Apertia · Library
How can an object in front of the focus plane be checked for geometric blur?
With 85 mm at f/1.8 focused at 2 m, a foreground leaf at 1.5 m blurs to 0.779 mm, while a background 2 m behind the subject blurs to only 1.124 mm.
The same formula in front of focus
The blur estimate does not care which side of the focus plane a layer is on. For a layer at sensor distance d, Apertia computes x = d − v and b = f² ÷ (N(u − f)) × |x − u| ÷ x, where u and v come from the focus distance. The absolute value makes a foreground layer positive. Because x is in the denominator, the disc grows quickly as a foreground layer gets closer to the camera, much faster than for the same gap behind the subject.
The app's bokeh preview arranges a subject and a background. For a separate foreground layer, the background-blur-geometry tool in this library accepts any layer distance from the sensor, including one in front of focus.
Worked example
The 85 mm model at f/1.8 focuses at 2000 mm, while the foreground layer is at 1000 mm from the sensor. The estimated diameter is about 2.413 mm.
- Foreground geometric blur diameter (mm)
- 2.412766
Estimate a foreground layer
- Set the focus distance to the subject, measured from the sensor plane.
- Measure the foreground object's own distance from the sensor, not its gap to the subject.
- Enter that distance as the layer distance with the same lens, aperture and sensor.
- Compare the disc with the background value to see which layer will dominate the blur.
- Keep the layer more than one focal length plus the image distance from the sensor, the model's lower limit.
Worked example: closing in on the lens
Full frame, 85 mm, f/1.8, focus at 2000 mm. A layer 200 mm behind the subject, at 2200 mm, gives 0.208 mm; one 200 mm in front, at 1800 mm, gives 0.257 mm. Further forward the growth speeds up: 0.779 mm at 1500 mm and 2.413 mm at 1000 mm. For comparison, a background a full 2000 mm behind the subject, at 4000 mm, reaches only 1.124 mm.
- Layer at 2200 mm
- 0.208 mm
- Layer at 1800 mm
- 0.257 mm
- Layer at 1500 mm
- 0.779 mm
- Layer at 1000 mm
- 2.413 mm
- For comparison: background 4000 mm
- 1.124 mm
Common mistakes
- Entering a negative distance for something in front of the subject.
- Entering the gap to the subject (500 mm) instead of the layer's own distance (1500 mm).
- Assuming equal gaps in front and behind give equal blur.
- Reading a large foreground disc as a prediction of how much of the subject will be hidden.
Geometry only
The disc size says nothing about the shape or transparency of foreground blur, or about lenses that render the front and back of focus differently. The DOFMaster equations listed below describe only the geometric defocus.