How do you choose a blur tolerance before calculating depth of field?

Where the default tolerance comes from

Every depth-of-field result in Apertia depends on the circle of confusion c, the largest blur disc still accepted as sharp. Each sensor preset carries a default of roughly the diagonal divided by 1500, the value used by lens makers' scales: 0.030 mm for full frame, 0.019 mm for APS-C, 0.015 mm for Micro Four Thirds, 0.011 mm for 1-inch and 0.008 mm for the iPhone main camera. The hyperfocal distance follows from it as H = f² ÷ (N × c) + f, measured from the lens, plus the image distance to reach the sensor plane.

Settings offers one alternative, Strict circle of confusion, which halves c for large prints and pixel-level viewing. It is a global setting, so every result, including the hyperfocal table and shot plans, becomes shallower.

Worked example

With an 85 mm model at f/1.8 and 2000 mm focus distance, a 0.015 mm tolerance gives about 26.083 mm of depth. The same setup at 0.030 mm gives about 52.173 mm; the changed criterion explains the difference.

Near limit (mm)

1987.047488

Far limit (mm)

2013.130501

Total interval (mm)

26.083013

Choose and record the tolerance

1. Decide how the photo will be viewed: small screen, normal print, large print or 100 percent crop.
2. Keep the default for normal viewing; turn on Strict circle of confusion for large prints or pixel peeping.
3. Note which setting was on when you read a hyperfocal distance or saved a plan.
4. After switching, refocus: a hyperfocal distance found with the default no longer reaches infinity under the strict setting.

Worked example: hyperfocal at f/8

Full frame, 35 mm, f/8. With c = 0.030 mm the hyperfocal distance is about 5174 mm, and focusing there keeps everything from 2605 mm to infinity within the tolerance. With the strict c = 0.015 mm it nearly doubles to 10278 mm, with the near limit at 5157 mm. If you stay focused at 5174 mm after switching to strict, the far limit is no longer infinity: the sharp zone becomes about 3461 mm to 10314 mm.

Default c 0.030 mm

H 5174 mm · sharp from 2605 mm

Strict c 0.015 mm

H 10278 mm · sharp from 5157 mm

Strict, still focused at 5174 mm

3461 mm – 10314 mm

Common mistakes

• Comparing two results made with different tolerance settings as if only the aperture had changed.
• Keeping an old hyperfocal mark after turning on the strict setting.
• Treating 0.030 mm as a guarantee of sharpness for every print size.
• Applying a full-frame tolerance to a smaller sensor; each preset has its own default.

A tolerance, not a measurement

Neither value is a measured threshold for a particular viewer, screen or print. The DOFMaster equations below show how c enters each limit, and the tolerance you choose should match how the photo will finally be seen.

Primary sources and related work

• DOFMaster depth-of-field equations