Michael Brennan Director of Photography

Michael Brennan Director of Photography

Michael Brennan Director of Photography

Michael Brennan Director of Photography

Digital Slow Motion The Beginning

 
August Musger an Austrian priest filed the first patent for a slow motion film camera in 1904.
100 years later digital slow motion cameras are coming of age and as with digital there are both compromises and surprises.
The first video slow motion camera was invented by NAC in the mid 1980s. A decade later they made the first digital colour slow motion camera the Memrecam Ci. With the help of UK TV engineers, Top Teks and Bob Sharpe from Image Diagnostics we turned this camera into a camcorder with the addition of a viewfinder, TV style zoom lens and shoulder pad.
 
In retrospect this was probably the first progressive scan camcorder!
NAC started back in 1958 and since have developed a wide range of extraordinary products NAC's new Ultranac captures an astounding 20,000,000 frames per second!
One of NAC's first laser technology projects was the Laser Beam Image Recorder for creating large format still pictures from satellite data. The same technology was used to create NAC's Laser Film Recorder. Sounds like an Arri Laser doesn't it? It is! Not surprising then that Arri unveiled their badged version of a NAC digital slo mo camera at IBC in 2003.
But NAC are not alone in digital slo motion and the established companies Vision Research who make the Phantom range, Photron and Redlake who merged with Kodak MASD, offer a wide range of cameras with different frame rates, resolutions and download speeds that make choosing the right camera for a film or TV job a difficult task.
Up until recently this technology has been used in industrial High Speed production lines, car crash testing, sports medicine, military, life science and animal sciences. There is no clear winner as to which cameras are best suited to TV or film work. New models are being released so here is a guide to digital slow motion that should help you find the right camera.
 
How Slow is slow?
Normal PAL video records 25 frames per second. NTSC 30fps. The digital slow motion systems record 100 to 3000 full frames per second. A full frame for the purposes of this article is 1000 pixels by 1000 pixels.
When one second of real time, recorded onto 1000 frames, are replayed at 25 frames per second the duration of the replay is 40 seconds. In other words, the camera stretches one second of reality into 40 seconds of slow motion.
The cameras record higher frame rates by reducing the size of the frame.
Most of the imagers are 4x3 aspect ratio so a reduction of height of the frame is not onerous if the shot is being used in a 16x9 project. The latest camera from Vision has a 1600x 1200 pixel imager at 1000 frames per second. Photron's Altima APX has a 1024x1024 sensor that will soon work to 3000 fps.
 
 
How does digital slow motion differ in using slow motion film cameras?
The main difference is that there is no additional expense that comes with a 200 foot load of film whizzing through a film camera in a few seconds for every shot. A typical slow motion film shoot is meticulously planned and executed for this reason!
Many TV projects cannot afford the regime that surrounds a shoot where the cost of rolling the camera is very high. Although there are film cameras that reach 8000 frames per second they are not in everyday use, not readily available and not suitable for every day TV production. They draw 35amps and are heavy. Surprisingly NAC made these film cameras too.
The more readily available of the Photosonic 16mm film cameras go to 500 frames per second. 450 fps is the slowest for a 35mm camera. There are one or two high tech exceptions.
There are probably more digital slo mo cameras already sold and still working than all the 500 fps film cameras ever built. So a 3000 frames per second compact, battery operated digital camera with a remote head is a significant development.
Another benefit is instant replay. It is usually very difficult to see the action with the naked eye. A typical 2000 frames per second shot will occur in a fraction of a second. Like 1/10th of a second!
By utilising instant replay the lighting and exposure can be fine tuned. The director can better evaluate what is and isn't working. Most of us are on a steep learning curve when it comes to analysing life at thousands of frames per second. As is the case with other types of digital photography the director has more control as he can see the picture whilst still on set.
So the DoP and director are able to explore and build an idea during a shoot.
Digital slow motion video cameras will encourage new ways of shooting. Extreme close ups that would be impossible for a camera operator to pull focus on, or would have the subject whizzing through frame can be achieved with a locked off camera. For instance, an extreme close up of the hand that releases a javelin is possible. It may take a few attempts to get the framing correct, but without the high cost of film this is not a problem.
without a cost penalty the director and DoP can to tryout new angles and daredevil shots that won't blow the budget, or end in embarrassing failure. Shooting from cranes, helicopters or tracking vehicles poses no unusual problems.
 
 
Quality
The perceived quality of the picture by the viewer is far greater than the technical specification suggests. When Television directors independently viewed a demonstration tape of the older lower quality systems the common response was that the quality was "superb''.
In fact the resolution and highlight handling ability of the old system was less than broadcast quality, equivalent to a single chip minicam. The filmic effect of slow motion creates a motion portrayal that we associate with film.
However the new cameras are a significant improvement on the old in respect of resolution and colour fidelity. Newer cameras have 10 bit colour per channel and dynamic pixel management. However colour fidelity is still far below broadcast cameras.
 
What's the catch?
At 500 fps the cameras need around 6 stops more light than regular video. By using super fast (F1.2) lenses some of this deficiency can be overcome, but it still leaves the camera 4-5 stops slower. So for TV sports outdoor events on a cloudy day present no problem. Indoor events however, need the lighting level to be increased to at least 2500 lux. This is a similar light level to the days of tube cameras. A typical boxing ring today is lit to 1000lux. A football stadium under lights is 500-750 lux.
If one wants to shoot above 500 fps then more light is required. One more stop to reach 1000fps and another for 2000fps.
 
