joan wrote:Chas Tennis wrote:(The video of a straight line rotating with a known rate can be used to measure the motion blur and calculate the camera's exposure time.)
Yes, I thought about this a bit yesterday and I think we don't even need to convert back to linear speed.
With a filled sector of a known angle on a rotating plate and measuring the apparent angle of the sector during video, we can directly infer the shutter speed. I came up with the following:
shutter speed = (measured angle / known angle) × angular speed
- shutter speed: in s-¹
- measured angle: in °. Angle measured on video frame.
- known angle: in °. Actual angle measured physically.
- angular speed: in °/s.
The advantage of this is that it's independent of the distance of the rotating plate to the camera, and doesn't involve any calibration of the space.
(We can even instead divide the angular speed by the "blur factor" to get the denominator of the more familiar "1/x of a second" notation).
I'll try to set up the experiment tomorrow to see if I can deduce the shutter speed of some cameras.
The same setup under various illuminations would help understand if a given camera has several shutter speed levels during video, if it adjust itself during a single shoot, how often does this adjustment occur, etc.
Have not considered an angular technique.
This still picture illustrates motion blur and how it increases with radius out from the center of rotation with linear velocity. There is no motion blur at the center.
http://www.dpreview.com/galleries/53108 … 00-8-9-rps
"To test the Jello Effect for a Casio FH100 the camera viewed a straight bar on a rotating disc, 8-9 RPS. Single frame. 10 Megapixels. The mechanical shutter has removed Jello Effect distortion. Ordinary motion blur increases out from the center of the disc. Compare to the electronic shutter images with distortion."
This still picture was taken with a mechanical shutter in the Casio FH100 at exposure time of 1/640 sec. [DSLR cameras also use mechanical shutters for most photos. Mechanical shutters turn out to be faster than the electronic 'rolling shutters' of CMOS cameras and might reduce distortions when compared to the electronic Jello Effect distortions.]
Added 1/23/2013 - The mechanical shutter in the Casio FH100 for stills is believed to be a leaf shutter. A leaf shutter, while it is open, can expose all areas of the sensor silmultaneously. The bar on the rotating disc above is not distorted, it's straight, with a leaf shutter. In contrast, focal plane shutters as used in most DSLR cameras expose different areas of the sensor at different times and will result in some distortion, for example, f a swinging golf club may be bent. Shutter- http://en.wikipedia.org/wiki/Shutter_%28photography%29
For illumination levels, include direct sunlight with no cloud blocking the sun.
For a rotating disc I used a sanding disc in a drill. A disc is also safer than a straight rigid object. My ceiling fan rotates at 2 rps. You can get the rps from the camera if the frame rate is fast enough (frame rate > rotation rate).
An easy-to-find rotating object would be a bicycle, use upside down and view the spokes - calibrate rps from the video if fast enough.
ADDED 12/11/2012 - Bicycle Test for Shutter Speed vs Motion Blur. Also can show Jello Effect distortions. For higher speed rotate the rear wheel using the pedals.
http://www.youtube.com/watch?v=lItCq5Gp6vw
Maybe the rotating disc should be white with a black bar so that the camera's AUTO exposure has more light and will select a faster shutter?
(Your post - Shutter speed units are seconds.)
