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Canon 7D Video Recording
High-definition video capture has rapidly become a must-have feature in this year's digital SLRs, with advanced amateurs and budget-minded professionals alike excited by the possibility of using the same camera body and lenses for both still and movie shooting. Most digital SLR video modes to date have had limitations in terms of control and output formats that have tempered this excitement somewhat, however.
The Canon EOS 7D's video functionality takes important steps in answering some of these criticisms, offering an unusually robust feature set that gives the photographer more control over their movies, as well as the ability to record audio from an external source. Both shutter and aperture are available for manual control, and the 7D also provides multiple frame rate options, including three that match the HD television timing specs. Like most digital SLR video modes, the Canon 7D omits continuous autofocus during video recording, which is likely more of a concern for the advanced amateur, with pros likely wanting to control focus manually regardless.
Canon EOS 7D Video: Image Size, Frame Rate, and Encoding
Video capability. The Canon EOS 7D offers 3 resolution levels for video recording, with various frame rates available depending on encoding and resolution.
The Canon 7D's all-new CMOS sensor records high definition video at a maximum resolution of 1,920 x 1,080 pixels with a 16:9 aspect ratio -- what's sometimes referred to as "Full HD", or 1080i/1080p. Both NTSC modes of 23.976 or 29.97 frames/second and PAL modes of 23.976 or 25 frames/second are available at full resolution. The full resolution can be downsampled in-camera to produce 1,280 x 720 (720p) resolution movies, with the NTSC mode using 59.94 frames/second, and the PAL mode offering 50 frames/second. Finally, a standard definition 4:3 aspect ratio mode captures 640x480 pixel (VGA) movies, again with 59.94 frames/second in NTSC or 50 frames/second in PAL. The numerous different frame rates match various broadcast television formats etc., removing the need to transcode to the intended output frame rate after capture.
Individual movie clips captured by the Canon EOS 7D are limited to a maximum of twelve minutes in the high definition 1080p and 720p modes, or 24 minutes in the standard definition VGA mode. (Canon didn't give a reason for this, but perhaps it's a matter of power consumption and sensor heating: We noticed that the body can get rather warm after an extended period of video shooting.) The Canon 7D records its movies as MOV files using AVC / H.264 compression, which is much more conservative of memory card space than the Motion JPEG format used by some cameras, and avoids some of the severe image quality loss suffered by AVCHD cameras when faced with significant amounts of change in image content between frames. (AVCHD uses a subset of the H.264 standard, among other things mandating a limit in recording bandwidth, which translates into a lesser ability to convey rapidly-changing detail.) The choice of H.264 comes with the requirement of greater processing power, though -- not only from the camera when recording, but also when playing back or editing videos. 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.
Here's a list showing what to expect for file sizes with the Canon EOS 7D's video recording: Canon 7D Video Options
H.264 Format (.MOV file container)
Resolution Encoding Frame Rate Clip Length File Size
1,920 x 1,080
(1080p Full HD)
NTSC 24p (23.976 fps)
12 minutes ~330 MB/min
NTSC 30p (29.97 fps) ~330 MB/min
PAL 24p (23.976 fps) (nt)
PAL 25p (25 fps) (nt)
1,280 x 720
(720p HD)
NTSC 60p (59.94 fps)
12 minutes ~160 MB/min
PAL 50p (50 fps) (nt)
640 x 480
(VGA SD)
NTSC 60p (59.94 fps)
24 minutes
PAL 50p (50 fps) (nt)
Canon recommends using a CompactFlash card with at least 8MB/s read/write speed to capture and playback movies.
Here are some examples of video from a preproduction version of the Canon EOS 7D, showing typical results under daylight conditions. Tests of our production units showed no difference.Canon EOS-7D Video Samples (Preproduction Camera Body)
1,920 x 1,080, 30fps
Programmed Exposure
(11.8 seconds, 66.4 MB) 1,920 x 1,080, 24fps
Programmed Exposure
(11.4 seconds, 62.2 MB)
1,280 x 720, 60fps
Programmed Exposure
(11.1 seconds, 60.3 MB) 640 x 480, 60fps
Programmed Exposure
(10.9 seconds, 28.7 MB)
1,920 x 1,080, 30 fps
1/800 sec Manual Exposure
(11.6 seconds, 63.6 MB) 1,920 x 1,080, 30 fps
1/4,000 sec Manual Exposure
(8.1 seconds, 45.2 MB)
1,920 x 1,080, 30 fps
1/4,000 sec Manual Exposure
(10.9 seconds, 60.9 MB)
(Too hot, Charlotte decides to take a break...) 640 x 480, 60 fps
1/4,000 sec Manual Exposure
(15.3 seconds, 40.5 MB)
Canon EOS 7D Video-Mode Focusing
As with most other video-capable digital SLRs, the Canon EOS 7D doesn't offer autofocusing during video recording. Instead, you can only trigger a contrast-detect AF cycle prior to the beginning of a recording by hitting the AF button on the camera's rear panel. You can manually focus the lens during a recording, though, and the true manual operation of AF on Canon'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 Canon EOS 7D doesn't have this problem, although it's possible that a third-party or older Canon lenses might produce audible noise while their focus was adjusted.)
