-  The Camera  -

A Practical Magazine for Photographers.

February 1908

The Camera Publishing Co.




The above cut represents a new and very ingenious device for testing the speed of shutters. The machine was designed by Mr. Fred Schmid, of the C. P. Goerz American Optical Company, and is in use at their New York factory. The essential parts of the device are: a hollow metal drum (D), per­forated spirally by a series of 200 holes making exactly one complete revolution around the drum;-a front board (FB) with a narrow slit (S) in it, equal in length to the distance from the first to the last hole of the spiral perforations and placed exactly at the same height as the axis of the drum; a mercury vapor lamp (L) ; a small electric motor (M) and such mechanical appliances as are neces­sary for revolving the metal drum at various required speeds.

The use of the apparatus is as follows: The lighted tube, supported horizontally, is placed inside the drum, the drum is then revolved at a given speed which is ascertained by a strik­ing bell mechanism (SB), giving one stroke for every five revolutions of the drum and thus permitting an accurate determination of the speed of rotation. The speeds, in practice, vary from three to eight revolutions per second and as the drum revolves each perfor­ation therein appears in turn before the slot in the front board. To a spectator who is watching the operation the slit appears as a continuous row of light-dots blending finally into one line of light.

The shutter to be tested is attached to a camera, placed in front of the apparatus, focused on the slit, and with the drum revolv­ing at a known speed and the shutter set for a given speed, an exposure is made. The photographic image which appears on the negative is a succession of dots of which the single white dot appearing on the slit in the reproduction is an example. The single and double dots appearing in the reproduction are openings in the front board above the slit and facilitate the counting of the dots by marking groups of five and ten in the negative.

In making the exposure a special plate holder is used which permits of the same plate being used for a complete test of all speeds of the shutter. This is brought about by means of a device by which the plate is raised or lowered in the plate holder.

The length of exposure is figured out from the negative in the following manner: Know­ing the speeds at which the shutter was set and the time value of each dot, derived from the speed of revolution of the drum, multi­plying the value per dot by the number of dots gives the actual length of exposure. If the value of each dot is determined at 1/1000 of a second and the shutter was set for 1/100 of a second ten dots would indicate the ex­posure to be 1/100 and the shutter to work accurately. However under same conditions should 20 dots appear in the negative the actual exposure world be 1/50th of a second and show the shutter working slower than marked.

An important feature in regard to this device is that it is equally well adapted for testing focal plane shutters as between the lens shutters. This is due to the fact that the record is produced on a straight horizontal line and will thus be correctly exposed by the slit in the focal plane shutter.

The device was designed in order to in­sure the accuracy of the XL Sector shutter manufactured by this company and it is by virtue of many tests made with this apparatus that they make the claim for their shutter that "you can rely on it perfectly."

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