This is a geometric thin-lens planner. Sharpness changes continuously; a depth-of-field boundary is where the defocus blur reaches your circle-of-confusion criterion. It is not a wall between perfectly sharp and completely blurred.

Use the actual focal length including any optical teleconverter effect. A smaller sensor does not change the calculation when actual focal length and CoC are held fixed. Format buttons only supply rounded conventional CoC values; they do not detect your camera, print size or crop.

With lengths in millimetres: hyperfocal distance H = f² / (N × c) + f. For focus distance s, near limit = s × (H − f) / (H + s − 2f), far limit = s × (H − f) / (H − s). At or beyond H, the far limit is infinity. When focusing at H, the near limit is H / 2. When focusing at infinity, the near limit is H − f.

For scene distances a and b, balanced focus is their harmonic mean, 2ab / (a + b). With a far subject at infinity, focus is 2a. The minimum geometric f-number is f²(b − a) / [c(2ab − f(a + b))], or f² / [c(2a − f)] for infinity. The tool then checks coverage using the selected nominal f-number. These are geometric choices, not an optimum for total image quality.

Stopping down increases geometric depth of field, but diffraction, lens aberrations, motion and focus accuracy affect the actual photo. None are predicted here. Use the result as a starting point and inspect real captures at your intended viewing size. This is not a macro, tilt/shift or focus-stacking calculator. Distances approximate the lens principal plane; a camera's sensor-plane mark or focus scale can differ.

Sources: DOFMaster optical equations and ZEISS on depth of field and the blur criterion.