Monday 14 October 2019

Why do mirrorless cameras have quicker autofocus than SLRs using live view?


With early 2013 cameras, it's generally accepted that using contrast detection autofocus on SLRs (ie live view on most SLRs) is something which is really suitable only for static, or close to static, subjects due to the slow focusing speed. On the other hand, the current best of breed mirrorless cameras (the Olympus OM-D E-M5 often being quoted here) have autofocus systems which are significantly quicker at achieving a focus lock, if not quite being up to the performance of phase detection autofocus systems.


My understanding is that both systems are using the same technology, so why is it the case that mirrorless cameras have much quicker autofocus systems than SLRs in Live View mode? Is it the case that the lenses for mirrorless systems are optimized for quick CDAF performance, and if so, what are those optimizations?


Edit: in response to one of the answers, I'm not thinking about how cameras like the Nikon 1 series or the Canon EOS M have quicker autofocus due to the use of phase detection elements in the sensor; I understand how using an entirely different technology will improve things - what I'm interested in here is how some manufacturers have made contrast detection autofocus much quicker than is apparently possible in SLRs. Similar reasoning applies to the Sony SLT series as that's again using PDAF rather than CDAF.



Answer



Major reason is that the DSLR lenses are optimized for Phase Detection. Every component of the lens is tailored towards quick movement and stopping the glass in precisely picked moment. Contrast detection on the other hand works best with stepper motors capable of quickly switching directions so that you can move lenses inside back and forth looking for highest contrast on the image.


Phase detection knows straight away where the focus is and how much should the lenses move to achieve perfect focus. Contrast detection needs to "find it". This forces different engineering solutions in lenses manufacturing.


Also the DSLRs are usually made with live view as an afterthought. Most of the manufacturers think of it as an manual focus assist. They don't try to create fast AF for video as they know that pro videophotographers usually relay on manual focusing (not that they have any choice with such performance...) while photographers usually use viewfinders anyway. Hence they rarely have dedicated processors for contrast detection, and if the main CPU is occupied with focusing - it doesn't perform as well as dedicated unit.



Also the statement in your question isn't entirely true. Sony's SLT DSLRs have AF that's much faster in live view than Mirrorless, as SLT design basically allows camera to fully utilize it's PDAF (Phase Detection Auto Focus) sensors all the time during live view. So you get DSLR-quality AF with Live view at the same time. Also the older generation of Sony DSLRs offered Quick AF Live View - which never flipped the main mirror up for Live View - instead it used secondary sensor in viewfinder allowing DSLR-quality AF for a price of additional delay before shooting photo (mirror had to flip up in order to capture the photograph).


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