Sony A99 - possible AF issues?

Started Sep 13, 2012 | Discussions thread
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Kappels Contributing Member • Posts: 796
Sony A99 - possible AF issues?

Over at Dyxum, David Kilpatrick, from Photoclub Alpha made IMHO a very important statement on page 13 of the below thread re the usability of the new AF system of the A99.

You might be aware that for the time being there are only a couple of lenses supported by the current firmware that can make use of the AF assist points and of the AF limiter function of the A99! Sony promises that more lenses will be supported in future, but they do not say which ones and when. See at the bottom of this weblink:

Here is David's statement on page 13:

I partially quote him here:
[QUOTE=photoclubalpha] ...

The issue with the lenses that can be used is not connected to the type of focus motor, or field curvature. It's related to the exit pupil distance and the geometry of the image-forming ray bundle (cone). There are three or more receptor pairs for each PD-on-sensor AF point. Each pair corresponds to a range of possible exit pupil size and distance factors.

Don't know if anyone here can remember, but back in the days of split image screens or microprisms, you got different interchangeable screens for wide angles, standard and tele. If you used the wrong screen, one half of the split image would tend to black out. PDOS works in a rather similar way. It has to be matched to the lens's optical design, to a degree. With today's zooms and retrofocus telecentric designs the rear exit pupil is not as simple to predict as in the old days.

This is why PDAF-OS is limited to certain lenses on NEX, as well. CDAF is dependent on the lens motor type and the focus algorithm, that's a different issue. PDAF may benefit from CDAF compatible lens focusing, but it's not likely to be dependent on it.

As I see it there are two possible futures - one, Sony just needs to create more detailed firmware tables to get the PDAF to work correctly with more lenses, and it will eventually work with all the current range. Two, that the lenses it works with now are almost the only ones where the exit pupil geometry is a good match to the range covered by the PDAF points; future new lenses will be designed so they are compatible, but if they can't do it now with an existing lens, they may never be able to.

I'll almost bet that PDAF will never be usable for the full range of any macro lens except internal focusing designs like the Tamron 60mm f/2 and that any totally up-to-date lens not included in the launch set of compatible lenses is causing Sony a problem - like the 16-35mm or the 24mm CZ. I can see no reason at all why such lenses, which are important to the A99 as a system, would be excluded unless the engineers hit a brick wall.

David[ QUOTE]

I am a committed Sony user, currently using the A900, A77, Nex5 and A100 (converted for IR use) but I have to say I am worried about this.

Missing support for important lenses such as i.e the Sony 70-200G lens ( I know it is older, but it performs flawlessly), leave alone all third party lenses makes me really wonder whether we ever can make use of the new af system of the A99?

Also, my understanding is the same issue occurs with the focus limiting option on the A99. It only works with some supported lenses.

I would really like to know from Sony which lenses they will include in future firmware upgrades before committing to buying the A99! I also would like to know how long we will have to wait for those updates. With the Sony A77 release, firmware updates for inclusion of more lenses were promised but so far there are many not included yet and we are still waiting!

The idea of not being able to use most of my lenses with the af sensor assist points is daunting and frankly then I would not see any improvement over the A77 and A900, in regard to the new improved AF!


 Kappels's gear list:Kappels's gear list
Canon PowerShot G10 Sony Alpha DSLR-A100 Sony Alpha NEX-5 Sony SLT-A77 Sony Alpha a99 +21 more
Sony Alpha a99 Sony Alpha NEX-5 Sony SLT-A77
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