax Kx's AVI format is much easier to deal with than H.264-encoded video (including AVCHD), at least currently: The more sophisticated encoding used in the H.264 standard requires quite a bit of processor power to pull it apart and put it back together again, so frame-accurate editing of H.264 requires a fast processor and capable editing program.

When it comes to playback, it's perhaps a bit of a toss-up between the two formats: AVI plays back much better on computers with relatively modest processors, while AVCHD files can be read directly by some television sets. Sadly, the Pentax K-x lacks any form of high-definition video output connectivity, so viewing videos on your TV in high-def requires either a suitable third-party device from which to play the video, or that the display itself include an SD/SDHC card slot and high-def Motion JPEG compatibility.

Here's a list showing what to expect for file sizes with the Pentax K-x's video recording:

Pentax K-x Video Options
Motion JPEG Format (.AVI files)
Menu Designation	Resolution and Frame Rate	Quality Level	File Size
0.9M	1,280 x 720 24 fps (720p HD)	High	304 MB/minute
		Medium	178 MB/minute
		Low	143 MB/minute
0.3M	640 x 416 24 fps	High	89 MB/minute
		Medium	62 MB/minute
		Low	46 MB/minute
(Measurements were made based on the sample videos below.)

At both resolutions, the medium and low quality settings produced almost identical file sizes. With all frame sizes, we felt that recording at anything less than the highest quality setting produced too many JPEG artifacts for our tastes: Smaller file sizes are good, but only if they provide the image quality we're looking for.

Here are some examples of video from the Pentax K-x, showing typical results under daylight conditions.

Pentax K-x Video Samples
0.9M Best Quality: 1,280 x 720, 24fps (30.3 MB)	0.3M Best Quality: 640 x 416, 24fps (14.9 MB)
0.9M Better Quality: 1,280 x 720, 24fps (26.6 MB)	0.3M Better Quality: 640 x 416, 24fps (8.2 MB)
0.9M Good Quality: 1,280 x 720, 24fps (28.5 MB)	0.3M Good Quality: 640 x 416, 24fps (6.1 MB)

Pentax K-x Video-Mode Focusing

Focusing in movie mode with the Pentax K-x is much the same as it is with most other video-capable DSLRs: You can trigger a contrast-detect AF cycle prior to the beginning of a recording by hitting the AF button on the camera's rear panel, but there's no autofocus during recording. You can manually focus the lens during a recording, though, and the true manual operation of AF on Pentax's lenses means you can do this more or less silently, simply by being careful about turning the focus ring. (We recently tested the Olympus E-P1, which uses "fly by wire" focusing, whereby the focus ring only instructs the camera to move the lens elements rather than moving them directly via a mechanical coupling. This meant that small clicks could be heard on the audio track every time the E-P1 changed the focus setting, regardless of how slowly we turned the focus ring. With true manual operation of its lenses, the Pentax K-x doesn't have this problem.) As is the case in Live View mode, pressing the Info button on the camera's rear panel lets you zoom in up to 6x when autofocusing, or 10x when focusing manually. This is helpful for confirming precise focus, but note that it is only possible before recording has started: Once the camera has begun recording, only the normal 1:1 view is available.

The omission of autofocusing capability during movie recording in the K-x likely comes down to a combination of two factors. Firstly, noise from the focusing motors would be picked up by the camera's internal microphone, although the degree to which this would be a problem would vary depending on the lens type in use. Secondly, the manner in which contrast detection autofocusing functions could also prove objectionable -- it's rather slow, and "hunts" around the point of focus. The K-x is far from alone among DSLRs in failing to offer AF function during video capture, but we must admit we'd prefer manufacturers let users make the choice as to whether or not they're willing to accept these potential drawbacks in their videos. Of course, manual focusing is still possible, but for most consumers, manual focusing during video capture is a skill they're not likely to master -- and nor would they want to do so.

As we've noted in other SLR reviews, the good news with focusing for video is that you can get surprisingly good depth of field in video mode by stopping the lens down, thanks to the relatively low resolution of the video image. With a pixel resolution of only 0.9 megapixels in the Pentax K-x's high-definition mode or 0.3 megapixels in its standard-definition mode, images that would be unacceptably blurred as 12 megapixel still shots look perfectly fine as video frames. This not only provides greater depth of field at any given aperture, but is also more forgiving of diffraction limiting at very small lens apertures. Diffraction at small apertures means you'd usually want to avoid f/16 or f/22 for still images, but again, the results generally look perfectly fine at video resolutions. Bottom line, with the K-x's lens set to f/16 or f/22 (assuming you're shooting under fairly bright conditions), you'll be surprised by how little focus adjustment is needed during a typical video recording.

