Frequently Asked Questions

I've noticed that a few of my movie frames are exactly the same as the immediately preceding frame. What should I do about this?

In Windows (and perhaps in other operating systems), if the video capture software can not keep up with the incoming video data stream, it inserts a duplicate of the previous frame rather than leave a "hole" in the movie. If you see this in a movie you are analyzing, it's best to just skip that frame and move on to the next frame in which you see that the object you are observing has really moved.

I think my vision is going! In many of my movies of rapidly moving objects I see two images of the moving object. Do I need new glasses?

Probably not! Much of the video we watch is "interlaced." This means that each full image ("frame") is made up of two exposures ("fields") separated in time by 1/60 second. These two exposures are "interlaced" so that together they look like a double-exposed photograph. You can fix this in a video editor (such as QuickTime Pro) by choosing "single field" and then "exporting" the resulting movie as a new file.

I just made a movie with two meter-sticks in it, one horizontal and the other vertical. I noticed that I get a different number of pixels-per-meter scale calibration depending on which one I use. Why?

With older video cameras, as well as with webcams and movies from still cameras, a single calibration using an object of known length in any orientation in the plane of the motion gave correct lengths for both horizontal and vertical motion.

Unfortunately, Digital Video (DV) cameras produce images on your computer that are stretched horizontally by about 12%. The images they produce are 480 pixels high by 720 pixels wide. On a computer screen, these images render a circle as an ellipse that is about 12% wider than it is high. To fix this problem, you need to compress (not crop!) the original movie from 480x720 to about 480x640 (or sometimes a slightly different value such as 480x660). You can do this in a video editor (such as QuickTime Pro) by resizing the movie either by dragging the movie-frame handle to the desired size or by typing in the new size when you "export" your edited movie as a new file. For critical data you will need to calibrate your camera by photographing two such meter sticks to determine whether you need to rescale to 640, 660, or some slightly different value.

For many pedagogical exercises you can (and should!) just ignore this correction.

I notice that you plug a number of commercial products on this website. Do you get paid to do this?

Sony, Canon, Apple, and Redlake don't send me checks! But I am a co-author of VideoPoint and I do software testing for Lenox and for Vernier. I describe how to use these products because they are the ones I am most familiar with and because I believe they are the best available for teaching physics to our students. As other similar products become widely available for commonly used lab computer systems, I'd be glad to add them to this website.