AF method

[abik]

Hi guys I m having canon 6d and I m wondering what effects will be taken by changing af method from flexizone to live mode or quick mode?

 

[trsullivan]

Live mode is the basically the same as FlexiZone, but with face recognition. For both of these modes, the camera is using the image sensor to focus, so on a 6D it is much slower and tends to hunt a bit. (Newer cameras live the 70D have better autofocus in live mode. )

FlexiZone shows a focus box that you can move around or zoom in on to choose your focus point.

Live view attempts to recognize faces and focus on the one you choose.

Quick mode is different. It's fast, but pauses live view while it focuses. When quick mode pauses live view, the camera drops the mirror back into place and uses the dedicated AF sensor that would be used during regular shooting. Once focus if achieved (usually very quickly), the mirror is picked up and live view resumes.

If you forget this later, the camera tells you briefly at the top of the LCD when you select different modes in live view.

 

[TCampbell]

Ultimately, putting your eye to the viewfinder to frame, focus, and shoot will be much faster. When using any form of "live view" (using the screen on the back of the camera) the camera is forced to use "contrast detect" auto-focus.

Imagine attempting to take a photo of a barcode... white background... black stripes. If the image is focused then some pixels are black and some are white. There are basically no "gray" pixels. You can have a black pixel immediately next to a "white" pixel. That means you've basically created very high contrast (instantly changing from black to white by merely travel to an adjacent pixels.) Now imagine deliberately de-focusing the camera. The "black" stripe fades to several tones of gray getting increasingly brighter until you eventually arrive at "white" pixel. Since the transition from black to white happens very gradually... that's low contrast.

The "contrast detect" AF system attempts to maximize the contrast ... which indicates it's optimized focus.

The other system (and the system we prefer) is the "phase detect" auto-focus system.

Imagine taking a print of a photo... cutting it in two down the middle, and then slightly mis-aligning the two halves. When you look at the pixels right along the cut, they don't match up. But as we slide the two halves up or down, eventually we find the spot where the pattern matches. The phase-detect system isn't cutting a print in two halves... but it does put the light through a prism to split it into halves. If the two halves converge and come "in phase" then the image is focused (at least at the point where the phase detect sensor is inspecting the image.) If the camera is out of focus then the two halves will be out-of-phase.

But what's more interesting about the phase detect AF system is that when an image is out-of-phase, the camera actually knows which "direction" it needs to adjust focus (does it need to bring focus in to a closer distance... or move it out to a farther distance) AND... it also knows exactly how much an image is out-of-phase -- it knows precisely how much to adjust focus. And THIS means that the moment it inspects the phase of the focus point... it instantly knows which way to go and how far to go... the camera snaps into focus very quickly. It does not "guess" it's way to better focus.

One more thing... your 6D has 11 auto-focus points when you use the viewfinder with phase-detect AF (if you put your eye to the viewfinder you are always using phase-detect AF ... when you use the LCD screen on the back to frame and focus you are always using contrast-detect AF.) You can tell the camera to use a specific focus point to lock focus.... or you can let the camera use any of the 11 points. When it uses any of the points it will always pick the AF point which can lock focus at the nearest focus distance (shortest distance from the camera). The phase detect system actually assesses the phase on all 11 focus points at the same time. It immediately knows which point can lock focus at the nearest focusing distance, and then adjusts focus to snap to THAT specific AF point. Basically the phase-detect AF system does not "guess"... it's very fast.

The "contrast" system actually has to test the contrast, adjust focus, evaluate if it got better or worse... re-adjust focus, and do many samples in order to optimize focus. This system can be rather slow. Point & shoot cameras typically can only focus using the contrast system. The reason many consumers complain about the delay when attempting to take a photo with a point & shoot camera is largely due to the lengthly delay of waiting for the contrast detect AF system to refine it's focus before it can take the photo.

If you get into the habit of using the viewfinder, you'll get much faster focus and shooting performance.

 

[abik]

Thanks a lot tcampbell. I realized that all those modes only matters if I use live view I.e the screen of my camera.

Definitely the phase detect through view finder is much faster and I use only this. Unless shooting where my cam is very low or very high so I may use live view.