Apertia · Library
How can a repeatable field photograph check a focus-model assumption?
Test one Apertia prediction: place markers at the 2639.736 mm and 3476.74 mm limits of a 50 mm f/4 focus at 3 m and judge them at output size.
Turn one model output into a testable claim
Apertia's result card gives a near limit, a far limit and a hyperfocal distance for one sensor preset, focal length, aperture, focus distance and circle of confusion. Each of those is a prediction you can check against a photograph, but only one at a time. A field check that changes the lens, the aperture and the camera position together cannot tell you which assumption failed.
The most direct test uses the limits themselves: put a flat, detailed target at the predicted near limit and another at the far limit, focus on a third target at the focus distance, and judge whether the outer two look equally soft at the size you will view or print.
A marker test
- Fix the camera on a tripod and note the sensor-plane mark on the body as the distance reference.
- Enter the kit, aperture and focus distance in Apertia and copy the near and far limits.
- Place three printed targets at the near limit, the focus distance and the far limit.
- Focus on the middle target, take the photograph and keep the original file.
- View all three at the intended output size and write down what you see beside the numbers.
Worked example: 50 mm at 3 m
With full frame, 50 mm, f/4 and the 0.030 mm circle of confusion, a focus distance of 3000 mm gives a near limit of 2639.736 mm and a far limit of 3476.74 mm, a total of 837.004 mm. The targets therefore go about 360 mm in front of and 477 mm behind the focus point. Switching on the strict circle of confusion halves c and narrows the interval, so the same test can show whether the standard or the strict setting matches your output.
- Near target
- 2639.736 mm
- Focus target
- 3000 mm
- Far target
- 3476.74 mm
Common mistakes
- Measuring the target distances from the front of the lens while the app measures from the sensor plane.
- Judging sharpness at 100 percent on screen when the circle of confusion was chosen for a normal print size.
- Adjusting the recorded numbers after the test so that the prediction appears to match.
Reading the outcome
Agreement in one setup does not calibrate the lens, and disagreement can come from focus error, a different recording area or the viewing condition rather than the formula. Keep the photographs with the inputs. The DOFMaster equations and the University of Arizona camera-systems notes below describe the model being tested.