STM was introduced mainly to give smoother, quieter autofocus during video recording. For still photography, autofocus happens before the shot, so that smooth focus transition is usually much less important.

That means STM is not automatically better for stills. The real answer depends on which two lenses you are comparing:

• Compared with some older non-STM versions of the same lens, an STM model may offer improvements such as faster autofocus and sometimes updated optics.
• Compared with higher-end ring USM lenses, STM is often not the better stills choice if you want the fastest autofocus; many ring USM lenses focus faster.

So for still photography alone, there is no general rule that STM is better than non-STM. Choose based on the specific lens’s autofocus speed, optical quality, and overall design rather than the motor type by itself. If you also shoot video, STM becomes more attractive.