Apertia · Library
How can a small distance-measurement error change a focus plan?
With 35 mm at f/5.6 on full frame, a 3 m focus guess that is really 2.7 m or 3.3 m moves the near limit between 1994 mm and 2298 mm; test both ends.
Why one distance is not enough
Apertia prints near and far limits to the millimetre, but they are only as good as the focus distance you entered. The calculator measures that distance from the sensor plane, converts it to the object distance u, and then applies Dn = u(H − f) ÷ (H + u − 2f) and Df = u(H − f) ÷ (H − u), adding the image distance back. Because H is fixed for a given focal length, aperture and CoC, any error in the distance shifts both limits, and the far limit moves much more than the near one.
The practical check is to bracket the guess: calculate at a low and a high plausible distance and see whether the details you care about stay inside both intervals.
Worked example
At an 85 mm focal length and f/1.8, the full-frame preset gives a narrow interval around a 2000 mm focus input. This example uses the preset 0.030 mm blur tolerance.
- Near limit at a 2000 mm input (mm)
- 1974.269372
- Far limit at a 2000 mm input (mm)
- 2026.442687
Worked example
Changing only the focus-distance input by 50 mm moves both boundaries. A printed total width alone would conceal that movement.
- Near limit at a 2050 mm input (mm)
- 2022.880213
- Far limit at a 2050 mm input (mm)
- 2077.891174
Bracket a distance estimate
- Write down your best estimate and how you got it, such as pacing, the lens scale or a tape measure.
- Choose a plausible error, for example 10 percent for pacing, and work out the low and high values.
- Set each value on the focus-distance ring with the same focal length, aperture and kit.
- List the near and far limits for all three distances.
- Keep only the details that stay inside every interval as safely covered.
Worked example: 3 m ± 10 percent
Full frame, 35 mm, f/5.6, focus estimated at 3000 mm. The calculator gives 2150 mm to 4991 mm. If the true distance is 2700 mm the zone is 1994 mm to 4203 mm; at 3300 mm it is 2298 mm to 5896 mm. A signpost at 2200 mm is covered for the low and middle cases but falls outside if the distance was really 3300 mm, while a fence at 4500 mm is lost in the low case. Only details between 2298 mm and 4203 mm are inside all three.
- Focus 2700 mm
- 1994 – 4203 mm
- Focus 3000 mm
- 2150 – 4991 mm
- Focus 3300 mm
- 2298 – 5896 mm
- Inside all three
- 2298 – 4203 mm
Common mistakes
- Checking only the total depth, which hides how far each limit has moved.
- Measuring from the front of the lens while the app expects the sensor-plane mark.
- Using a wide bracket for a measured tape distance, or a narrow one for a guess.
- Ignoring a subject that moves between focusing and the exposure.
What the bracket does not cover
The comparison changes the distance input only. Autofocus error, focus shift and real-lens behaviour are separate sources of uncertainty that the DOFMaster equations listed below do not model.