A method of estimating the speed of a camera shutter is as follows. Attach to the side rim of a bicycle wheel a piece of tinfoil. Invert the bicycle, place it in the sunshine, and focus this wheel sharply. Put a plate in the camera ready for exposure, and set the shutter at its lowest speed, using as large a stop as possible. Revolve the wheel so that it makes one revolution per second, or fifteen revolutions in a quarter of a minute, and when this speed is obtained let go the shutter. Now make a time exposure on the same image, but on another plate with the wheel at rest. The first plate on development will show a blurred arc where the image of the bright tinfoil moved across the plate. The proportion the movement bears to the complete arc is the speed of the shutter expressed in fractions of a second. To find the degree of movement, measure on the negative showing the wheel at rest the width from side to side of the tinfoil, and subtract this from the extension of the arc. Now ascertain with the compass how many times the remainder is contained in the circumference of the wheel image and the answer is the fraction of a second exposure that the shutter gives.