Apertia · Library
How should focus distance and background distance be entered separately?
With 85 mm at f/1.8 focused at 2 m, a background at 3 m blurs to 0.755 mm and one at infinity to 2.198 mm; enter each layer by its own sensor distance.
Two distances, one formula
Apertia's blur estimate needs the focus distance and the background distance, both measured from the sensor plane. It converts the focus distance to the object distance u and image distance v, then computes the blur disc for a layer at distance d as b = f² ÷ (N(u − f)) × |x − u| ÷ x, where x = d − v. For a background at infinity the second factor becomes 1, so the disc settles at f² ÷ (N(u − f)).
In the bokeh preview the background slider sets that second distance, and the screen shows the disc as a percentage of the frame's short side. A description such as three metres behind the subject has to be turned into a total distance before it fits this model.
Worked example
With the 85 mm full-frame model at f/1.8 focused at 2000 mm, a layer at 5000 mm produces an estimated 1.343 mm blur diameter. This is a geometric diameter, not a bokeh-quality score.
- Geometric blur diameter at the image plane (mm)
- 1.34277
Enter the layers correctly
- Measure the subject from the camera's sensor-plane mark and set it as the focus distance.
- Add the subject-to-background gap to get the background's own distance from the sensor.
- Keep focal length, aperture and sensor unchanged while you try different background distances.
- Read the blur diameter in millimetres and, in the preview, the percentage of the frame.
- Use the infinity setting only for a background that is effectively very far away, such as a skyline.
Worked example: moving the background back
Full frame, 85 mm, f/1.8, focus at 2000 mm. A wall at 3000 mm gives a disc of 0.755 mm, 3.15 percent of the 24 mm short side. At 5000 mm it grows to 1.343 mm (5.59 percent), at 10000 mm to 1.774 mm (7.39 percent), and with the background at infinity it reaches 2.198 mm (9.16 percent). Moving the wall from 3 m to a point 8 m behind the subject does not grow the blur in step with the distance; the growth slows as the background approaches the infinity value.
- Background 3000 mm
- 0.755 mm · 3.15%
- Background 5000 mm
- 1.343 mm · 5.59%
- Background 10000 mm
- 1.774 mm · 7.39%
- Background at infinity (limit)
- 2.198 mm, 9.16% of 24 mm
Common mistakes
- Typing the gap behind the subject (1000 mm) instead of the background's total distance (3000 mm).
- Measuring the subject from the front of the lens and the background from the sensor, mixing two references.
- Expecting the disc to keep growing in proportion to the background distance.
- Comparing percentages between sensors without noting that each uses its own short side.
What the diameter leaves out
The result is the geometric size of a defocused point, not how the background will look. Highlight shape, onion rings, aberrations and portrait-mode processing are outside the thin-lens model described in the DOFMaster and University of Arizona references below.