Nikon Z Mirrorless Video Focusing Question

Started Feb 22, 2019 | Questions thread
beatboxa Veteran Member • Posts: 8,400
Re: Nikon Z Mirrorless Video Focusing Question

Richard Frederick wrote:

The Nikon Z Mirrorless Talk forum doesn't much care about video, so I thought I would ask here.

When using Nikon native lenses for Z cameras:

For Stills: As I understand it the system auto focus at f5.6, and then changes aperture for the exposure. Apparently this is to give the Phase Detect (PD) system a useable and perhaps optimum aperture.

This is not correct, and somewhat describes a DSLR's PDAF system (though even then it would be incorrect).

On the Z system--at least with the current Z6 & Z7 cameras, the aperture is what it is. Opening up the aperture does give more light to PDAF sensors, unlike in a DSLR.

Here's how to think about DSLR's PDAF, and why it's "stuck" at F/5.6:

The PDAF sensor is masked to only see at pre-defined positions & sizes, like 2 different holes, at F/5.6 apart.

The system will compare the parallax from the images between the "red" & "blue." It's like 2 small apertures for the PDAF sensor, within the lens aperture.

Opening or closing the lens has no effect on the PDAF positions--unless the aperture is so small that it blocks these positions. Once it does, the AF system is completely blind because you've blocked all light.

But on-sensor PDAF works as follows:

The lens will project the entire image to the sensor. Some of the light makes it to individual pixels that are half masked (half blocked:  see the black masks over the pixels above)--allowing them to only see light from roughly half of the lens.

Parallax is then compared between sets of pixels (not individual pixels). So if you should have a straight line, and it doesn't go straight across those set of pixels, it is out of focus.

And in this case, opening or closing the lens does affect the PDAF brightness. By F/5.6, there probably isn't enough combination of exposure & parallax information for this to be useable--nor is there probably a large degree of focus shift. So the Z's don't go slower than F/5.6, though this will go wider.

(Unmasked pixels (such as the one in the middle) would see all of the light).

In reality, Nikon & others probably use a combination of this PDAF and CDAF. As the lens stops down more, I'd expect the CDAF to become more dominant.

For Video: The lens must remain at shooting aperture when focusing. I have been under the impression that at higher apertures, say f8.0 - f22.0, either PD auto focus would not work well or, if used Contrast Detect (CD) autofocus would be slow and may hunt.

I did a test and my Z7 seems to focus very quickly and accurates at these higher (8.0 - 22.0) apertures. If you would like to see for yourself, the following URL will take you to two short videos shot at those apertures.

Can anyone explain how Nikon pulls this off?

Nikon probably uses a combination of PDAF & CDAF.

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