All the video-DSLR's are broken

ben
Sun, 23 Aug 2009 00:05:18 +0000

Discussing the current state of video-capable DSLR cameras

Note: There's a summary for the impatient!

In August 2008 Nikon released the D90, the video DSLR that shot HD video.

The concept was apparently aimed at news-gathering, where reporters who would usually have to carry both a DSLR for photos and a separate video camera, could instead carry a single device capable of both shooting stills and, at the press of a button, shoot HD video.

For completely different reasons, these VDSLRs also appealed to film makers - a wide range of changeable, affordable lenses and large sensors capable of creating "filmic" shallow depth of field.. but, film-makers and news-gatherers have very different needs, and all VDSLR's thus far have been, in various ways, broken..

Starting with the D90, the first video-capable DSLR - it can shoot "720p24" - 1280pixels by 720pixels, progressive scan - this isn't the highest of the common HD resolutions (that being "1080p" - 1920x1080 progressive), not ideal, but perfectly adequate. It records at 24 frames per second, the same as film, which some people regard as an essential part of the "film look"

The D90's main fault lies with it's CMOS sensor, a fault that effects most cameras based on the chip, to varying degrees - the so called rolling shutter effect..

"Rolling shutter" is basically the result of how the CMOS sensor captures each frame - unlike a CCD which exposes and captures each "pixel" at the same time, a CMOS sensor captures one line at a time. This doesn't sound too much of a problem, until things start moving in front of the lens..

The white bar in that image is a perfectly vertical window frame, shot with the camera panning from left-to-right. Basically the top line of the frame is captured, then a tiny delay, then the next line and so on. The result is a skewed image, which can look visually bad (particularly with strobe lighting), and can make it very difficult to do things like 3D tracking, and integrating computer-generated elements.

This page describes the effect well (along with a few other CMOS and CCD artefacts)

Of all the video-capable DSLR cameras released, the D90 has had the most visible rolling shutter problems. As I said, this is a problem with almost all CMOS-based cameras, even the $17,500USD RED One (albeit much less noticeably)

The Panasonic Lumix GH-1 promised to be a very interesting VDSLR - shooting at 24fps (and 25 and 29.97), allowing manual control of exposure settings/audio, and has autofocus.. but it records interlaced footage - interlacing being a technique developed for televisions around 1926, and is simply an unnecessary pain to work with nowadays.

The GH-1 can produce very nice looking video, although I think it has been somewhat been overshadowed by Canon's first video-capable DSLR..

The Canon 5D Mark II is probably the most commonly used VDLSR currently. It is reportedly being used on the sets of the TV show "24", rumoured to be used on the Iron Man 2 set and many others, commonly as a "B cam" or "crash cam". There is also large communities based around the camera, such as Cinema5D

The cameras main "selling point" is its Full Frame sensor - the same size as 35mm film, something before only available on much more expensive cameras (such as the Panavision Genesis. This larger sensor allows for shallower depth of field (larger sensor means wider field-of-view - to get the same field of view as a smaller sensor you use a longer focal length, and a longer focal length means a shallower depth of field) - another important part of "the film look"

Shallow depth of field was obtainable previous by the use of 35mm lens adapters, such as those made by Redrock Micro, but these had drawbacks, mainly the loss of light (about 1 stop of light), requiring extra lighting. They can also be quite cumbersome and expensive. It is debatable wether full-frame VDSLR's could signal the end for 35mm lens adapters

The Canon 5D Mark II is an extremely promising camera - good, larger sensor which excellent low-light performance (much better than the RED One even), but it is not without it's flaws.

From it's release, until about 6 months after, there was no way to explicitly set exposure settings in video mode, such as shutter-speed, aperture and ISO - there was workarounds discovered some of which involved shining lights at the camera to lock exposure, and slightly untwisting the lens to lock the aperture!

This problem has since been resolved with a firmware update, allowing full manual control over all settings in video mode, but other problems remain..

The 5D Mark II records at 30fps, which aside from not being the filmic 24fps, causes problems in PAL countries such as the UK, where the lighting operates at 50Hz - this causes very visible flickering at all shutter speeds but 1/50th of a second. Another drawback is the frame-rate differences make it difficult (or impossible) to integrate the 5D with 25fps footage (30fps is difficult)

It is possible to convert 30fps to 25 and 24fps through optical flow retiming methods, but such techniques are both slow and can cause artefacting, particularly on fast-motion.

