Glossary
Infinity focus
Infinity focus means a lens can still bring very distant subjects to a sharp focus, which requires the lens to sit no farther from the sensor than its design distance.
Also called reaching infinity, infinity hard stop
How it works
If an adapter is too thick, the lens sits too far back, and the closest it can focus becomes nearer than infinity. You gain a little close-focus ability and lose distant subjects entirely. Landscape work becomes impossible.
If an adapter is slightly too thin, the focus ring reaches infinity before hitting its hard stop. This is harmless and in fact common, and it is why some adapted lenses focus past the infinity mark on the barrel.
Reputable adapters are machined to the exact difference between the two flange distances. Very cheap adapters sometimes miss by a fraction of a millimetre, which is the usual cause of an adapted lens that cannot quite reach infinity.
Why it matters when buying
Test a new adapter on a distant subject on day one. If infinity is unreachable, the adapter is too thick and no amount of focusing technique will fix it.