Apertia · Library
Where should a close-focus distance be measured from?
Measure from the sensor-plane mark: for 85 mm f/2.8 at 2 m, a lens-plane formula overstates depth of field by about 10 percent, 89.102 versus 81.18 mm.
The reference Apertia uses
Every distance you enter in Apertia, and every distance it shows, is measured from the sensor plane — the line usually engraved on the camera body as ⦵, which is also where lens focus scales count from. Internally the app converts that distance d into the lens-side object distance u = (d + √(d² − 4df)) ÷ 2 before applying the depth-of-field formulas, then adds the image distance back to the results.
The app's own development notes record why this matters: an earlier version counted the distance from the lens, which overstated depth of field by about 10 percent for 85 mm at 2 m and by about 29 percent for 200 mm at 2 m. Many simple calculators still use the lens-plane convention, so their numbers will not line up with Apertia's unless you know which reference each one expects.
Worked example
For a 1000 mm sensor-to-subject distance and a 50 mm thin lens, the object-side distance is about 947.214 mm. The remaining distance belongs to the image side of this model. It is not a measurement of a particular lens barrel.
- Model object distance (mm)
- 947.213595
Measure a repeatable distance
- Find the sensor-plane mark on the camera body; if there is none, use the manufacturer's stated position.
- Choose a fixed, high-contrast point on the subject to measure to.
- Measure from the mark to that point with a tape and note the unit.
- Enter that value as the focus distance in Apertia, with the real focal length.
- When checking another calculator, find out whether it expects distance from the lens or from the sensor.
Worked example: the size of the convention error
Take full frame, 85 mm, f/2.8 and the 0.030 mm circle of confusion, focused 2000 mm from the sensor. Apertia converts that to u = 1911.043 mm and returns a near limit of 1960.272 mm and a far limit of 2041.452 mm, a total of 81.18 mm. A formula that treats the same 2000 mm as u gives 1956.441 to 2045.543 mm, a total of 89.102 mm. The difference, 7.922 mm, is about 9.8 percent of Apertia's interval — enough to matter for a close portrait where an eye and an ear must both be sharp.
- Sensor-plane convention · total
- 81.18 mm
- Lens-plane misuse · total
- 89.102 mm
- Overstatement
- about 9.8%
Common mistakes
- Measuring from the front of the lens hood and entering it as the sensor-plane distance.
- Subtracting the length of the lens barrel to estimate u; the principal plane of the model is not the front glass.
- Mixing feet and millimetres between the measurement and the input; Apertia can display either unit, but the reference stays the sensor plane.
What the convention fixes and what it does not
Using the sensor plane makes inputs repeatable, but the thin-lens split into u and v does not locate a real lens's principal planes or calibrate its focus scale. The DOFMaster equations linked below are written in lens-side distances, which is why the conversion step exists.