The full story
Why your autofocus needs calibrating
Two illustrated guides: how autofocus really works on DSLR and mirrorless cameras, the many small things that knock it out, and what FoCal users’ results show about how much it matters.
68%of lenses on Nikon DSLRs needed a significant adjustment (4 units or more)
55%of lenses on Canon DSLRs did too
46%of F-mount lenses on Nikon Z mirrorless cameras needed a noticeable adjustment
17%of native Z lenses did, on cameras that “don’t need calibrating”
Averages over all the cameras of each kind in FoCal IQ, from FoCal users’ calibration results.
4 chapters
Your DSLR’s autofocus
How a DSLR focuses, what goes wrong, how to fix it, and the evidence from FoCal users.
- How DSLR autofocus worksThe light paths inside a DSLR, and why the AF sensor and the image sensor have to agree.
- What goes wrongThe AF module, the mirrors, the sensor, the lens mount and the lens: tiny movements, soft pictures.
- The fixAF Microadjustment and AF Fine-Tune put the focus back where it should be. FoCal works out the value.
- The evidenceFoCal IQ results for popular cameras and lenses, and what they mean for yours.
6 chapters
Your mirrorless autofocus
No separate AF sensor, so no need to calibrate? Not quite. The sensor, the lens and the AF processor, one at a time.
- Why mirrorless still needs itThe usual argument, what the data says, and the parts of the autofocus system that can still be off.
- How phase detect worksMicrolenses, masks and shifting images: how one exposure tells the camera how far out of focus it is, and which way.
- The AF sensorMask placement and microlens errors a fraction of a hair wide, and the fixed errors they cause.
- The lensMounts, adapters, decentred and tilted elements, worn motors and calibration data from the day the lens was made.
- The AF processorOpen and closed loop focusing, fast shooting, and how small errors become a fixed offset.
- The evidenceFoCal IQ results for Nikon Z cameras, with native and adapted lenses.
By Richard Meston at Reikan. First published as two series on the FoCal blog in 2023, and updated with current FoCal IQ figures.
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