Is AF-C basically a myth for NEX?

Started Feb 4, 2013 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Is AF-C basically a myth for NEX?

dpyy wrote:

dpyy wrote:

I have a NEX6 camera and I've been missing a lot of moving shots for candid portraits in the street. Mostly because they are all out of focus. So I started training for this particular scenario. So first of all, I'm using a fast lens Z24 that allows me to go up to 1/200 or 1/300 with decent exposure.

Then secondly, I started using focus tracking. This way I can mark the person's face and then recompose. For the most part focus tracking is not the problem.

So finally the problem really is with AF-C. In burst mode, what I would expect is that the camera should constantly try to refocus after each shot, the box would turn green, makes the beep sound, and then take the shot. It kind of does this, but what ends up happening is that it would beep but the subject is still out of focus. So basically it lies...

The question is, anyone was able to use AF-C successfully for moving objects and burst mode? I'd imagine this is a very typical scenario. For example, the good old train-moving-towards you scenario where you would need to continuously change focus.

(P.S. I can take a video demonstrating this effect if I'm not explaining this clearly)

Ok so I've actually went out and did the test, and the suggestions in this thread doesn't really help. Here's how I've setup the test:

  • I move towards and back with the center focused on a object while holding down shutter
  • Used a lens that support PDAF (Z24)
  • Used Aperture bigger than F/6
  • Autofocus mode in AF-C (Center)
  • AEL w/ shutter set to OFF (though I don't notice a difference either way)

So with that in mind, here are the tests. I've only uploaded one image per test because otherwise there would be too many photos.


Test 1 - 1/250, F1.8

Video 1 -


Test 2 - So maybe DoF is too shallow therefore I increased it to 3.2 (1/250, F3.2)

Video 2 -


So the problem as you can see is that there are a lot of photos that are out of focus with the burst mode. So now the question is, how do I take photos of a moving object? I don't mean a speed bike or anything. Just a regular human being with candid shots? I wasn't even moving quickly forward and backward with the tiger but yet it fails so often!

The DOF is very shallow, even at f/3.2. The small area in focus in this case is slightly behind the eyes. A more effective way to increase DoF is to increase the focusing distance. Assuming you took this image at about 0.5m, the DoF at f/1.8 would be about 3cm which grows to a little over 5cm at f/3.2.

IMO, the dual AF system set up in NEX (CDAF-PDAF hybrid) is more about reducing the time CDAF would take to focus on its own, rather than replace CDAF with PDAF. So, the improvements are not going to be noticeable especially when shooting action. But, the camera should grab focus faster than it would if PDAF weren't active (or were absent).

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