Picked up one of those cheap shutter speed testers to check out my old Spotmatics, and the readings were all over the place, like 1/500 reading as 1/250 on a camera I just serviced. Turns out the optical sensor needs a perfectly centered light source, which is impossible with most vintage lenses. Anyone else had better luck with the audio-based testers instead?
I always figured the cheap optical ones were good enough for a rough check but you're right, the sensor placement thing kills them on real lenses. What finally got me to switch was testing a leaf shutter. The audio tester caught a 1/125 that was really running at like 1/90 by comparing the two clicks, and the opt one showed it dead on. Now I just record the sound with a phone app and eyeball the waveform.