Strobing propellers and wagon wheels

Edward Smith, of Napier, asked:

When I was young, at movies watching westerns, the spokes on wagon-wheels sometimes turned slow, stopped or went backwards.

This was the stroboscope effect caused by film frames being presented to our eye in rapid succession.

Now I watch aircraft flying on TV I see the same thing but now the image is digital so there should not be this stroboscope effect. So why do propellers slow or stop? John Chrisstoffels, who teaches film-making at the University of Canterbury and who filmed some segments of the Rutherford documentary, responded:

The human eye cannot respond faster than about 30ms (milliseconds) in determining between two single images presented at that speed.

In pre-cinema times, it was already a popular form of entertainment to create the illusion of movement with a series of still images presented to the viewer in quick succession (Zoetrope, Praxinoscope). We might still do this today by making a ‘‘flip book’’ by drawing a small animation in the corner of an exercise book.

The invention of modern cinema underwent a number of refinements between 1895 and 1927. As well as introducing sound and colour film, the frame rate was also increased from 18fps (frames per second) to 24fps in order to reduce the ‘‘flicker’’ which was caused by the dark interval between successive frames.

The 24fps measure is still the standard international projection frame rate used today in digital cinemas. However, television in New Zealand is 25fps, as the frame rate is tied in with the mains frequency for our electricity supply (50 Hertz), so that one frame shows for one half cycle, ie 1/25th of a second. Other countries with different mains frequencies will have different frame rates for their television.

The strobe effect in 24fps cinema is best achieved in the example of a spoked wheel making 24 revolutions per second captured by a movie camera at 24fps. The resulting film will give the appearance as if the wheel is standing still.

The digital electronic image still uses the same principles of old celluloid. The modern digital camera still shoots 24 frames per second for movies or 25 frames per second for TV. Each frame is shown for 1/24th or 1/25th of a second.

There are however some differences between capturing a film image and an electronic image. The ‘‘film’’ image is simply a shutter that opens and a ‘‘photograph’’ is taken on to a light-sensitive celluloid negative.

The ‘‘electronic’’ image is taken when light sensor behind the lens is digitally scanned. Usually each pixel is scanned from top left to bottom right in very quick succession. This can also cause a peculiar phenomena with fast moving objects like propeller blades. For example, at www.youtube.com type in /watch?v=LVwmtwZLG88.

Recently some film-makers such as Peter Jackson have been experimenting with 48 frames a second to create a crisper picture and smoother panning shots. This doesn’t eliminate the strobe problem, it only changes the formula.

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