My but you can sure cram a lot of mis-information into a few sentences.
The highest possible frequency for a 555 would be defined by the rise time + the fall time of the output waveform.
Those are each 100ns, so 200ns total, which is a frequency of 5MHz.
Of course in a real circuit you could never achieve such fast operation reliably, but you can get to 1MHz.
The problem with the 555 is that it uses external resistors and capacitors to set its operating frequency.
While precision resistors are readily available, capacitors better than 5% are very difficult to find.
Add to that the switching time errors within the device, and you'd be lucky to hold 6% without high cost.
The actual 555 timebase circuits I've encountered usually spec their accuracy at +/-10%.
I've never encountered any crystals that operate at 200MHz.
Perhaps such do exist, but I've never found them in any equipment I've worked with.
And the position of the sensor has nothing to do with the measurement of exposure time.
A good time meter has three sensors, one in the middle and a pair at diagonally opposite corners.
This allows you to get three times for each test, regardless of the direction of shutter movement.
- Leigh
Ref: 555 datasheet from TI: www.ti.com/lit/ds/symlink/lm555.pdf
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