Pentax K-x Video Exposure Control

Many video-capable SLRs only offer automatic exposure in their movie modes, but the Pentax K-x gives you a choice of either automatic or aperture-priority exposure mode. In Auto aperture mode, the camera adjusts the aperture as needed, but doesn't report the current value on the LCD screen. (We'd like to see a future firmware update add aperture display.) In manual aperture mode, the current aperture is displayed in the lower left corner of the LCD screen, and can be changed prior to movie capture by rotating the command dial. This is a great feature that's as yet available on only a few video-capable interchangeable-lens cameras. The ability to control depth of field is very nice, giving you that much more creative control over your videos. Note that regardless of whether controlled automatically or manually, the aperture is fixed as soon as movie recording commences.

In common with most other video-capable SLRs, the Pentax K-x offers the full range of white balance settings in movie mode, including four different options for fluorescent lighting, and Kelvin values ranging from 2,500 to 10,000. You can also select any of the K-x's Custom Image modes, including Bright, Natural, Default, Portrait, Landscape, Vibrant, Muted, and Monochrome. As with still-image shooting, settings for saturation, hue, high/low key adjustment and contrast can be adjusted independently for each mode. A sharpness setting is available as well, which we were slightly surprised to find actually did affect the sharpness of the video images, to a very noticeable degree. (Note that turning the sharpness up too high will make the compression artifacts much more evident, though.)

One movie-mode exposure parameter that the Pentax K-x unfortunately doesn't let you control is its shutter speed. The shutter speed appears to be tied to its frame rate, and regardless of the light level involved, rapid motion is always blurred to about the level you'd expect with a 1/24 second exposure time. (As noted above, the video frame rate of the K-x is 24fps in all resolution modes.) Shutter speed control is pretty rare in video-capable interchangeable-lens cameras; as of this writing, only a handful of models offer this feature.

Pentax Kx Video: Image Stabilization

The Pentax K-x's body-based image stabilization can be used when recording movies. Not only does this make any lens you're recording with into an IS model, but we found that the image stabilization worked exceptionally well when recording videos. Because the Pentax K-x's IS system provides such a high degree of stabilization, there's a bit of a "floating" feeling when recording videos with IS enabled: As you move and pan the camera, the video image tends to lag a little behind your motions, "catching up" when you come to a rest. A related IS effect is that the subject can drift up or down or from side to side slightly during recording. This can be slightly disconcerting, but the upside is that your videos will be much more stable than otherwise, even holding the camera at arm's length, or when shooting with a long telephoto lens.

Pentax K-x Video: Audio recording

Like most competing SLRs with video recording capability, the Pentax Kx sports only an internal, monaural microphone that can record an audio track, with no provision for an external microphone. Internal mics are somewhat problematic, in that they're prone to picking up noise produced by moving your hands on the camera or actuating any controls while recording.

In the case of the Pentax K-x, the issue can be exacerbated if you choose to use the body-based image stabilization system during movie recording If the K-x is jostled a good bit during recording with IS enabled you can hear the IS in the audio track, but not when it's being held relatively steadily: The light clacking sound that's picked up on the audio track in the face of significant camera movement seems to be the result of the sensor unit bumping up against its stops. If the camera is held even reasonably carefully, we were hard-pressed to hear any IS-related noise at all, even in a quiet office environment. In normal usage, you thus shouldn't hear any noise from the IS system, unless you're panning quite a bit, or if unstable footing results in a lot of camera movement.

We found that the Pentax K-x's built-in mic picks up up ambient sound pretty well, with sound quality and sensitivity both being good. The design seemed to be omnidirectional, picking up noise from nearby subjects behind the camera almost as well as it does those in front. It also seemed to be less sensitive to sounds picked up from focus adjustment or hand movement on the body than some other cameras we've tested.

Pentax K-x Movie Recording User Interface

The Pentax K-x's movie mode is accessed via a separate position on the camera's mode dial, rather than being initiated from within still-picture Live View mode. There's no separate control button to start/stop movie recording: Pressing the shutter button begins recording, pressing it again stops it. Some SLRs have a dedicated button to start/stop movie recording, but we find the K-x's use of the shutter button more intuitive - important, for a camera that's aimed at less experienced photographers. Without a dedicated button, the decision to promote the movie mode to its own position on the mode dial makes sense

Like the K-7 before it, the K-x groups setting adjustments for its movie mode in a sub-menu accessed via the Movie option on screen 3 of the Record Menu. We still found this a little awkward when making frequent adjustments to video settings, as the extra level of menu selection meant more button-pressing to make the changes than would have been required if they'd been available from the main shooting screen via a "quick menu," as some cameras have. That said, the more commonly accessed options for white balance and Custom Image settings were easily accessible via the 4-way control buttons on the camera's back panel.