For pack shots at 2000 frames per second where a good depth of field is required the level of heat generated by the lights is fierce. Fibre optic devices, strobes and a new range of HMI lights are in common use. The new HMIs are able to reach correct operating temperature immediately. These are small units that produce an enormous output for a few seconds.
Sun glasses and a fire extinguisher regularly adorn a high speed set!
On a dull day or indoors additional lighting is required.
It is not always practical to light an entire stadium to 5000 lux but consider that one may only need to light a very small area since we only need to light an area where a second or so of action takes place.
As a rough guide switch in 1000th of a second shutter on your regular TV camera. This is a very general guide as to whether additional lighting may be required at 500 fps.
 
How the cameras work
The cameras use single chip imaging devices that are capable of working at higher frame rate than that of normal video. The technical issues involved are heat dissipation and moving the huge amount of data from the chip to storage. Most of the sensors use a Bayer filter.
The transfer and record rates from chip to DRAM exceed 2 gigabytes a second.
 
The pictures from either the CCD or CMOS chip are passed to the storage medium until the capacity of the memory is filled. The system is set so that the memory is continually being overwritten. That is, once the memory is full the pictures are repeatedly recorded over until the operator wants it to stop. In other words the camera can be on for every moment of a tennis match but only saves frames when triggered.
 
The operator can stop the recording in 3 ways;
Immediately; thus saving the previous images in the DRAM.
Middle; will continue to overwrite the first half of the existing images in DRAM with new images
End; will carry on recording until DRAM has been completely overwritten with new images.
 
Once the recording is stopped the pictures are replayed from the volatile DRAM memory to a computer for storage. By volatile I mean if the battery dies or the computer crashes you lose the shot. Some cameras have a reasonable quality composite output to watch the image as it is transferred. This is the bug bear of digital slow motion. Even though your shot took just 1 second to record it takes 2 minutes to download!
 
 
Capacity
The storage capacitIes vary from camera to camera and is best noted as a number of frames at a particular resolution. So a camera with a capacity to store 2500 frames at 1kx1k may have a capacity of 5000 frames at half that resolution.
A recording capacity of just a few thousand frames may sound pitifully small. At high frame rates it is just a few seconds of recording time. This may not sound like much but remember that the camera is continually recording, 24 hours a day if you wish, so consider that you are selectively saving 2500 frames at a time. The neat trick is that if nothing is worth saving then you save nothing. The downside is that the camera is out of action for the length of time it takes to replay. The download time from DRAM to computer is indeed an import factor to understand when you are considering which camera to use.
 
These cameras are not designed to document an entire football match at 1000 fps. At high frame rates explosive action or extreme close-ups make interesting shots. A wide shot of a soccer player kicking a ball at 500 frames per second would take a minute to watch. Rather boring. The real interest is in a close-up of the boot hitting the ball, which may take place in just one tenth of a second.
 
Files
The cameras output their own custom files from camera head to computer.
Once safely stored on disk the files can be decoded and then transferred to Tiffs or JPEGs. The raw files are also available and offer the most scope for grading. Some of the cameras do not replay the shot smoothly until they have been decoded. TIt is painful to have to work through each manufacturers software, if only they could agree a standard codec and software package! There is usually a image analysis software package with the system.
 
Bodies
Camera bodies come in all shapes and sizes. There are self contained cameras, remote head cameras and camera heads that need a computer to record. All of the cameras need some type of computer to download and store the data. Many models are designed for car crash testing where they are rigged on the car crash sled. These cameras are built to withstand 100 to 150gs. Special hi G lenses are also available. As the imagers use single chips 35mm lenses can be used.
 
 
Shutter Speeds
The key to good resolution of slow motion images is good lighting and a fast enough shutter speed to freeze the action. The cameras are capable of shutter speeds of up to 1/500,000th of a second if you can find the light!
Even though the frame rate is high the subject can still be blurred!
 
For instance a bullet with a velocity of 1000 feet per second will move 1 foot in 1/1000 of a second. So if we select a shutter speed of 1 1/10,000th of a second the bullet will still move an inch or so in that time, it will still be blurred! A shutter speed of 1/50,000th of a second gets us in the ball park.
Next question is how many frames do you want to capture of the bullet? ie how long a shot? A five second shot is 125 frames. The angle of view will dictate the length of time the bullet is in shot. So if you want a shot that extends in front of the barrel by 3 feet we know that the bullet will cover this distance in 1 1/300th of a second. We need to record 125 frames in 1/300th of a second. That is a frame rate of 37500 frames per second! Ouch. Tricks for this shot include slowing the velocity of the bullet to 500 feet a second by using less of a charge, using a longer lens from front on to the barrel so you see another 3 feet of the bullets flight. Needless to say the bullets in "Matrix" were cgi.
 
I have been waiting impatiently for digital slo motion to take off for a decade. It was 1994 when NAC introduced a reasonable quality 500x400 pixel camera that ran at 500 frames per second. It wasn't quite good enough for TV.
It took a further ten years to develop a 1k x1k camera that runs at 1000 fps. It still can't compete with film in respect of colour fidelity or contrast. That is probably another decade away.
Increases in sensitivity download time and frame rate seem likely in the next 5 years as the rate of development is quickening.
Don't expect film like reproduction of skin tones or artefact free recordings The cameras are mass produced (by HD standards) to fulfil scientific test and measurement criteria. An approach was made a few years ago to one of the manufacturers to adapt their camera for broadcast TV use. $2 million dollars was available to tweak the existing camera and produce 6 TV slow motion cameras. The manufacturer wasn't interested.
We may have to wait a few more years for a better colours, in the meantime the interest for today's audience is in the extraordinary timeless quality found in the detail that surrounds us all when seen at 3000 frames per second. For some a spiritual experience that perhaps was an underlining interest for August Musger
 
Michael Brennan
Printed in High Definition Magazine 2004