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 2.1 megapixels in the Canon EOS 7D's highest-resolution 1080p Full HD mode, 0.9 megapixels in 720p HD mode, and just 0.3 megapixels in VGA mode, images that would be unacceptably blurred as 18 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 EOS 7D'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.
Canon EOS 7D Video Exposure Control
Many video-capable SLRs only offer automatic exposure in their movie modes, but the Canon EOS 7D gives you a choice of either automatic or manual exposure modes. In Auto mode, the camera adjusts the shutter speed and aperture as needed for a correct exposure, keeping things simple. 3.0EV of exposure compensation is available in 1/3 EV steps, to ensure exposure is as intended. Auto exposure mode is selected whenever the mode dial is set to anything other than "M". In Manual mode, simultaneous control of both the shutter speed, aperture, and ISO is possible. This is great news -- the ability to control depth of field or freeze action is very useful, giving you significantly more creative control over your videos. (Given the slow default shutter times of most video-capable digital SLRs, the 7D included, a higher shutter speed to freeze fast motion is almost a necessity for good-quality video of anything moving.) Note, though, that although you can control shutter speed, this doesn't prevent the so-called "Jello effect", more properly known as rolling shutter artifacts. Like other digital SLRs, the progressive manner in which the Canon 7D clocks data off its sensor means that sudden camera or subject movements can cause distortion, although this is less of an issue for the 7D than in some cameras.
Canon EOS 7D Video: Audio recording
External Mic. The EOS 7D's Mic jack resides on the camera's left side under a split rubber panel, alongside the camera's other various connectors.
Like most competing SLRs with video recording capability, the Canon 7D sports an internal microphone that can record an audio track. Internal mics are somewhat problematic, though, in that they're prone to picking up noise produced by moving your hands on the camera or actuating any controls while recording. We haven't noticed pronounced differences in how much camera-handling noise various models' internal mics pick up; the Canon 7D's seems fairly typical in that regard. In a big bonus for serious video users, the Canon EOS 7D also sports a microphone jack under a rubber flap on its left side, to which you can attach an external stereo microphone. As of this writing, relatively few video-capable SLRs offer options for external audio input. In addition to its internal microphone and external mic jack, the EOS 7D also allows audio recording to be disabled entirely through screen 4 of the Record Menu.
Canon EOS 7D Movie Recording User Interface
The Canon EOS 7D's video mode is accessed via a dedicated mode switch on the camera's rear panel, which selects between Live View and Movie modes. A Start / Stop button in the center of this mode switch is used to initiate recording when the switch is in the Movie position. Setting adjustments in movie mode are made via screen 4 of the Record Menu, and again are only available when the camera is in Movie mode -- otherwise they're replaced by options relating to the camera's Live View mode. Still images can be captured in Movie mode, interrupting the movie for about 1 second. Autoexposure is used, unless manual exposure is chosen. Flash is not supported.
In Playback mode, the Canon 7D allows you edit out or trim the first and/or last scenes of a movie, in one second increments. You can save the trimmed movie to a new file (if there is space on the card), or choose to overwrite the original. Movies can be played back at normal speed, or in slow-motion, with adjustable playback frame rate. There are "VCR" like controls for advancing to the next or last frame, or playing from the previous or first frame.
Canon EOS 7D Video Quality and Artifacts
As compared to the video from other cameras we've tested, we found the Canon EOS 7D's video to be relatively immune to motion-induced compression artifacts that we've seen when recording in AVCHD mode (as noted, the 7D uses the broader H.264 spec). Its rolling shutter artifacts also don't seem quite as pronounced on some cameras, although they're still very evident when you pan the camera quickly while recording.
Here are some examples of what we found in the EOS 7D's movie files:Canon 7D Video Quality Samples
1,920x1,080 -- 24 fps 1,920 x 1,080 -- 30 fps
Unlike some competitors, the Canon 7D's highest resolution does conform to the HD dimensional and frame rate specs (both 24 fps and 30 fps), so it's usable for recording high-detail video for display on an HDTV. When I first looked at this shot, I thought it was back-focused, but on further scrutiny, decided that the issue is simply that Charlotte and Marti both have much softer detail that gets stepped on more by the H.264 compression than does that of the contrasty plant foliage in the background. It'll depend a lot on the screen size of your TV of course, but the good news is that this kind of softness isn't nearly as apparent during playback than it is when pixel-peeping a crop from a frame-grab. This crop is also from 1,920 x 1,080 resolution video, but this time at 30fps. Given that the data rates seem nearly identical between the two modes, but one has 20% more frames per unit time, I expected to see at least some image quality decrease at 30fps. Sometimes I can convince myself I'm seeing a little difference, but suspect it's just because I think there should be one. If there is one, it seems it'd take a pretty well-controlled lab test to smoke it out, vs these fairly casual samples. One thought, though: Could camera shake play a role here?
