Shutter Speed Tester

Background

I wanted a cheap tester for checking the speeds of my large format shutters. A quick search of the Internet turned up the approach of using a home-made circuit plugged into the audio card of a computer and using freely available software for the timing. A neat approach and if I find the site again, I'll give the author the credit! I've simply adapted the components to suit what I could obtain. Total cost of components, including a box, was less than £10 ($15). You will also need a torch and a computer. This tester can also be used for roll film and 35mm cameras where the back can be opened and the shutter tripped (not all cameras allow the shutter to operate when the back is open, so check your model). I've tried to test the shutter speeds of digital SLRs using reflected light off the sensor, but so little light is reflected that the basic circuit needs adapting with an amplifier to make it work with a sound card setup. Since I don't really want to check digital camera shutters, I have not persued this adaptation.

Principles

By changing light shining through a shutter into and electrical impulse suitable for a computer, we can measure the time between the opening and closing of the shutter in software. Large format shutters have speeds between 1/500th second and 1 second, and this circuit will easily cover this range. I've also used it to check nominal speeds up to 1/2000th second. You will need some experience at building basic electronic circuits - this page is not a step-by-step how-to guide.

Circuit description

Test setup with torch, camera/lens and circuit

A torch is shone at the shutter on test (there is no need to remove the lens elements) with the phototransistor positioned on the other side. Both torch and phototransistor are approximately on the optical axis of the lens. When the shutter is tripped the light causes the phototransistor to conduct, the LED turns on (the LED is present as a useful indicator to the operator, especially during alignment of the torch/lens/sensor) and the capacitor charges, which is seen as a change of voltage to the computer's sound card. When the shutter closes, the capacitor discharges, which is seen as another change of voltage at the sound card.

The component values are not critical, but the values shown keep the current drawn from the battery to a minimum (typically 230µA when the shutter is open, 0.9µA when dark). I originally chose a battery voltage of 3V in order to drive a low-powered laser module but I found that a laser was unnecessary and added to the cost of the circuit. A torch is convenient and more tolerant of misalign than a laser.

I built the circuit into a small box for convenience, but if you really want to reduce the cost, a simple breadboard circuit will do.

Circuit diagram

circuit diagram of shutter speed tester

Component list

All components were purchased from Maplin Electronics

Software

Since we are using the microphone input we need audio software that will display and measure the waveform between a pair of cursors. If you already have audio software, try using it. I use the excellent open source software Audacity.

screenshot of Audacity

Example screen shot of waveform in Audacity. The important time is shown in tho bottom display bar, in this case (0.00.010468 min sec), which is 1/0.010468 i.e.a 95th of a second (the shutter was actully set to its highest speed of 1/200th second, so the shutter is giving a one stop over exposure at this setting!)

Operation

  1. Using a room with low ambient light level, align the torch, lens and phototransistor.
  2. Switch on the circuit, set a shutter speed of about 1/4s and trip the shutter - check that the LED lights when the shutter is open.
  3. Plug the jack plug into the microphone input of your computer and start Audacity.
  4. In Audacity, click the Record button.
  5. Trip the shutter 5 times in quick succession.
  6. In Audacity:
    1. click the Stop button
    2. scroll back through the recorded waveform to the first pair of peaks, corresponding to the opening and closing of the shutter
    3. align the cursor with the start of the opening peak and drag to the start of the closing peak
    4. record the duration in seconds from the bottom of the cursor window
    5. repeat for each pair of peaks

Results

We are looking for three things in a shutter:

  1. accuracy
  2. repeatability
  3. lack of bounce

Accuracy

A well made and maintained shutter should be within ±10% of the marked times in the centre part of its range. The highest couple of speeds and the lowest speed are often less accurate, with the highest speed being 50% or more slower than marked. Older or well-used shutters will be less accurate throughout their range, with individual speeds varying by 25% to 50%. ±25% may well be acceptable, but a 50% error represents 1/2 stop, which may be cause for concern. But now that you know about it, you can compensate when setting your exposure.

Repeatability

But just as important is the variation of times shown for each speed, which is why I suggest timing at least 5 clicks of the shutter. If your shutter's 1/2s marking actually gives 3/4s it doesn't matter so much: once you know this you can compensate by setting a smaller aperture. What matters is that it consistently gives 3/4s. If there is too much variation, you won't know what exposure you are giving your film. If repeatability is worse than ±20% (a variation of over 1/3 of a stop) in the speeds that matter to your photography, you should seriously think about having the shutter serviced since repeatability is a more serious issue than known inaccuracy.

Lack of bounce

Shutter bounce is a small secondary exposure occuring after the main picture is taken and usually occurs at high shutter speeds. In focal plane shutters you will see an irritating pale stripe along one edge of the picture's frame, and you will need to get the shutter service to get rid of this bounce. In leaf shutters you will only notice the effect if you are photographing moving objects, where the secondary exposure may cause a ghost image.

It is quite common for leaf shutters to bounce at their highest speed, and this circuit will detect the bounce as a second pair of peaks shortly after the main exposure. This is caused by the shutter blades re-opening briefly as the last of the shutter spring's energy is released. [Note that this bounce may not be recorded if you use a slightly misaligned laser instead of a torch as a light source]. Strictly speaking, the bounce time should be added to the main opening time to get a total time for the shutter. In practice, if the bounce time is the equivalent of 5 stops less than the main exposure, which equates to about 1/30th of the main time, no ghost image will be recorded on the film. If the bounce time is excessive, say 1/8th or 1/4 of the main time, which is only 3 or 2 stops down from the normal exposure, this will make a significant difference and could leave a ghost image. Since most people avoid using the highest shutter speed, this may not be a problem for you, and in any case, a service may not help. What's most important is that you know about it!

shutter bounce

Screen shot showing shutter bounce. In this example the bounce (in darker grey) is approximately 25% of the duration of the main exposure,though the shutter does not fully open for this time. This amount of bounce is likely to produce a slight ghosting on rapidly-moving objects.