Making sense of the STM lenses and the 650d AF

Started Jun 9, 2012 | Discussions thread
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Re: Stepper Motor = Sigma "Linear" Motor in new Sigma 19mm + 30mm
In reply to scorrpio, Jun 13, 2012

scorrpio wrote:

WilbaW wrote:

scorrpio wrote:

PDAF, however, ( ) knows exactly which way it needs to go and how far, and as long as lens is properly calibrated, accurate focus is achieved with a single lens repositioning.

That's a myth which has been comprehensively disproved. See -

You can focus on your jumping squirrels till the cows come home.

I use Post-It notes - they stay put and don't eat the birdseed.

My post had no mention of either open or closed loop,...

You gave a concise description of an open-loop control process - "PDAF... knows exactly which way it needs to go and how far... with a single lens repositioning."

... and frankly, that particular aspect of AF system is not under discussion here.

Why did you mention it?

The wiki link was more to show the concept of PDAF sensor.

It does that fine, and I can see why you would take it as conforming with the myth, but it doesn't tell you anything about the (closed-loop) control process.

And the fact that phase system is a lot more precise.


One thing I know for certain is when I aim my active focus point at a well-lit, stationary, high-contrast object, and press AF-ON, the lens focus ring moves exactly ONCE, and focus is confirmed. Does it check? Likely. Does it have to correct? If it does, it is by such a tiny amount and done so quickly, it's imperceptible. Does it check midway and adjust? If it does, it does it so darn fast the whole process looks like one fluid motion.

Yep, you need to constrain it to one step at a time to know what it's actually doing. Try the Intermittent Illumination Test.

By comparison, using live view, same target, same lens, same camera, the lens racks to the general vicinity of being in focus, and then does a bunch of small jumps before flashing that green rectangle on the LCD.

Yeah, since CD AF needs a static image, the iterations are apparent.

In terms of the squirrel test, CDAF is like trying to get said critter with a .22 from the hip, watching where bullets hit and adjusting accordingly.

Interesting analogy, works for me. Though I can see some people getting confused by using an example of an open-loop process to explain a closed-loop process.

And PDAF is like using a scope. Sure, you still gotta check for a dead squirrel under the tree (closed loop) but if the scope is well zeroed, all it takes is one shot.

Wrong, PD AF is like getting said critter with a machine gun. You're doing the same thing as you did with the 22 (iterations), but you're doing it pretty much continuously.

It's impossible to work out what's happening with PD AF by observation of normal operation and intuition. You have to test it, like in the tests I gave.

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