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Old 05-08-2013, 07:31 AM   #11
Chas Tennis
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Join Date: Feb 2011
Location: Baltimore, MD
Posts: 3,204
Default Video Considerations for the Serve

The impact parts of the serve are too fast for standard frame rate video cameras. At 30 fps, a frame is taken every 33 milliseconds (0.033 second); at 60 fps, a frame is taken every 17 milliseconds (0.017 second).

This high speed video at 420 fps shows all that happens in the 20 milliseconds before impact.
https://vimeo.com/65434652

The racket goes from the edge on to the ball to ball impact in about 20 milliseconds. Likely, at 30 fps for this 20 milliseconds you would get only one or no frames and with 60 fps one or two frames. The very rapid internal shoulder rotation can be seen by the rotation of the elbow bones & upper arm - more apparent later in the video, after ball impact.

This video & its thumnail from behind show the critical angle between the arm and racket at impact. This rapidly changing angle is visible for only a few milliseconds around impact as the racket is moving very fast.

https://vimeo.com/27528701



From the side view, most people looking at serve videos don't notice that the racket is at an angle to the arm at impact.

You don't have a high speed video camera? Check, because a lot of cameras have high speed video modes such as the Canon Powershot cameras.

Some compromise tricks to get the most information from your video camera:

1) Always shoot in direct sunlight (sharp shadows are cast) because the light levels are roughly 100X those of indoor tennis courts. Your video camera most probably uses automatic exposure control and it will usually adjust exposure by using faster shutter speeds for high light levels. Result - the video will have much less motion blur and you might be able to accurately see where the arm, racket, ball, etc are located. Most smartphones use very fast shutter speeds in direct sunlight so that the images of fast moving objects are very sharp.

2) Some new DSLR's have manual exposure control or shutter priority (auto) shutter speed adjustments for video. Some offer very fast shutter speeds such as 1/4000 second. These cameras might also have 60 fps.

3) View from high & directly behind the server so that the camera views along the ball's trajectory. That is both an informative view and since the racket is moving away from the camera at impact the motion blur will be greatly reduced at the time of impact- lower motion blur.

4) For 30 or 60 fps cameras, repeat the same serve until you catch one where you can see ball impact as shown in the above video thumbnail.

5) Consider any Jello Effect distortions produced in video cameras of very rapidly moving objects, especially so with high resolution HD smartphone cameras.

You can get a lot of information, mostly for the slower body parts, from your current videos. But the motion blur and low frame rate will always obscure the fastest parts of the serve, especially those in the last 0.05 second leading to ball impact. It could be very misleading to develop your serve based on videos that can't show what is happening at impact.

Last edited by Chas Tennis : 05-12-2013 at 11:58 PM.
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