Focus there and everything from half that distance to infinity counts as sharp. For a 50 mm lens at f/8 on full frame it is 10.9 m.
A smaller hole deepens the sharp zone without limit. A 50 mm at 3 m has 0.58 m at f/2.8 and 1.76 m at f/8, until diffraction softens the whole frame.
Keep the subject the same size and the lens barely matters: 293 mm at 24 mm from a metre away, 287 mm at 200 mm from 8.5 m.
At that same framing the 200 mm blurs distant lights over eight times as much, because it has a 71 mm hole against 8.6 mm.
At 3 m with a 50 mm at f/2.8, 45 percent of the sharp zone is in front of the subject. Near the hyperfocal distance the far limit runs away.
Past this, stopping down softens everything. APS-C reaches it at f/14, Micro Four Thirds at f/11 and a one-inch sensor at f/8.
Sensor, lens and blur limit all shrink together, but the wavelength of light does not. Small sensors gain depth of field and reach the wall sooner.
A 50 mm at f/2.8 has an 18 mm hole and a 200 mm has 71 mm. The hole sets how far the background melts.
A 200 mm at f/11.2 has the same 17.9 mm hole as a 50 mm at f/2.8, and at the same framing four times the depth of field.
Many lenses lose focal length as they focus closer, so a 70–200 can behave like a 140 at its nearest. Close-up figures drift first.
At a fixed distance, yes. But walk back until the subject is the same size in the frame and almost all of the difference cancels. What you still see is the background: the long lens magnifies it, so it looks far more blurred even though the sharp slab is the same thickness.
A decision, not a measurement. It is the largest blur circle you are prepared to call a point, and the traditional figure is the sensor diagonal divided by 1500, roughly what an unaided eye resolves on a 10×8 print at arm's length. Two calculators can disagree and both be right. Switch the standard here to see how much it moves.
It gives the widest range that is acceptably sharp, but it puts infinity right at the edge of acceptable, so distant mountains are exactly as soft as you said you would tolerate. If the far detail is the subject, focus on it instead.
Because the far limit is u(H−f)/(H−u), and as the subject distance u climbs towards the hyperfocal distance H the bottom of that fraction goes to zero. It is a real feature of the geometry, and it is why hyperfocal focusing feels like a trick.
A lens forms a real image only of something further away than its focal length. Inside that distance the light never comes to a point behind the lens, so there is no sharp zone to work out. A real lens stops well before that.
The size and spacing of the discs are worked out for this lens. Their look is not: real discs have cat-eye shapes, bright rims and hard or soft edges that depend on the lens, and brightness here is eased so that large discs stay visible.
A thin lens in air, with the subject distance measured to the lens. Real lenses are thick, many focus internally and their focal length changes as they focus, so close-up numbers drift first.
The only blurs are defocus and diffraction. Lens aberrations are not modelled, so the best aperture on a real lens is usually a stop or so away from the pure diffraction crossover. Sensor resolution and the anti-aliasing filter are absent too, and the Airy disc uses a wavelength of 550 nm.
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