Originally posted by Na Horuk Anyway, if going by eye works better for you, then just go by eye. That's the way it should be done. You don't need some machine's approval
What they said ^ ^ ^
I read an article yesterday (should have kept the reference) about AF strategies and limitations. The last sentence was sweet. To paraphrase, it said something to the affect that "The real truth is that PDAF (by nature and physics) truly sucks for critical work, but since it is fast and adequate most of the time we put up with its failings."
Here are a few bullet points:
- Focus confirmation performance uses the PDAF (auto-focus) sensor and provides the same performance (accuracy and precision) as autofocus with the same lens
- A manual focus lens may exhibit front/back focus with the PDAF sensor the same as an auto-focus lens
- Except for a few newer lenses (e.g. Irix), per-lens PDAF fine tuning is limited to auto-focus lenses. That is because most manual focus lenses lack the "data" contact on the mount that supplies the camera with a lens id code.
- Even with the f/2.8-sensitive center AF point on the K-5II, precision lags for lenses faster than f/2.8 when compared to magnified live view or a well-calibrated split-image finder and will be no better than an f/2.8 lens
- Note the word precision in the point above. Precision is the ability of the system to assign the same point of focus consistently. The system may be well-calibrated for accuracy, but if precision is poor, missed focus will be more common. This will be more obvious when shooting at wider apertures. At narrower taking apertures, DOF will often cover the deficiency unless one is pixel peeping.
- The stock focus screen has its own precision issues, but discussing it fully here would be a deep hole
- Magnification for manual focus with the stock screen has proven valuable for many users
- Replacing the stock screen with one having focus aides (e.g. split-image) or an enhanced matte surface (e.g. Canon S-type) has potential to work very well, but often requires calibration using shims
- Focus using magnification in live view (with or without focus peaking) is the Gold Standard for both accuracy and precision for manual focus with a given lens
- No amount of focus technique will make a soft lens sharp
- Most fast primes (f/2.0 and wider) are fairly soft wide open
- A soft lens is harder for both you and the AF system to bring to acceptable focus
Emphasis should be placed on the point above regarding magnified live view. When fine focus is essential, one can do no better than live view with magnification. The sad part is that using the feature without a tripod is a waste of effort.
As noted in comments above, practice helps with manual focus as does learning what to expect from a particular lens. In the dim past (before pixel peeping), photographers using hand-held 35mm cameras photographers were a bit less picky simply because few took the time to examine their negatives under magnification. Ignorance was bliss and we were generally quite blissful.
Steve