Roger Cicala at LensRentals has published an update
to his investigations into autofocus. Although specifically mentioning Canon, the article mentions principles that would affect most if not all designers.
- Contrast-detect is more accurate than phase-detect
- Contrast-detect is "about as accurate as the most careful manual focusing"
- improvements in autofocus accuracy depend as much on improvements in lens drive stepper motor accuracy as on camera hardware
- introduction of closed-loop AF systems in ultrasonic-driven lenses using rotation detectors has increased focus accuracy
- the further apart the pairs of high precision (cross-type) AF sensors, the more accurate the focus
What this means:
- improvements in both camera body AF systems and lens stepper motor accuracy work together to improve overall AF accuracy
- more accurate camera AF systems demand updated lenses with more accurate AF stepper motors, i.e. if you upgrade a digital camera body you may need to upgrade all your lenses to take advantage of the camera's better AF capability (if you rely on, or need to use AF)
- existing AF lenses may become obsolete with each new generation of improved AF camera bodies
Seems to me the cost of periodic rangefinder adjustments in digital M bodies using manual focus lenses, is a lot less than the upgrade cycle of digital autofocus systems. As sensor resolution increases focus becomes more critical. It's reasonable to assume future mirrorless models with 40+ megapixel sensors (X-Pro2?) may require a complete lens set upgrade in order to guarantee focus accuracy if you rely on AF
. As Roger Cicala shows, this is already happening with Canon - the 5DIII's improved AF is only effective when paired with newer lenses with improved steppers.