Some disappointing film slides arrived in the post this week, an ugly jumble of well exposed and limp, underexposed frames.
Until recently, I’ve been shooting monochrome negative film with wide exposure latitudes, masking a suspected problem with my camera – a medium format classic, the Hasselblad 500CM and Carl Zeiss 80mm lens.
Presuming all was well, I shot a roll of color slide film, which does not take kindly to incorrect exposure.
Fortunately, I keep a basic journal detailing the shutter speed and aperture for every shot taken, which highlighted a potential timing issue with the shutter speed of 1/8 second (125 milliseconds), leading to the underexposed frames.
Before paying to have the lens serviced, I wanted to assess the scale of the issue – if only a small subset of shutter speeds were affected, I could then take note and adjust my future exposures or ignore those speeds entirely.
Using Audio Waves to Test Shutter Speeds
Having spent all my money on a Hasselblad, I didn’t feel like shelling out for a professional shutter speed testing device. So, armed with the equipment I had on hand, I decided to devise a rudimentary measuring technique to highlight any substantially erroneous speeds.
For this method, I used an audio recording to measure the shutter actuation. Then, I used a visual analysis of defined peaks along the waveform as the shutter snaps open then closes to measure the shutter speed.
The beauty of this method is its simplicity, requiring only two items; the camera and means of recording audio (Audacity in my case).
I took the offending Hasselblad 500CM and Carl Zeiss 80mm f2.8 CF T* lens, removed the A12 back, and mounted the camera onto a tripod above the built-in microphone in my laptop.
With Audacity open, and a shutter cable release attached, I was ready to begin testing the lens.
Shutter Speed Testing Method
Set up the camera with the shutter close to the microphone.
If present, the mirror lock-up feature reduces noise enabling better isolation of the shutter.
Begin recording, release the shutter, stop recording.
Expand the waveform and identify the time between the open and close peak.
Repeat the exercise for each shutter speed.
After calculating the deviation* between expected and recorded shutter speeds and plotting the results it is clear that, while my lens is not functioning correctly at speeds under a 30th of a second, it is acceptably consistent at most faster speeds.
This is exceptionally useful information, as the lens in question employs a different mechanism for shutter speeds between a 15th and 1 second – implying that the issue lies in this mechanism.
I found this method of recording shutter speed audio allows for ballpark analysis of speed deviation and consistency for shutter speeds up to 500th of a second.
I’m not suggesting that it offers the precision or accuracy of a dedicated professional tool, but as shown here, can aid in detecting issues with a shutter mechanism before sending it off for service or repair.
I sent the lens to Aperture in London for a full CLA and just received the camera back and it is now functioning properly.