Two month’s ago (!), inspired by this posting (
Shutter speed tester - PentaxForums.com) about measuring shutter speeds for a film camera, I put together a simple system to make such measurements, using my Pentax ME Super A as the device under test (DUT in electronics jargon).
(Aside - the photoplug tester used by forum member Vendee is almost certainly using an integrating detector with a modest time constant - I could duplicate those swoopy light curves when I used my photodetector in an integrator circuit.)
Here’s a shot of the setup: I configured a cheap green laser pointer module from Ebay to shoot through the ME Super with its film back open and on to a high-speed photodiode light detector. The photodiode response (scope trace goes high when light shines on the diode) was displayed on a storage oscilloscope, from which I could make screen dump copies of the displayed light curves. The overall response time of the system is at the microsecond (millionth of second) level.
The scope also has a pair of cursors that can be placed over the time response trace. I set those at the half-power levels at the start and end of the light pulse that came through the shutter and read directly the time interval. This interval is how long the shutter is open. Typically, I made 2 or 3 measurements of the shutter time. The values given here are the average of those measurements.
For longer durations, more than a 1/30 second or so, the rise and fall of the light curve is very sharp at the time resolution of the scope when showing the entire open trace. For shorter shutter speeds, the rise/fall times are resolved, and at the very shortest times (1/1000 and 1/2000 seconds), the shutter is barely “open” for the given duration of the exposure.
Here are the results in tabular form, with the nominal shutter speed (i.e. the speed shown in the camera display - the ME Super shows the shutter speed in the viewfinder display), the nominal speed given in milliseconds, the measured shutter speed in milliseconds, and the ratio between them, in the sense of measured divided by nominal.
Nominal | Nominal | Measured | Ratio | |
(sec) | (millisec) | (millisec) | | |
1/2000 | 0.50 | 0.44 | 0.88 | |
1/1000 | 1 | 0.87 | 0.87 | |
1/500 | 2 | 1.82 | 0.91 | |
1/250 | 4 | 3.82 | 0.96 | |
1/125 | 8 | 9.80 | 1.22 | |
1/60 | 16.7 | 15.4 | 0.92 | |
1/30 | 33.3 | 30.6 | 0.92 | |
1/15 | 66.7 | 60 | 0.90 | |
1/8 | 125 | 119 | 0.95 | |
1/4 | 250 | 239 | 0.96 | |
1/2 | 500 | 475 | 0.95 | |
1 | 1000 | 961 | 0.96 | |
2 | 2000 | 1916 | 0.96 | |
4 | 4000 | 3820 | 0.96 | |
And here are a few of the scope screen dumps.
For a quarter of a second (the vertical purple lines are the time measurement cursors) the edges are nice and sharp:
Originally, I wondered why there were those wiggles at the beginning of the trace (they were on most of the traces). Then, I realized that the camera was sitting on the board to which the rest of the system was mounted. The shutter slap was bouncing things around, in particular the detector diode with respect to the laser beam. When I picked the camera up off the board before I fired it, things were much better:
At short shutter speeds, the traces are not quite so distinct. Here’s what you see for 1/500 of a second. The open and close parts of the trace take place over a few hundred microseconds (the time divisions in this image are 200 microseconds):
And, at 1/2000 second, all you get is a bump, with about 400 microsecond rise and fall times:
Here’s a plot of the results. The speeds are consistently shorter than nominal, by about 5-10% (around a tenth of a stop) and even shorter at the two fastest speeds, except for the 1/125 speed. This is also the speed that is set for flash exposure. I measured the 1/125 setting for both the normal speed setting as well as the X (flash) position on the dial. In both cases, the measured speeds were comparable and longer than nominal. I wonder if Pentax gave themselves a bit of leeway here to make sure that for flash exposures, the shutter really would be open “long enough” for the flash.
Of course, these values apply only to my vintage ME Super (I'm the original owner), which has never been CLA’d and was last used seriously for a film class I took back in 2007 or so.
The rise and fall times tell us something about how fast the shutter curtains are moving. An
a priori estimate of the curtain speed can be made by noting that the flash duration setting corresponds to a speed of 1/125 second. At shorter speeds, the second curtain starts its motion before the first curtain has had a chance to fully traverse the 24 mm vertical frame. So, we estimate that it takes around 1/125 second for a shutter curtain to move 24 mm. This corresponds to a curtain speed of
24mm/(1/125 sec) = 3000 mm/ sec (pretty fast!)
How should this affect my scope traces? Well, my green laser pointer produces a beam of light which is slightly bigger than 1 mm in diameter (with an unknown, but probably not too far from Gaussian, profile). If I naively assume a uniform dot 1 mm across, it should take a shutter curtain about 1 mm/3000 mm/sec = 333 microseconds to slice across the beam. This is remarkably close to what I see.
This image shows zoomed-in views of both the open and close light traces for a moderate shutter speed (1/60 second). (The close trace has been reversed and overlaid on the rise trace, with an eyeball best fit.) The whole process takes around 400 microseconds - very consistent with the estimate above for how long it should take a curtain to move across the laser beam. These timings are also consistent with the “bump” shown above for the 1/2000 second measurement.