Axx AF sensor ?

[stevecim]

Hi All

was looking at images of the AF sensor of the A77 and was wondering why they only cover centre of the frame? (i can see why in my A350/550 has the second mirror for the AF only covers the centre) does anyone think there would much benefit of having AF sensors
covering ~ 90%-100% of the frame? seems to me the SLT design would be perfect for full frame sensor array?

cheers, Steve

[KevinBarrett]

The AF points of the a77 are distributed about as widely as those of the a700, there's just more of them in the same area so that they look light a tighter grouping. Note also that the a700 had a 95% coverage viewfinder, whereas this camera's EVF covers 100%. That extra real estate shows up at the periphery where it visually shrinks the AF cluster even further. Now, why doesn't it have an AF point distribution suitable for a FF camera? Possibly because the narrower light path within the camera body (perhaps even the semitransparent mirror itself) would not be so friendly to those peripheral PD-AF points. More probably, it is because Sony do not want to provide AF points that would function poorly with the camera's APS-C dedicated kit lens. Yes, these could be turned off when a DT lens is detected, but I imagine it's not the kind of *cough* streamlined interface that Sony wanted.

[David Kilpatrick]

Try a Canon 7D if you want to know why an AF sensor with points widely distributed near the edge of the frame is not a good idea. Shoot a landscape with the 7D set to default setting (factory settings) and it will focus on the grass near your feet. It's been a real problem for Canon owners who do not want to mess around selecting points, changing AF behaviour, or diving into complex setup menus for every different shooting situation.

Clustering the AF points more centrally does at least prevent the sort of wildly wrong focusing the 7D has been prone to.

David

[stevecim]

Thanks kevin, david....