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Passive autofocus systems like contrast detection and phase detection don't work well in low-light environments and on low-contrast areas.
But I have a lens that says "ULTRASONIC" on it, and it suffers from the same limitations of passive autofocus.
Why does my lens with active autofocus have trouble focusing in low-light environments and on low contrast areas?
"Ultrasonic" (or "USM") has nothing to do with active vs. passive autofocus. It's a type of drive motor for actually moving the focus elements, in contrast to e.g. stepper motors ("STM"). Contrast detection and/or phase detection are actually part of the sensor assembly and are the source of the data that gets converted into instructions to tell the drive motor how far to move - regardless of whether it's a USM or STM lens. Also, regardless of whether you have contrast detection, phase detection, or both (or even something completely different...), low light and low contrast lead to lack of detail that a sensor can detect to derive instructions for the focus motor, kind of like how your eyes have trouble seeing in low light or with no details/edges to discern things, only worse - sensor technology is not nearly advanced as the human eye.
