Apertia · Library
Why compare a blur circle with frame dimensions as well as millimetres?
The same 0.4427 mm blur disc is 1.84% of a full-frame frame but 2.84% on APS-C, because Apertia divides by the sensor's short side for its portrait preview.
Millimetres versus share of the frame
Apertia computes the blur disc on the sensor in millimetres, then divides it by the sensor height, the short side. The reason is the preview: the bokeh screen shows a vertical 3:4 portrait frame, so its width is the sensor's short side. An early version divided by the long side and drew the disc about a third too small; the app now uses the short side, and the paywall and preview label it as blur in percent.
The diameter itself does not depend on the sensor preset; it depends on focal length, aperture and the two distances. The percentage does, because a smaller sensor gets enlarged more to reach the same output size.
Worked example
The diameter divided by the 24 mm short side of this sensor is about 0.100532. That is approximately 10.053% of that specific frame dimension before a later crop or resize.
- Image-plane diameter (mm)
- 2.412766
- Ratio to the 24 mm portrait width
- 0.100532
Read the percentage
- Set focal length, aperture, focus distance and background distance in the bokeh preview.
- Note the diameter in millimetres from the library tool if you need the absolute size.
- Read the percentage and remember the denominator: 24 mm on full frame, 15.6 mm on APS-C, 13 mm on Micro Four Thirds.
- If you crop the photo later, divide the diameter by the cropped short side instead.
Worked example: one lens, two sensors
A 50 mm lens at f/2 focused at 2000 mm, with a background at 6000 mm, gives a disc of 0.4427 mm on either sensor. On full frame that is 0.4427 ÷ 24 = 1.84 percent of the short side. On the APS-C preset it is 0.4427 ÷ 15.6 = 2.84 percent, so the same lens and distances look more blurred in an APS-C frame of the same print size, while also showing a narrower view.
- Blur disc, both sensors
- 0.4427 mm
- Full frame, short side 24 mm
- 1.84%
- APS-C, short side 15.6 mm
- 2.84%
Common mistakes
- Quoting the percentage as a quality score; it is a size ratio with a stated denominator.
- Dividing by the long side for a landscape photo and comparing it with the app's portrait percentage.
- Assuming a smaller sensor gives less blur with the same lens, focus and background distance; in this comparison the ratio rises.
- Leaving out a later crop, which makes the disc a larger share of the remaining frame.
Scope of the ratio
The ratio describes the uncropped preset frame. Screen size, print size and viewing distance change how large the blur appears to a viewer, and the thin-lens relations in the DOFMaster and University of Arizona references below do not model that viewing step.