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DIAGRAM showing stroboscopic effects of wheels revolving before motion picture camera oper¬ ating at 24 fps.: Fig. 1, spokes stationary. Fig. 2, spokes moving backwards. Fig. 3, spokes doubled in number.
spoke (Fig. 2), the wheel will appear to move backwards, because the eye would sooner believe that one spoke has moved a short distance backwards than that the previous spoke has moved nearly the distance to the next spoke.
If in the l/24th second the wheel has moved exactly half the distance from one spoke to the next (Fig. 3), the wheel will appear to have double the number of spokes, which will be sta¬ tionary. At any speed less than this, the illusion will be correct.
Mathematically, these facts can be ex¬ pressed by simple equations, in which V is the speed of the wheel in revolu¬ tions per second, and n is the number of spokes. Spokes will appear stationary when
24
V = —
Wheels That Still Turn Backward
At any speed slightly less than this, the spokes will appear to move back¬ wards. The spokes will appear to be double in number when:
12
V — —
Avoiding the trouble is simply a matter
The requirement for a correct illu¬ sion is:
of mathematics or use of a shutter-reflex type of camera, says British technician.
By R. HOWARD CRICKS, F. B.K.S., F. R. P. S.
IN all TYPES of films, from features to to newsreels, one still sees the ele¬ mentary fault of wheels standing still or going backwards. Several particularly flagrant examples occurred in “The Card*' (British production), where the effect was aggravated by the fact of Alec Guinness’s quaint vehicle appear¬ ing in full side view. Even “The Great¬ est Show on Earth,” with all its won¬ derful special effects, was not free from the fault, which in most war films is displayed also on the tracks of tanks.
When an aeroplane propeller is swung it passes through the phases of appear¬ ing stationary, and then of going alter¬ nately backwards and forwards, until it settles down to a blur.
In actuality films, the fault is gen¬ erally unavoidable; but in all staged productions its avoidance is merely a matter of a little forethought, plus either some elementary mathematics or a shutter-reflex type of camera.
The fundamental cause of these stro¬ boscopic effects is quite simple. If in the l/24th second from one exposure to the next a wheel turns the exact
distance from one spoke to the next (Fig. 1), then the spokes will appear stationary. If in that time it turns slightly less than the space of one
12
V <
Tl
In theory, stroboscopic faults can also occur at higher wheel speeds, but move¬ ment blur usually makes such faults unnoticeable.
As an example, if a 12-spoked wheel turns at exactly two rps, the spokes will appear stationary; if it turns at 1^ rPs they will appear to move backwards ; if (Continued, on Page 409)
The above article is reprinted from “British Kinematography” magazine, London, England.
WHERE spoke-wheeled vehicles are photographed at an angle, as in this scene from "Quo Vadis," the stroboscopic effect, although present, will be greatly minimized on the screen.
390
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American Cinematographer
September, 1952