What is the best m43 for focus tracking?

Started Mar 31, 2013 | Questions thread
JamieTux
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Re: what is the best m43 for focus tracking?
In reply to clengman, Apr 5, 2013

clengman wrote:

JamieTux wrote:

Don't forget that PDAF does have one very strong trump card...  It knows whether something is in front of or behind the current focus point.

This was the reason for my initial question. I understand that PDAF systems have a definite advantage with regard to C-AF because can calculate at each instant whether the subject is in front of or behind the focus point. That allows the camera to react more quickly to subject motions in the z-axis. Though, apparently the differences in C-AF performance are shrinking.

Apparently so - although in my own experience erratic motion (even in a single direction) is better with a PDAF system (I think!)

It doesn't explain to me though why a PDAF system would be better at tracking C-AF where the problem is tracking subject motion in the x and y directions. I still don't understand entirely why a PDAF system would have much of an advantage for that task

I see a slight advantage in that if the camera sees a blob of light that seems to move as a unit in all three axes, it's likely to be a discrete object. The CDAF system can look at "blob" movement in two axes, but it's more difficult (but not impossible) to "see" the blob move in the z-axis. (A CDAF system could look at the changing size of the blob for instance, or it can record a value for local contrast across the blob along with a value for focus distance and use that information to plot a z-axis trajectory in which the blob has maximum contrast.)

I disagree a bit here...  Taking your blob example - as a point gets more out of focus it grows (and gets lower contrast) so that might make it appear to be moving towards you (and only in the z axis)

To split the blob up and look at the contrast on the blob you have to first define the correct blob as the one that has smaller blobs of detail - doing that with the amount of data that the sensor is giving would require huge amounts of analysis - on a PDAF system it's easier because they are dealing with a much smaller data set - they are simply looking at what the colour and brightness is of the blob is and where it's movign to in the x and y axis (this was the game changer when Nikon stole a march with the D3/D300s AF module).

Then you have the fact that for a contrast AF system to know that it is in focus it must have to overshoot (until contrast is reduced) and then jump back - you can see this happening if you put your camera in AF-C and focus on a stationary object.

So I would think the real break through will be when someone works out the algorithm to make the blobs and then runs the AF algorithms on just those.  Standard binning based on zones of the camera wouldn't work I don't think, it would need to be image specific.
To me its like using fourier analysis to turn a piece of music into a set of midi instruments - the advantage we have is that we only use that analysis for a specific job instead of trying to recreate the whole image.

Otherwise I agree - simplify everything so that the computer has less work to do and it will be faster

I guess this is the compelling argument, but certainly, someone at Olympus must know that if they do some sort of binning operation before they attempt the tracking operation that they can produce a faster tracking algorithm. Perhaps they wanted to develop their system to take advantage of a more complex set of information knowing that it would not be as fast or reliable in early iterations and just count on faster processing in the future to eventually bring the speed of the system up to par?

I think I gave my thoughts as to why that is above  Hoping that the information processing will keep up or overtake the information gathering seems like a risky strategy to me - I would think that if they were working that way the latest 12mpix bodies would have tracked better than the first 16mp ones - but maybe the difference would have been insignificant and gone unnoticed.

I still think that the speed improvements will be coming from reducing data intelligently.

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