A6000 vs. A77ii Continuous AF - Very confused

Started May 14, 2014 | Discussions thread
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Re: PDAF vs CDAF, separate or on-sensor, in a nutshell
In reply to 123Mike, May 19, 2014

123Mike wrote:

Corpy2 wrote:

EinsteinsGhost wrote:

Corpy2 wrote:

someguy50 wrote:

I believe a77ii will be faster. Constant focusing due to SLT, and like the a6000, built for speed.

Even taking into consideration the lack of in-body stabilization, this does not account for a doubling in price.

Its cheaper than E-M1 (to put that in perspective). So, in a way, a77 II might be a steal... although priced in line with its direct competitors: 70D and D7100.

I have no doubt that the a77ii seems priced right, and if its autofocus capabilities are as people are predicting, perfect for objects in motion, and far better than the em-1.

My issue is my belated discovery of the a6000, for about half-price, and smaller, which seems (to my current ignorant self) to be providing virtually the same feature set, minus the in-body stabilizartion, and a cryptic reference to the two types of autofocus methods.

I'm not sure why I have to know the 2 methods in detail, but if these two methods create different AF capabilities between the a6000 and the a77ii, that impacts the feature sets, and is therefore important to know.

A simple way to look at it is that CDAF looks at the contrast of an image. It then moves the focus around looking for the best contrast it will see at a given point or multiple points. This is slow, but it can be very accurate.

PDAF is by seeing depth, and therefore being able to gauge distance, that allows it to calculate how much the focus should move.

The A77ii as do other SLTs (and DSLRs and SLRs with auto focus), use a separate PDAF sensor from the imaging sensor. They either reflect the light or partially reflect the light to that PDAF sensor. The result is in decisive quick focusing decisions, without hunting or seeking.

The A6000 uses a special new sensor that has PDAF right on the imaging sensor. This technique existed for a little while (not long), but up to this point has been a bit dicy. The camera could only deduce a bit of information that would prevent the focus from moving in the wrong direction. But recently, this has become good. The A6000 can do effective and precise auto focusing through it. I don't know if this still does involve looking at contrast. Maybe. But that's ok, and in fact, this would be an advantage, because it would ensure the sharpest possible shot, without depth perception errors. What matters is that the focus is decisive and moves the focus in one go. It does that. Another advantage of on-sensor focusing (be it CFAF, PDAF, or combination of both), is that is no point of micro-focus-adjust. The system just moves the focus until it sees what it wants to see. With a separate PDAF sensor there can be errors, and you can correct for that using that micro-focus-adjust.

That's it in a nutshell I think.

Wow... Thanks for that Mike. I have been a hobby photographer, on and off, for many years and I found that response to be very informative... I knew what the different between CDAF and PDAF was, but it was nice to see why different cameras use one over the other and how each AF system is employed in the different camera formats.

Thanks... very well done.

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