Headline - DSLRs have imprecise focusing due to their mirrors

Started Apr 5, 2014 | Discussions thread
Martin.au
Martin.au Forum Pro • Posts: 11,904
Re: Closed vs open loop focusing
2

Sante Patate wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

CD is a closed-loop system: it has to try a change and see if contrast increases, then try again, and again ... until contrast does not increase any more. That makes it slow, and prone to hunt. You can make it faster by reducing the number of times it tests and checks, but that reduces accuracy. But however many checking cycles your system has, it will always have them, so the accuracy will be consistent.

Phase detect AF is inherently faster than contrast-detect, because the PD system knows exactly where the lens elements have to be to achieve focus. To maximise speed, PD is (AFAIK) always open-loop: the system works out where the lens elements need to go and moves them there, but it does not then check to make sure it was right - unless the user activates the AF again. (Focus tracking systems are often called closed-loop, but that is misleading: the system is constantly making new guesses, but each guess is open-loop). Because PD AF is open-loop its performance is more variable, and it is prone to occasional large errors.

It is purely the fact that CD systems are closed loop that gives them an advantage in accuracy. PD systems could be made closed-loop, but that would defeat the purpose of AF because it would make it slower.

Both are closed loop. You can test it by starting the focus on a subject, then quickly moving the subject during the focus group's movement, and seeing if it focused on the background, or on the spot where the subject was.

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