What is focus shift?
Focus shift is an optical phenomenon where the plane of sharpest focus moves as you stop down the aperture. You focus at f/1.4 and it looks sharp, but at f/5.6 the focus has shifted slightly forward or backward, resulting in a softer image than expected.
Why focus shift happens
Focus shift is caused by spherical aberration. At wide apertures, light rays from the edges of the lens focus at a slightly different point than center rays. These aberrations create a soft glow that masks the true focus point.
When you stop down, edge rays are blocked by the smaller aperture, removing the aberration. The true geometric focus point may differ from where aberrated focus appeared wide open.
Not all lenses exhibit focus shift. It depends on the optical design and how well the manufacturer corrected spherical aberration.
When focus shift causes problems
Most noticeable with fast primes (f/1.2, f/1.4) when you focus wide open then stop down for the final shot.
A particular concern in studio photography where you might focus at f/1.4 for visibility, then shoot at f/5.6 for depth of field.
In video with a fixed aperture, focus shift is not an issue because you shoot at the same aperture you focused at.
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How to work around focus shift
Focus at your shooting aperture: Use the depth of field preview button or set your aperture before focusing.
Use Live View: Mirrorless cameras always show the image at shooting aperture in the viewfinder, so focus shift is visible and can be compensated.
Test your lenses: Shoot a target at different apertures without refocusing and check if the focus point moves.
Focus shift is primarily a concern for DSLR users who focus through an optical viewfinder at wide apertures. Mirrorless users are largely immune because the EVF shows the actual shooting aperture view.