Pier

keep the far lights sharp. how near can the subject stand?

drag the subject along the pier and the lens follows; turn the ring to stop down -- find where sharp reaches the horizon

A lens is only ever focused at one distance, but a zone around it looks sharp, and that zone is lopsided: it reaches much further behind the subject than in front. At one particular distance the far edge reaches infinity, so everything from half that distance to the horizon falls inside it. Photographers call it the hyperfocal distance, and it answers a precise question -- how near can the subject stand while the horizon is still acceptably sharp. It is a limit rather than a sweet spot: focus exactly there and infinity sits on the far edge of the zone, which is the blurriest thing the standard calls acceptable, which is why most landscape photographers focus a little past it and give up a little of the near end. Stopping the aperture down thickens the zone and pulls that limit closer, which is why a wide lens at f/8 is sharp almost everywhere. The distances shown are the real ones for a 24 mm lens on a full-frame camera, using the 0.03 mm circle of confusion every depth-of-field table for that format assumes -- a number that comes from what a person with good eyes can tell apart on an 8x10 print at arm's length, so it is a convention about looking, not a fact about the lens. The blur discs in the photo are drawn larger than life so a phone can show them.

Concepts: optics, depth, light, perception.

An interactive you play with one thumb, on Wundr.