Sports Analysis: The Camera Specs

In the last blog Video Analysis of Sports 101 we said that we would discuss what makes a good camera for video sports analysis, so here it is.

If you already have a digital video camera there is no need to go out a buy a new one simply based on our recommendations here. If you follow the setup rules we discussed in Video Analysis of Sports 101 you will be able to obtain great footage that you can analyze.
However, if it's time for you to pick up a new DV camera, then getting the right camera can really help you obtain excellent sports analysis footage and make your video analysis clearer.

Speed and Size
Like most sports, a good sports analysis camera needs speed and size. There are a few different speed and size measures of a camera and they are discussed below.

Frame Speed
Most off the shelf DV cameras have a standard frame speed of 30 frames per second or fps(NTSC) in the North America and 25 fps (PAL) in most of the rest of world. What this means is that the camera records 30 pictures every second, one picture every 0.03333 seconds. 30 frames per second is the speed at which most regular television is recorded and viewed. When these 30 frames are played back at full speed every second, our brain is unable to distinguish between the individual pictures and we see a flowing video.
However, if we want to analyze a tennis serve then we need to slow down our video and look at each one of these 30 frames of every second and this is where a problem may arise. As the serve is generally a quick motion, we will probably find that if we only record 30 fps then the arm will look like a blur during the serve motion and we will be unable to analyze it properly. To solve this problem we can record the motion at a higher frame rate, say 60 fps and get double the amount of pictures every second. This will give us more data to analyze and reduce the possibility that the motion will be blurred.
In general the more frames we can capture per second the better our analysis can be. Luckily though there is a limit. In most sports when analyzing the human body movement 250 fps is sufficient. If we then want to include say the golf club or tennis racket and analyze its movement, we once again need higher speeds maybe up to 500 fps.

Many sports TV programs now use high speed cameras to analyze sports. Here is some recorded high fps video. A lower fps camera would often show blurring as we watched each frame like this.


Unfortunately though there are very few off the shelf commercial cameras that can record high frame rates. You can find some cameras that can record 60 fps, but if you want to go higher, costs can increase substantially. We think a 60fps camera is a perfect start for doing your own analysis.

Shutter Speed
We can get away with the usual 30 fps as long as we have a high shutter speed. Shutter speed determines exposure time, or how long the cameras sensor is open to receive light and therefore record the frame. In sports analysis the higher the shutter speed the less blurred our image will be. However there is a trade off - a faster shutter speed will allow less light onto the cameras sensor and therefore if we use a high shutter speed we need to make sure that we have sufficient lighting as we discussed in the last blog. When looking at a camera for sports analysis, look at the shutter speed and whether it is manually adjustable. A camera that automatically adjusts its shutter speed to the lighting conditions can make you forget the importance of ensuring good lighting on your subject.

Resolution
We all know about resolution from high def televisions. High def television can only display those high definition images if they were recorded by a high definition video camera. The resolution of a video camera is a measure of how many pixels our camera sensor has. Luckily for us most video camera manufacturers are now making HD cameras with about 1920x1080 pixels or 2.3 Mega pixels. Of course a higher resolution is always better just like it is on our TVs.



Lens Choice
There are 2 important aspects of a lens for sports performance capture, aperture (f-stop) and focal length and optical zoom.

Aperture

The aperture or f-stop is a measure of the amount of light the lens of the camera will allow through. A lower number will allow more light into the camera and a higher number less light. Most DV cameras have f-stops as low as 2.5 which is good enough for sports analysis purposes. Once again just like with shutter speed there is a trade off. A lower f-stop can often result in a lower resolution image. This does not mean that the cameras sensor has less resolution but that there is a trade off with lighting and quality. Once again this points to making sure you have a good light source for your recordings. With a good light source you can use a lower f-stop and therefore obtain a better resolution image.

Focal Length
The focal length is the distance from the lens to the camera's sensor. A smaller focal length means that the camera will capture a wider angle of view and see more in the frame, while a large focal length allows the camera to see more details further away. Focal length and optical zoom therefore are linked. Optical zoom of course allows us to zoom in or away from an athlete.

Most DV cameras come equipped with optical zoom and will read something like 49-735mm, which is the range of focal lengths that can be achieved with this camera.

The more optical zoom we have the further away we can be while still recording excellent footage for analysis. Now its up to us to use that optical zoom effectively by positioning our athlete correctly in the field of view. Check out Video Analysis of Sports 101 for details.

Also don't get tricked by digital zoom. Digital zoom is not our friend and if used can reduce our actual sensor resolution considerably. It is far better to reposition our camera closer to the athlete than make use of digital zoom.


