[Guest]

Thnaks,

Tracking moving object could be beter at least in theory. AF cover area could be advantage for someone.

Honestly Im tired to read canon advertizing about this revolutionary??? system. They sounds like the Nikon 1 advertisement one year ago.

Back to the really techical part. I have questions

1. what is the base distance in canon 70d hibrid PD?

2. Is it constant across focus

 

Yes, indeed, the new sensor is technically something really new (not just a shrunk dSLR, like the Nikon 1). For daily use

of a dSLR, it perhaps makes not such a big difference nowadays, since there are plenty of good performing cameras out there.

But technically, yes, it is very novel, and has the potential for further innovations. If its PD AF gets as good as the viewfinder PD AF, then the days of the mirror box are counted, provided good EVF come along.  Further, the dual pixel technology has also the potential to increase dynamic range dramatically, if different ISOs can be applied to the two pixels during imaging (like the Magic Lantern dynamic range expansion recently introduced for some Canon cameras).

 

According to Canon, the area of the sensor that can be used for AF is about 80% horizontal and 80% vertical.

 

http://www.canon.com/news/2013/jul02e.html

http://www.learn.usa.canon.com/app/pdfs/...uality.pdf

http://www.canon.com/camera-museum/tech/report/2013/08/

 

So, you can calculate the distances, i.e. 22.5 x 15mm.  80% is 18mm  x 12mm.  if a point at the edge of this 80% is to be calculated to be in focus, the maximal spread of the out of focus signal could be 3mm (vertical), the point at the edge at 80% is 1.5mm away from the edge, so the out of focus phase signal could be spread +/- 1.5mm. In the center of course, bigger spreads are possible. 

 

This sensor area will of course not change, irrespective of lens or focus distance. What is only relevant is the angle of the incident light, which can change of course depending on focal length, aperture, and focus distance (with simple old lens designs, the lens is just moved further away from the sensor, so the angles will become more narrow). 

Canon says, apertures up to f11 can work with the new PD. 

 

I guess one can try to calculate how much out of focus something can be to work, if one uses the 1.5mm out of focus blur above as starting point.   Say, a 50mm lens, at 1:10 (0.1x) magnification, a point at ~50cm distance could be maximally blurred to 15mm (radius), if the lens is out of focus.    At the bottom here, http://en.wikipedia.org/wiki/Circle_of_confusion it shows how one can calculate the blur radius c, so one could run a couple of different examples through to get some idea how well it works. Anyway, as pointed out, in the center, larger blur radius would work for lenses such as f1.2 or f1.4.  One notes, that in most cameras, for such large aperture lenses, also conventional PD AF is usually limited to central areas.