This was a quick hack to measure camera shutter speeds, motivated by my finding out that my first SLR (an Olympus OM-10 purchased secondhand a few months earlier) had a shutter problem. After I had the camera repaired, I used this to check that the shutter speeds were OK, and later used it to test various mechanical shutters on other cameras.
The pushbutton sets the count to zero and arms the circuit. When the phototransistor sees light above a certain level (the threshold is adjustable with the trimpot) the counter starts running, and when it next drops below the trigger level the count stops and holds until the circuit is re-armed.
I had lots of the TIL311 displays (LED dot-matrix hex displays with built-in decoding and latch) so the count starts at 00.000000 seconds and runs up to 99.999999 seconds. That's right, up to 100 seconds in steps of one microsecond. The 684 microsecond count shown in the picture resulted from my digital camera's flash. It looks kind of dim in the picture since I scanned it on my flatbed scanner, and the LED displays aren't very bright compared to the scanning lamp.
No schematics, as it's a pretty trivial circuit, I just made it up as I went. I think I spent about one day on this.