Apertia · Library
Meters and millimeters in camera focus calculations
Apertia can show distances in metres or feet; 10 ft is 3048 mm, and with 50 mm at f/4 that moves the near limit from 2640 mm at 3 m to 2677 mm.
How the app handles units
Internally Apertia works in millimetres: focal length, sensor size, circle of confusion and every distance from the sensor plane. Settings only changes how distances are displayed, either metric or feet. In metric the app shows centimetres below 1 m and metres above. In feet it shows inches below 1 ft, feet and inches such as 9′ 0″ below 10 ft, and decimal feet from 10 ft. The default follows the device region, with feet for the United States, Liberia and Myanmar.
The focus-distance ring itself runs on a logarithmic scale from 0.15 m to 100 m. The library tools are stricter: they take millimetres only and reject a distance inside four focal lengths, but they cannot tell a correct number from a wrongly converted one.
Compare the same scene with a mistaken unit
Every row below uses a 50 mm lens, f/4 and the full-frame preset. Only the supplied focus distance changes. The last two rows deliberately demonstrate a unit mistake; they are not suggested camera settings.
| Written value | Unit | Multiply by | Entered distance (mm) | Calculation | Near limit (mm) | Far limit (mm) |
|---|---|---|---|---|---|---|
| 3 | m | 1000 | 3000 | Calculated | 2639.735949 | 3476.740429 |
| 300 | cm | 10 | 3000 | Calculated | 2639.735949 | 3476.740429 |
| 3000 | mm | 1 | 3000 | Calculated | 2639.735949 | 3476.740429 |
| 3 | mm | 1 | 3 | Rejected | Unavailable | Unavailable |
| 3000 | m | 1000 | 3000000 | Calculated | 20739.996088 | Infinity |
Audit a distance before you rely on it
- Write the measured value and its unit exactly as measured.
- Convert once: metres × 1000, centimetres × 10, feet × 304.8, inches × 25.4.
- Check that the converted value is what the field or ring shows.
- Confirm the measurement starts at the sensor-plane mark, since conversion does not move the origin.
- Keep Infinity as its own choice rather than typing a very large number.
Worked example: 3 m against 10 ft
With 50 mm at f/4 on full frame, focusing at 3000 mm gives 2640 mm to 3477 mm. Ten feet is 10 × 304.8 = 3048 mm, and focusing there gives 2677 mm to 3542 mm. Treating “3 m” and “10 ft” as the same distance shifts the far limit by 65 mm. The feet display shows 3048 mm as 10.0 ft, while 9 ft, 2743.2 mm, still appears as 9′ 0″.
- 3000 mm
- 2640 – 3477 mm
- 10 ft = 3048 mm
- 2677 – 3542 mm
- Display of 2743.2 mm · 3048 mm
- 9′ 0″ · 10.0 ft
Common mistakes
- Rounding 10 ft to 3 m when a narrow depth of field depends on the difference.
- Typing a distance in metres into a millimetre field, such as 3 instead of 3000.
- Converting feet with 300 instead of 304.8.
- Reading a feet display in the app and a metric result in a library tool without converting.
What a valid number proves
A result only shows that the input was inside the model's accepted range. It cannot confirm that the number matches your scene or that it was measured from the right point; the DOFMaster equations listed below assume a correct sensor-plane distance.