Phase detect AF video - I still don't get the "why".

Started Jul 9, 2012 | Discussions thread
sybersitizen Veteran Member • Posts: 8,622
Re: Phase detect AF video - I still don't get the "why".

Shield3 wrote:

sybersitizen wrote:

Shield3 wrote:

No one is saying or expecting the PDAF to work @ F/8 are we? I simply pointed out that my kit lens will AF @ F/5.6 @ 55mm in video mode so I should be able to set my F/2.8 lens to F/5.6 and have AF work.

... I get it, it ain't gonna work @ F/8. I'm not on here saying "Gee, my AF should work at F/22!"

That was you 16 months ago. And I agreed with you that a small aperture limitation exists.

I also agreed with you - after I realized my one error thinking there was some difference between a maximum aperture f/5.6 lens and a fast lens stopped down to f/5.6 - that there is no large aperture limitation and never was. Done with that part, okay? Okay? Okay? Okay?

I'll tell you what. I'll go rent a Sony a77 and a 16-50 lens today. I will TAPE the aperture level open in such a way that the lens is FORCED to F/11. Would you like to place a wager that it will AF in video mode? Name your wager price.

That's you today.

Tell me why 16 months ago you believed the small aperture limitation is real, but now you don't.

That's before I saw it work first hand @ F/1.4. Probably about the same reason you thought the angle of incidence made it not work.

At the end of the day, there wasn't an absolute technical reason for this limitation. Sony designed it that way. Period, end of story.

Sony certainly designed the large aperture limitation that way. Their lenses can obviously autofocus when they're wide open because they do it for us all the time. And Sony certainly designed the f/3.5 limitation that way, because we know their lenses can autofocus at apertures as small as f/6.3-f/8. But you have not proven that Sony designed the small aperture (smaller than f/6.3-f/8) limitation that way.

With that settled, for simplicity let's now eliminate everything from the discussion except the small aperture limitation, which experts say or imply is/was common to all camera systems using traditional PDAF - but which apparently does not exist with on-sensor PDAF.

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