Focus shift or focus shift usually occurs when the lens aperture is narrowed from the maximum aperture of the lens. For example, if you focus on an object at a maximum aperture of f/1.2 and then change it to f/2.8, the object that was previously in focus will become out of focus.
The object's right eye focuses well at a maximum aperture of f/1.2 (left);
Focus shifts away from the subject's eye when the aperture is narrowed to f/2.8 (right)
Source: https://bisnisfotografi.com
Cause of occurrence
Focus shift usually only occurs when you shoot using auto focus (AF) mode. The cause of this is that when the aperture becomes smaller, the amount of light entering also decreases. The focus of the lens decreases when only a small amount of light enters. The maximum aperture of a lens is usually a very wide opening and more than enough light enters so that when the aperture is reduced, it greatly affects the focusing performance of the lens.
Sometimes you have used the maximum aperture but if the light conditions are very low, it will be difficult to recognize whether the object is in focus. This can also cause a shift in focus.
Usually lenses that are susceptible to focus shift are lenses with large openings such as f/1.0, f/1.2 and f/1.4 which are usually found on prime lenses. Lenses with a larger maximum aperture of f/1.8 or f/2 usually don't experience focus shifts too often.
How to overcome focus shift
You can use manual focus, but usually it is still more accurate if you use autofocus. To overcome this, it is recommended to keep using a wide aperture. But if you still want to use a narrower aperture, when you lower the aperture you can adjust the focus again in manual mode. Or you can change your lens to a narrower maximum aperture.