1,920 x 1,080 -- 30 fps
1/4,000 sec shutter time 1,280 x 720 -- 60 fps
Now, this shot, captured with a 1/4,000 second video shutter time does look sharper. I guess no surprise, in that the effective shutter times of ~1/30 second above would require careful holding to get a crisp image in a still shot, but I really wouldn't have expected to see any blurring at video resolution from camera shake. Hard to argue with the crop above, though: When specifically looking for detail in video images, we need to be looking at shots captured while on a tripod. (I suspect though, cameras like the Canon 7D will much more commonly be handheld when shooting video.) Here's a sample at 720p resolution. It's hard to adjust for the difference in pixel size visually, but edges in the Canon 7D's 720p-resolution videos do look a little sharper than those in its 1,080p ones. (It also appears that there's a little sharpening of some sort being performed here as well.) As noted above, we found that the video data rates for the 7D's 720- and 1080-resolution videos were virtually identical. As it turns out, this is relatively consistent with the amount of data involved: The 1080 frames have 2.25x as many pixels in them than do the 720 frame, but the frame rate is 2x higher in the 720-res video. So the amount of compression being used is very similar, albeit just slightly favoring the 720 resolution setting.
640 x 480 -- 60 fps No Motion-Induced
Compression Artifacts
At the lowest video resolution, things again get crisper, this time noticeably. Pixel count drops by 3, going from 720 to 480 resolution, while the data rate drops in half. The net result is less detail lost to compression artifacts, but it's still easy to see them in still crops like this, at least in the darker shadows. There's plenty of blur here (shot in programmed exposure mode at 1,080 resolution and 24 fps), but no motion-induced compression artifacts. The AVCHD compression/format produces lots of strange artifacts in this situation, but the 7D just gives us straight blur, albeit plenty of it. Check out the examples below that show the effect of shutter speed, though...
What a difference shutter speed control makes!
In the video programmed exposure mode, the Canon 7D seems to default to using a very slow shutter speed, even when given plenty of light to work with. This is pretty much a worst-case example, although it's at 30 fps vs 24 fps, which seemed to do a bit worse yet with motion. Contrast the shot at left with this one, shot with a shutter speed of 1/4,000 second (which required cranking the ISO up -- to ~800, as I recall). This is far from a scientifically controlled test, but I tried to catch a frame where Charlotte was again snapping her head forward very rapidly, to grab the Frisbee. There are obvious JPEG artifacts visible here, but the difference in motion blur (or the lack thereof) could hardly be more dramatic.
Rolling Shutter Artifacts
Essentially every video capable digital SLR 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 Canon EOS 7D, with its variable frame rate, this means that image data for the last row of a given frame is captured and read out anywhere from 1/24th to 1/60th 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 visual impact of rolling shutter artifacts on the Canon EOS 7D seemed a bit less noticeable than in some other recent digital SLRs, but as noted, it's still a factor.
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 AVC / H.264 image compression used by the Canon EOS 7D is one of the more compute-intensive formats, and its 1,920 x 1080 (1080p) resolution means there's a lot of data in each frame to deal with at full resolution. The net result is that you'll want a relatively recent and powerful computer to play full-res high-def video files from the EOS 7D on your computer. At lower resolutions, the requirements will be more modest. We found that we could run the 7D's video acceptably at half size on an older G5 Power Mac with dual 2.3GHz processors, so long as nothing else was running simultaneously, so it definitely seems less processor intensive than full HD video from many other cameras, including some using Motion JPEG compression.
You can of course view your movies on a TV, either via the composite A/V Output, or on an HDTV via the HDMI output. Both terminals cannot be used simultaneously.
Canon EOS 7D Video Mode: The Bottom Line
Overall, the Canon EOS 7D seems to be a great video machine, albeit still without continuous autofocus capability during movie recording. It should be an excellent, cheaper (and hopefully more available) option to the EOS-5D for people looking for high definition video from a digital SLR, particularly one with a Canon lens mount.
Note: The above sample videos were taken with a preproduction unit, however tests of our production units showed no difference.
< Bu mesaj bu kişi tarafından değiştirildi AytacFx -- 8 Mayıs 2011; 3:52:52 >
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