Shutter Speed Tester

I wanted a shutter speed tester to check the shutters on the cameras I tinker with and decided to build my own. This is a description of it, not detailed maybe, but enough to help anyone thinking of doing the same thing perhaps.

The tester consists of three main parts; a counter and display, an oscillator to drive the counter and an optical sensor to gate the output of the oscillator. The basic principle of operation is that the optical sensor sits behind the shutter and a light is projected through the camera. When the shutter opens and the light reaches the sensor, the gate opens and the counter counts up. When the shutter closes, the gate shuts, the counter stops counting and the number shown on the display represents the period the shutter was open for.

Counter/Display
I decided to buy a ready made counter/display module. It wouldn't have been difficult to build one but I didn't think the saving was worth the effort. The module I used came from an ebay seller by the handle of e3b6a5y. I chose a 6 digit counter to give me the range I wanted without having to have range switching.

Oscillator
I opted for a simple RC oscillator though a crystal one would be more stable. I finally settled on a frequency of 10KHz because I had problems with the counter module running at the speeds I would have liked (ideally 100KHz). The final circuit is shown here. The two pots allow coarse and fine adjustment of the frequency.

Optical sensor
I searched for a suitable circuit I could use and came up with these circuits by David Johnson. One of the criteria I had was fast response and the more complex of these promises that so that's what I used. The optical sensor I used was a darlington pair photo transistor. I had to adjust the value of the middle resistor of the potential divider chain as I decided to use a 9V supply instead of the 5V shown.

The whole tester was built into a box with a power switch and a push button to zero the counter. The optical sensor was positioned at one end and a small hole drilled in the box in line with the sensor.

Problems
The main problem I had was the counter/display module not counting fast enough. The module I bought was specified up to 50KHz but the fastest I could get it to count at was just a few KHz. An email to the supplier produced a modification which did allow the module to count up to 100KHz but it was clearly missing counts and I had to reduce the frequency down to 10KHz to get a reliable operation. With a 10KHz clock frequency, an accuracy of 10% with a shutter speed of 1/1000 second is achievable - I considered this to be good enough.

The other problem I had was with inaccuracies when measuring fast shutter speeds. It turned out this was down to two different things; the over sensitivity of the tester (or at least, the optical sensor) and the size of the hole I had drilled which, although only 2mm, was comparable, it seems, with the size of the slot formed between the two shutter blades at fast shutter speeds. These problems were overcome by turning the hole into a slot using two bits of black tape and by adjusting the brightness of the light I was using before making the test (see below for details).

Use
Operation is pretty simple. The tester is sat behind the camera shutter and a light source (I use a torch) is shone through the lens. The camera shutter is opened using a B or T setting and the aperture of the lens is closed down (you could also alter the position of the light source) until the counter just stops counting. The aperture is then opened up again until the counter just starts counting. The shutter is closed, the counter reset and the shutter fired normally. The resultant count is converted to time (count/10000 seconds). Note: avoid using a mains (AC) light source as the flickering of the light will effect the result.

Update: I've found I can get better results with fast shutter speeds by using a laser as the light source. The laser is fired through a very small pinhole made in aluminium foil so that the beam width is a small as possible.

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