The Pentax K-x's Record Menu options for video recording are:

Pentax K-x Record Mode Menu Movie Options
Top-Level Selection	Second-Level	Notes
Recorded Pixels	- 0.9M [16:9] (1,280 x 720) - 0.3M [3:2] (640 x 414)	All modes record at 24fps 0.9M mode is compatible with display on an HDTV, although the K-x doesn't offer any HD connectivity
Quality Level	- *** (high) - ** (medium) - * (low)
Sound	- On - Off
Movie Aperture Control	- Auto - Fixed	"Fixed" lets you set aperture via the rear-panel control dial before movie capture starts. The aperture cannot be adjusted during movie recording, regardless of this setting.
Shake Reduction	- On - Off

Rolling Shutter Artifacts

Essentially every video capable DSLR currently on the market exhibits motion-related distortions called rolling shutter artifacts. These are caused because the image data is captured and then read off the chip sequentially by rows, rather being captured all at once. In the case of the Pentax K-x, with its 24fps frame rate, this means that image data for the last row of a given frame is captured and read out up to 1/24 second after the data for the top row was captured. The effect on moving objects is similar to 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.

The shots below show some screen grabs from our evaluation of the Pentax K-x's rolling shutter behavior.

Pentax K-x Video - Rolling Shutter Artifact
Reference: Static frame	Rolling Shutter, 0.3M resolution

The upside for the K-x relative to rolling shutter artifacts is its excellent IS system. While you can avoid the worst effects of a rolling shutter by panning slowly, even minor jiggling of the camera due to hand-holding can produce a jelly-like appearance in the video output from some models. The K-x's IS system stabilized the video well enough that we experienced no such effects when the camera wasn't being panned, so long as the scene didn't contain fast-moving subjects. If our shooting required both stationary shots and rapid panning in the same segment, though, we had to shoot with the IS system disabled to avoid clacking noise in the audio track from the internal mic.
(Unfortunately, if your shooting for any given segment will involve both panning and steady camera positions, you'll generally need to shoot with the IS system disabled, to avoid clacking on the audio track and somewhat erratic framing in response to rapid camera movements.)

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 Motion JPEG image compression used by the Pentax K-x is one of the less compute-intensive formats, though, and its 1,280 x 720 resolution means there's a good bit less data to deal with in each frame than in videos with 1,920 x 1,080 (1080i) resolution. The net result is that you should have no problem playing video files from the K-x on your computer, as long as it's a relatively recent and reasonably powerful model. We don't have a specific benchmark for this, but if your computer is less than 3 years or so old, it should have no problem with the K-x's video streams. (By contrast, to play AVCHD or other keyframe-encoded video formats at full 1,920 x 1,080 resolution can demand a very powerful CPU and video card, to keep up.)

That said, the Pentax K-x's video codec doesn't appear to be terribly efficient, as it produces very large file sizes at its best quality setting. (And as noted earlier, its lower quality settings introduce an unacceptable level of JPEG artifacts into the image stream, so you'll want to use the highest quality setting almost exclusively.) So, while you might be able to play its videos on an older computer, you'll almost certainly need to increase your hard drive capacity. With video files occupying 300 MB/minute, it's a good thing that terabyte hard drives are so cheap these days. (At that pace, a terabyte hard drive could hold around 50 hours of video footage. That's a lot of footage if you primarily use the K-x for casual "video snapshots," but if you're a serious video user, it can still go by pretty quickly.)

Pentax K-x Video Mode: The Bottom Line

Overall, the Pentax K-x's video mode is a mixture of strengths and weaknesses. On the positive side, the K-x's video imagery was noticeably more crisp-looking than that of most SLRs we've tested to date. Its rolling shutter artifacts were a bit more obtrusive than those of 30fps video modes in some competing models, but less problematic than those seen in cameras shooting at only 20 frames/second. We found the K-x's body-based IS provided excellent stabilization during video recording, with very little impact on the audio recorded by the in-body microphone, but the strong IS also produced a "floating" feeling in videos, and sometimes made it hard to keep the subject completely stationary within the frame. Unlike the K-7, the K-x has no audio input jack, so you can't easily mitigate focusing noises or isolate sound to just a single microphone.

Video in SLRs is still somewhat at the bleeding edge of technology, so no camera we've seen (yet) does it perfectly. For the average user, though, the K-x's video is certainly good enough to avoid having to lug along a digicam on trips, just for the sake of capturing video snapshots. Recording in its highest quality setting at 720p resolution, its output will be acceptable for many more serious users as well; but those who are more serious will do better with the Pentax K-7 which records at 30 fps and has an external stereo microphone jack.