It finally happened...
Some disappointing 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 & Carl Zeiss 80mm lens. Presuming all was well, I shot a roll of colour 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 8 (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.
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 to hand, I decided to devise a rudimentary measuring technique to highlight any substantially erroneous speeds. The beauty of this method is its simplicity, requiring only two items; the camera and means of recording audio (Audacity in my case). I also decided to use a tripod and shutter release cable to better position the lens over my laptop. Essentially, the audio recording facilitates measuring of the shutter actuation, through visual analysis of defined peaks along the waveform as the shutter snaps open then closes.
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.
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.
*(Expected Speed - Measured Speed) ÷ ((Expected Speed + Measured Speed) ÷ 2)
Deviation within 1/3 of a stop (33%) is typically considered acceptable
To reduce the effect of outliers & observe shutter consistency I recorded 7 iterations of each shutter speed – yes, 70 individual recordings…
For cameras lacking mirror lock up, the waveform will contain extra peaks as the mirror moves
Measurement becomes more challenging as speed increases, as the waveform is compressed