[Guest]

 

Thanks for the input,

I agree with your and Canon explanation about advantages of their new system. While such

system offers

 

1.huge AF cover area - nice feature for video

2.solved/reduced split prism blackout problem – it allows to work with A=11. Nice feature for video

 

However such advantages should comes at certain cost. In this case I suspect the they reduced the maximum “baseline distance” used in their PD. If this is true – the new system offers the potential of bad AF accuracy esp with fast glass :-)



 

E.g Canon 10D,20d,30d…..60d offer   2,8 max baseline only for AF central point.



 

Pro camera

has 2,8 AF area only around image center and they have huge AF sensor



 

There is no camera that offers baseline distance higher that 2,8. That is why all SLR

suffer somehow with focus accuracy for 1,4 and faster lenses. E.g. Canon and

Nikon specifies very good the AF accuracy with such fast glass. E.g. Canon 1D III+

50/1,2 L



 

There are some physical lows – You cannot make sharp AF point at image corner with big baseline

/2,8/.



I still puzzling the new canon split focus design - what is the output of such convex desing insted of rounded prism

 

Theory,  theory….. The better way to prove it is to search for real life experience. E.g wait for Dpreview




With Kind Regards,

Miro

 
I tried a quick example for blur size based on the bottom of the wikipedia link I gave. Magnification given as 1:10  (let's say for a 50mm lens, object is ~ 50cm in front), the other object (that is blurred) is at infinity.  Aperture is 50mm (e.g. a 50mm f1.0 lens, or a 100mm f2.0 lens).
This gives the following graph (if I didn't make any mistakes), with the x- axis being the aperture (1 = f/1,  11 = f/11), and the y axis is the circle of blur in that condition.
 
[sharedmedia=gallery:albums:126]
 
At f1.0  you have a circle of 5mm, at f2.0 a circle of 2.5mm, at f5.0 a circle of 1mm, at f10.0 a circle of 0.5mm
 
Thus, at the 80% edge of the 70D, you should be able to AF with f5.6, but not with f2.0.
In the center no problem.
I think the reason that only the center point is sensitive for fast lenses with normal PD AF is that all these PD AF sensors sit in the bottom of the camera. Only a fraction of the light in the center is redirected down, and in that little space down there, it would be 
difficult to install the larger fast PD AF sensors also towards the edge of the image. I think it's simply a practical limitation why you don't have fast PD AF outside the center.