We hope these tips help if you are looking to choose a new DV camera for sports analysis. Remember though, any camera can work; but if you want more detail you need to ensure that you recorded the detail with your camera. You cannot analyze details you do not have.

We welcome any comments or suggestions on our blog. Have a great week.

Dudley Tabakin

Video Analysis of Sports 101

Welcome to the Video Analysis of Sports blog. As this is our first article, we will take this opportunity to introduce ourselves.

We (Dennis and Dudley) have a combined 20 years experience in motion capture and biomechanics with a special interest in sports performance and injury prevention. Through our consulting company Sadaka, LLC http://www.sadakallc.com ,we offer services to sports professionals, sports equipment manufacturers, hospitals and motion analysis software and hardware companies. Some of our past clients include Titleist, FootJoy, Warrior Hockey, National Pitching Association, NASA, Shriners Childrens Hospital and Vicon Motion Systems.

Video analysis of sports performance is often the domain of professional athletes and large companies or research institutions. This analysis is done with expensive 3D video camera equipment and complicated software to ensure high speeds and accuracy. However there are some basic techniques that can be used by any individual to evaluate their own performance and get great results using a simple digital video camera and a little understanding of biomechanics. The video camera will require a small cost (we will discuss what makes a good camera in a future posting), but we will supply you with much of the biomechanics understanding for free on this blog.

Reviewing video clips can be a great way to provide coaching feedback to players, as well as a means to emulate one's favorite athletes. Whether you're a professional all-star, high school athlete, or weekend warrior, seeing yourself from a different perspective can help improve your performance and/or minimize your chances of injury.

So in this first posting, we'll start right at the beginning and describe what features make a sports clip suitable for video analysis of an individual, and what you can do to ensure the clips you film will meet those requirements. Future postings will demonstrate the various levels of analysis that can be performed on suitable videos, using a range of available software, from basic free software to detailed 3D analysis.

What makes a good sports clip?
A good sports clip of an individual shows the athlete's full range of motion (ROM) during an action, with the camera positioned such that angles can be accurately calculated (this will be discussed in more detail later). Filming the athlete's ROM can be accomplished by simply zooming out or moving the camera backwards until all the desired features are in the viewer. In general, it's recommended to have the camera as close to the athlete as possible and still capture the entire ROM, as this will make the most use of the camera sensor's resolution. For example, if we were to capture a basketball player dunking the ball we would want to ensure that our camera can see the complete action from take off until the player puts the ball in the net. Our camera would therefore need to see the ground at the take off point as well as some space above the rim of the the net.

Another important factor in obtaining a great sports clip is to make sure that your camera is stationary. There are some motion capture software available that can handle a moving camera, but for now we suggest that the camera is mounted on a tripod and is not moved while the video clip is being filmed. This means that you should ensure the full ROM (described above) can be captured in this one shot.

How you can film useful footage:
Camera placement
In general, most sports clips we watch on tv are best suited for analyzing strategy and teamwork because they provide an overall picture of many or all of the competitors and the playing field. However, when it comes to analyzing an individual's performance or technique, it's not only important to focus the camera on the single athlete, but the angle we film from must also be taken into account. It's best to use "orthogonal angles", meaning the camera is directly in front of, behind, or to the side of the athlete. Golf coverage usually does a nice job of this, providing shots of the golfer either "face-on" or "down-the-line". Here's a great face-on view of Tiger Woods' swing:



And here is a down-the-line shot of his swing:



Notice these cameras are level with the golfer; they're not filmed at an upward or downward angle, meaning it's possible to accurately calculate some angles, if that's what is desired from the analysis.

Background/contrast
Although the above clip is a beautiful view of Tiger's swing, the footage could have been improved by having him wear clothes that were a different color from the background. Black clothing with a black background can make it difficult to accurately evaluate the amount of hip and shoulder rotation - two commonly analyzed characteristics of a golf swing. Not that having a solid black background is a bad thing though; on the contrary, if you can manage to film your athlete with a solid-colored background, you are well on your way to filming some nice footage for analysis.

Lighting
As is true with taking most photos, it's ideal to position the camera and athlete such that the light source (e.g., the sun) is behind the camera and illuminates the athlete. This will provide a crisp view of the athlete, and will allow you to use a more open setting on the camera's aperture.

Now that you're up to speed on the basics of filming great video for analysis, grab your video camera and start practicing! Follow our regular blog postings, and see how you can create your own analyses of your videos, using a variety of software packages.

-Dennis Ho