The Higher Speeds.—A focal-plane shutter usually includes beyond the sec. And a few calculations will show that in purely instantaneous work within a few feet of the plate these high speeds are frequently called into action. A man walking only three miles an hour moves 41ft. per second. During that second his image will have travelled across the focussing screen if he is within, say, eight feet of the lens. A trotting horse is estimated to move through 39 ft., and a galloping horse about 5o ft. Express trains (not to mention birds, whose flight is at a speed treble that of the locomotive) soon exhaust the capabilities of a shutter at -atrtr sec.
In photographing rapid motion at athletic sports, races, and the like, the subject is, if possible, taken when approach ing in the direction of the camera, and not crossing the field of view at right angles. The range of pictures at a com paratively low speed may be thus considerably increased—an important matter when the weather is dull and the light in consequence rather weak.
If the speed of a shutter to take an object moving at a given rate has to be estimated, the distance of the object must be divided by too times the focus of the lens, and then the rapidity of motion of object must be divided by the result. Or if R be rate of motion of the object, D the distance and F the focus of the lens, then, in order that the circle of confusion may not exceed Th inch, S ooF x The subjoined table will give a few suggested speeds for the shutter, gathered from our own experience or from reliable sources. These speeds have no reference whatever to the light conditions prevailing at the time of exposure. In the A column is given the figure for objects in motion parallel to the direction of the camera ; in B, objects crossing the field of view at right angles.
A. B.
sec. sec.
Street scenes (no apid motion) . . . .
„ „ cart traffic .....
Animals grazing, birds feeding . . . . to a Man walking three miles an hour . . . 41-6 1.15 Children building castles, paddling, etc. . . to Coaches, six miles an hour . . . . . rya ten „ >,. . . . . Cyclists (not scorching) . . . . . th Galloping horses . • • • sh nata Large birds flying, ducks, gulls, etc. . . . Athletic sports . .....
Small birds flying raw These speeds are calculated for a point about so times the focal length in distance from the lens. For nearer points
the speed of shutter must be increased in proportion. For instance, a child skipping at, say, 25 ft. from the lens would probably give a sharp image at sec. ; at 8 ft. from the lens the highest speed of the shutter would perchance be vainly called into operation.
With many subjects there is a critical instant when a successful snapshot may be taken at a comparatively low speed and yet convey the most vivid impression. For in stance during a high jump, and still better in skipping, motion is actually less just when the athlete is attaining the full height of his leap, and before he has commenced his descent. We have seen a capital picture of a batsman's middle stump falling, which was taken at sec.; the ball is not very sharp, but could easily be touched up, and is, after all, a minor consideration.
Arrested Illotion.—In the hands of a master, like Mr. Adolphe Abrahams, and particularly in the portrayal of incidents occurring at athletic sports, rowing contests, and yacht races, the records of the instantaneous shutter some times rise to the dignity of true pictures. More often they are merely triumphs of a highly perfected instrument. We are shown snap-shots of an express train travelling at sixty or seventy miles an hour ; the spokes of the wheels are all sharply defined ; there is no sign of motion whatever. We might be gazing at a stationary locomotive engine discharg ing smoke in a high wind. Leaping horses look as if they were paralysed and hanging in the air by invisible wires ; the winning eight have apparently posed themselves rather awkwardly after splashing the water somewhat. Portraits of ladies skipping are exceedingly uncomplimentary ; their mouths are generally wide open and their whole aspect displays little of the grace of motion. We sigh for the old fashioned sporting print. For this is a field in which the camera must fail in nine cases out of ten. The successive action of the muscles as each is called into play cannot be recorded on a single instantaneous plate ; and we must be content to leave the artist to express motion conventionally, or resort to the continuous record on the cinematograph.