Monday, March 9, 2009

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.

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.


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

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