See how rate stability can change between full wind and low power reserve — and what good isochronism means in real ownership.
Isochronism is the movement’s ability to keep a similar rate as amplitude/torque changes over the power reserve. Better isochronism = less timing drama between fully wound and nearly empty.
Isochronism = rate change with power-reserve / amplitude level
Positional error = rate change with orientation (dial up, crown left, etc.)
What is isochronism in a watch? It is how stably the watch keeps time as the mainspring winds down and amplitude changes.
Why does my watch run differently when low on power? Torque falls near the end of reserve, which can shift rate if isochronism is weak or the movement is due service.
Is some change normal? Yes. Large swings are more concerning than small, usable variation.
How do I test it simply? Compare average timing over a fully-wound day versus a low-reserve period, under similar wearing conditions.