Educational data logger

Introduction

This experiment uses an ADC-100 oscilloscope to measure the shutter speed of a camera. Light is shone through the camera, the amount of light blocked by the shutter is then detected by a photo–diode.

Equipment required

  • Torch
  • Camera
  • ADC-100
  • Battery
  • 2 MΩ resistor
  • Photo-diode Siemens SFH2030 or equivalent

Carrying out the experiment

Before we can take any measurements the circuit in figure 1 has to be built. The photo–diode has a minimum resistance when the shutter is open, this gives an output of 6 V to the ADC-100. It is important that the light falling on the photo–diode is sufficient to ensure a voltage drop of 6 V across the 2 MΩ resistor.

Circuit diagram of light sensor

Figure 1: The light sensor circuit

From figure 2, you can see we have a light source (torch) mounted at one end of a board. At the other we have the light sensor mounted on a swivel bar, enabling the rig to be changed for different camera sizes. The light sensor consists of a 35 mm film canister with the photo–diode glued into a hole and a component box with the light sensor circuit inside. Figure 2 also shows two masks, one for 35 mm cameras the other for 2.25".

figure 3

Figure 3: The basic arrangement

figure 2

Figure 2: The basic arrangement

Analysing the results

An ideal shutter would open instantly, remain fully open for the exposure time and then close immediately. In practise this ideal condition (see figure 4) is mostly closely achieved for long exposure times.

ideal shutter response

Figure 4: Slow shutter speed response

For short exposures,the time for the shutter to open fully at the beginning of an exposure and to close fully at the end of the exposures becomes a significant proportion of the total exposure time. In such cases the exposure measurement takes the shape of figure 5. The effective exposure is given by the interval between the half–way point on the rising edge and the half–way point on the falling edge. This is the time the shutter would have had to be fully open to pass the same amount of light. The ratio of the effective opening time is a measure of the shutter efficiency. Shutters should have an efficiency of at least 60% at full aperture.

figure 5

Figure 5: Fast shutter speed response

The tolerance in effective exposure time for the diaphragm shutters is ±20% for exposures longer than 1/125 and ±30% for exposure times shorter than 1/125. A focal plane shutter should be accurate to within ±25% for exposure times longer than 1/250 and accurate to within ±33% for exposure times shorter than 1/250. In practise the absolute accuracy of a shutter speed is not important as its consistency. If a shutter is consistently slow, allowance can be made for this, but not if the shutter speed varies from one exposure to the next.

A test of shutter speeds can also detect shutter bounce, where the shutter opens again slightly after completing the exposure.