The rolling shutter on the 5D Mark II is an issue (the above rolling-shutter example image was taken from my 5D Mark II), but not nearly to the level of the D90.

The final problem with the 5D Mark II is the audio controls, or lack thereof - the built-in microphone has an automatic gain control, and there is no way to disable it. This means when it is quiet, the gain is automatically increased so you hear background noise. This also applies to any external mics (connected via the 3.5mm jack). There are a few workarounds to this..

The first is to use "dual system" sound - basically recording the audio on a separate device, and synchronising this while editing. The second is using a device like the BeachTek DXA-5D, which sends an inaudible tone to the 5D's mic-input, effectively disabling the auto-gain-control (although this can cause problems with noise)

The final option, and by far the most impressive, is to use the Magic Lantern Firmware - a third-party modification of the 5D Mark II's firmware, which (among other things) disables the AGC. There is series of videos demonstrating 5Dm2's audio options

There are other problems with the 5D mark II, like the lack of a HD video output while recording (for field monitors, or "video village" splits for directors to watch), lack of exposure and focus assistants like false colour (which use colours to indicate correct exposure levels), "peaking" (which highlights correctly focused areas),

The D90, the GH1 and the 5D Mark II are probably the video-capable DSLR's of most note currently.. As I started out by saying, they all just slightly broken in their own ways. I really don't think it would take much to produce a brilliant camera that would "win" the "VDLSR race" - the 5D Mark II really close (and the Magic Lantern firmware is pushing it ever closer)..

I think if any of the rumoured cameras (like the Canon 7D) can check the following boxes, they will do very well:

Full frame sensor

1080p

24fps

Manual exposure controls

(Optional) auto-focus

Audio level controls

HDMI output while recording

There is a final camera I want to mention, the video-DSLR's biggest "rival" - RED's latest plan, the Scarlet. I could double this articles length by writing about RED, but I will try to keep this short..

It was originally going to be a 3K (3072 x 2048pixels) camera with a fixed lens, for about $3000USD ("3K for $3k"), but this plan was scraped when Nikon and Canon released their video-DSLRs.

The Scarlet plan then changed to became a modular system, ranging from a ~$3000USD, 2/3rd inch, capable of recording up to 3K in a RAW format (essentially "uncompressed"), from 1fps (timelapse), through 120fps (slow motion), to the utterly insane, $53,000 "Epic 617", which records up to "28k" (for comparison, in this image, 1080p, the current HD standard is that tiny blue box)

As I said, I could write a lot about the Scarlet, how its pros/cons versus the video-DLSRs (things like complication of a RAW workflow, the 2/3rd inch sensor versus full-frame etc), but this article is already far too long, and the camera hasn't been released yet, but it will certainly be an interesting 12 months..

Summary

Since this was a rather long article, here is a bullet-pointed list for the impatient!

Nikon D90

For:

Shoots 24fps

Deserves merit for being the first "video-DSLR" to be released

Against:

Very visible rolling shutter problem

Shoots 1280x720 (720p), not 1080p

Manual focus

No manual exposure controls

Panasonic Lumix GH-1

For:

Manual controls from the outset

The included lens was designed specifically for video, such as smoother focusing for...

..Autofocus - something neither the 5D Mark II or the D90 has

Against:

Interlacing

Smaller ("Micro Four Thirds") sensor than the 5D Mark II

Canon 5D Mark II

For:

Full frame sensor

Manual exposure controls

Reasonable compression (H.264 at about 37mbit/sec)

Against:

No HD video output while recording

No exposure/focus assists

No audio controls (ignoring Magic Lantern)

No auto focus

Scarlet 2/3" cinema

For:

Changeable frame rates (including 24/25/29.97 and 120fps for slow-motion)

Records to RAW format (no image compression)

Reasonable price (comparable to video-DSLR prices)

Is a dedicated video camera (with stills capture as a "bonus")

Against:

It's not currently released!

Not a full-frame sensor (cheapest full-frame camera body will be $9,750USD, or the Super35 for $7000USD)

Records to RAW format (requires more complicated post-production workflow, or at least a very different workflow to what people are used to)

3K is arguably overkill for a majority of projects (but the option is always good)