From the " Wellcome Exposure Record and Diary." The formula and table given below indicate the shutter speeds necessary to secure negatives sufficiently sharp for direct printing.

For enlarging it is better to give 1/2 to 1/3 these exposures, or to work further from the object. The figures are no guide to what is the correct exposure for the plate.

If D = distance of object in feet, F = focal length of lens, S = speed of object in feet per second, and E = exposure for an object moving across the field of view, then

Shutter Speeds For Moving Objects 83

The following table gives in round figures the shutter speeds necessary for various moving objects, using the ordinary quarter-plate lens of about 5 in. focus. The column A is for objects moving directly towards the operator, B for objects moving obliquely towards or from the camera, that marked C for objects moving directly across the field of view.

Distance of Object, 25 ft., unless otherwise stated.

A.

B.

C.

Street groups (no rapid motion)

1/5

to

1/10

Pedestrians (two miles per hour) . . } Animals grazing ................ }

1/20

1/40

1/60

Pedestrians (three miles per hour)

1/30

1/60

1/90

Pedestrians (four miles per hour)

1/40

1/80

1/120

Vehicles (six miles per hour)........

1/60

1/120

1/180

Vehicles (eight miles per hour)......

1/80

1/150

1/250

Cyclists and trotting horses ........

1/160

1/300

1/500

Foot races and sports ............

1/240

1/500

1/700

Divers ........................

-

1/600

1/800

Cycle races, horse galloping ......

1/300

1/750

1/900

Yachts (10 knots per hour) at 50 ft.

1/60

1/120

1/180

Steamers (20 knots per hour) at 50 ft.. .

1/120

1/240

1/360

Trains (30 miles per hour) at 50 ft.

1/150

1/300

1/450

Trains (60 miles per hour) at 50 ft.

1/300

1/600

1/900

At 50 ft. the exposure may be double that at 25 ft.

At 100 ft. the exposure may be double that at 50 ft.

For lenses of greater or less focal length than 5 inches multiply above figures by 5 and divide by the focal length of the lens in inches. Thus for a 6-inch lens 1/30 X 5/6 = 1/36